Steel structures design - complete notes, Notas de estudo de Engenharia Civil

Baixe Steel structures design - complete notes e outras Notas de estudo em PDF para Engenharia Civil, somente na Docsity! HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 1 1.0 INTRODUCTION According to published literature, iron was primarily used for making weapons in ancient times. The great Indian epics, Ramayana and Mahabharatha, contain evidence that our forefathers knew about the usage of iron long before many other countries knew about it! Iron is thus very native to India! This is a logical conclusion because, our war-centric epics date back to several thousand years BC. The backdrop of these epics revolves around the eastern, central and southern parts of our country, where there are still huge deposits of iron ore. Not only during Vedic times but also in medieval times, our country has been an epitome of iron wonders. A review in the subsequent sections shows that in modern times too, our country has good examples of construction in steel. Under compelling reasons, both economic and strategic, the western countries brought about the industrial revolution during the last century. Possibly because our country was under the colonial rule at that time and also due to a mood of complacency, our country failed to catch up with the western industrial revolution. During the last 50 years our country has continued to lag behind in infrastructure development and consequently poor consumption of iron and steel. Published studies by the Steel Construction Institute (U.K) have established that countries which have a higher rate of growth in Gross Domestic Product (GDP), have proportionately higher consumption of iron and steel. Soon after independence, our country had to gear itself to meet the demands for development and industrial growth and in the first few Five Year plans made reasonable strides in the area of production and usage of iron and steel. Due to various reasons, steel consumption in our country has been stagnating during the past 2-3 years. Further, steel industry is facing stiff global competition through imports. There is also considerable under utilisation of installed capacity for steel production. For sustenance of steel industry, extensive usage of structural steel in the construction sector is an important requirement. Our country has to live up to the global competition as we have done in Information Technology! In this chapter, we will first discuss about the historical development of iron and steel in the world and India. Since the present days are the days of inter-disciplinary approach to engineering solutions, we will first review the metallurgical aspect of structural steels and then proceed to discuss, the mechanical properties of steel, which are very relevant to structural designers. The approach of treating the metallurgical and mechanical aspects of steel together helps the designer in structural steelwork, to use steel effectively in tune with its performance requirement. Later, we will briefly review the production process of steel, which gives an idea about the different structural steels being produced. We will also review the special variety of steels (such as stainless steels and cold rolled steels). © Copyright reserved Version II 1 - 1 I .exe HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 2.0 HISTORICAL DEVELOPMENT Ancient Hittis were the first users of iron some 3 to 4 millenniums ago. Their language was altered to Indo – European and they were native of Asia Minor. There is archaeological evidence of usage of iron dating back to 1000 BC, when Indus valley, Egyptians and probably the Greeks used iron for structures and weapons. Thus, iron industry has a long ancestry. Wrought iron had been produced from the time of middle ages, if not before, through the firing of iron ore and charcoal in “bloomery”. This method was replaced by blast furnaces from 1490 onwards. With the aid of water- powered bellows, blast furnaces were used for increased output and continuous production. A century later, rolling mill was introduced for enhanced output. The traditional use of wrought iron was principally as dowels and ties to strengthen masonry structures. As early as 6th century, iron tie-bars had been incorporated in arches of Haghia Sophia in Istanbul. Renaissance domes often relied on linked bars to reinforce their bases. A new degree of sophistication was reached in the 1770 in the design of Pantheon in Paris. Fig. 1 World's first cast iron bridge - Coalbrookadale bridge at Shropshire, U.K (Source: John H. Stephens, The Guinness book of Structures (Bridges, towers, tunnels, dams), 1976) Version II 1 - 2 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS colonial past. The “Rabindra Sethu" Howrah Bridge in Calcutta stands testimony to a marvel in steel. Even after its service life, Howrah Bridge today stands as a monument. The recent example is the Second Hooghly cable stayed bridge at Calcutta (Fig. 2), which involves 13,200 tonnes of steel. Similarly the Jogighopa rail-cum-road bridge across the river Brahmaputra (Fig. 3) is an example of steel intensive construction, which used 20,000 tonnes of steel. There are numerous bridges, especially for railways built, exclusively using steel. As far as production of steel in India is concerned, as early as in 1907, Jamsetji Nusserwanji Tata set up the first steel manufacturing plant at Jamshedpur. Later Pandit Jawaharlal Nehru realised the potential for the usage of steel in India and authorised the setting up of major steel plants at Bhilai, Rourkela and Durgapur in the first two five year plans. In Karnataka Sir Mokshakundam Visweswarayya established the Bhadravati Steel Plant. Today we also have a number of private sector steel plants in India. The annual production of steel in 1999-2000 has touched about 25 million tonnes and this is slated to grow at a faster rate. However, when compared to countries like USA, UK, Japan, China and South Korea the per capita consumption of steel in India is extremely low at 27.5 kg/person/year. By way of comparison, rapidly growing economies like China consume about 80 kg/person/year. 3.0 METALLURGY OF STEEL There is a definite need for engineers involved in structural steelwork to acquaint themselves with some metallurgical aspects of steel. This will help the structural engineer to understand ductile behaviour of steel under load, welding during fabrication and erection and other important aspects of steel technology such as corrosion and fire protection. To this end, in the following sections, we shall discuss briefly the metallurgical composition of steel, its effect on heating and cooling and the effects of alloying elements such as carbon, manganese and other additive metals. 3.1 The crystal structure and the transformation of iron Pure iron when heated from room temperature to its melting point, undergoes several crystalline transformations and exhibits two allotropic modifications such as (i) body centred cubic crystal (bcc), (ii) face centred cubic crystal (fcc) as shown in Fig.4. When iron changes from one modification to the other, it involves the ‘latent heat of transformation’. If iron is heated steadily, the rise in temperature would be interrupted when the transformation starts from one phase to the other and the temperature remains constant until the transformations are completed. The flat portion of the heating/cooling curve in Fig. 5 exemplifies this. On cooling of molten iron to room temperature, the transformations are reversed and almost at the same temperature when heated as shown in Fig. 5. Iron upto a temperature of 910°C remains as ‘ferrite’ or ‘α-iron’ with ‘bcc’ crystalline structure. Iron is ferromagnetic at room temperature, its magnetism decreases with increase in temperature and vanishes at about 768°C called the Curie point. The iron that exists between 768°C and 910°C is called the ‘β-iron’ with a ‘bcc’ structure. However, in the realm of metallurgy, this classification does not have much significance. Version II 1 - 5 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Between 910°C and 1400°C, iron transforms itself into ‘austenite‘ or ‘ γ -iron’ with ‘face centred cubic’ (fcc) structure. When temperature is further increased, austenite reverts itself back to ‘bcc’ structure, called the ‘δ-ferrite’. Iron becomes molten beyond 1539°C. The different phases of iron are summarised in Table 1. (b) Face centred cube (a) Body centred cube (bcc) Fig.4 Crystal structure of Iron 200 400 600 800 1000 1200 1400 1600 Temp 0 C δ α 0 β bcc 768 0 C N o n -M a g n et ic Heating γ bcc fcc 1539 0 C 1400 0 C 910 0 C Cooling M a g n et ic Time Fig.5 Allotropy of Iron Version II 1 - 6 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Table 1: Various forms of Iron Stable Temp. Range 0 C Form of matter Phase Identification symbol >2740 Gaseous Gas Gas 1539-2740 Liquid Liquid Liquid 1400-1539 Solid bcc δ-ferrite 910-1400 Solid fcc γ-austenite <910 Solid bcc α-ferrite It is interesting to note that a given number of atoms when arranged as fcc crystals occupy slightly less volume than when arranged as bcc. Due to this reason, there would be a slight volume reduction when iron transforms itself from ferrite to austenite. As shown in Fig. 4, both bcc and fcc structures have interstitial hole positions (inter atomic spaces) which are at mid point of the cube for bcc and at mid point of the cube edges for fcc. In γ-iron or austenite, more volume fraction of interstitials can be accommodated than in α-iron or ferrite. Atoms of elements such as carbon, nitrogen, hydrogen and boron, whose atomic diameter is smaller, would occupy these inter atomic spaces. Such an arrangement is called an ‘interstitial solid solution’ as shown in Fig. 6. In other words the solute atoms are -Ferrite -Carbon Fig.6 Interstitial solid solution of Carbon in Iron accommodated in the interstices (inter atomic spaces) of the crystal lattice of the solvent. If we take the example of carbon, since the interstices of fcc are larger than the bcc, the solubility of carbon in austenite would be more than its solubility in ferrite. 3.2 The Iron-Carbon Constitutional Diagram When carbon in small quantities is added to iron, ‘Steel’ is obtained. Since the influence of carbon on mechanical properties of iron is much larger than other alloying elements, we would study the fundamentals of Iron-Carbon alloy in a little elaborate way. The atomic diameter of carbon is less than the interstices between iron atoms. The carbon goes into solid solution of iron. As carbon dissolves in the interstices, it distorts the original crystal lattice of iron. The iron crystals, which were centred originally at the intersection of symmetry axes of the iron crystals, get distorted as seen from Fig. 6. Version II 1 - 7 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS eutectoid point (2) eutectic point and (3) peritectic point shown in dotted circles in Fig.7. Generally carbon content in structural steels is in the range of 0.12-0.25%. Upto 2% carbon, we get a structure of ferrite + pearlite or pearlite + cementite depending upon whether carbon content is less than 0.8% or beyond 0.8%. Beyond 2% carbon in iron, cast iron is formed. Table 2: Metallurgical terms of iron Name Metallurgical term % Carbon(max) Crystal structure α - Iron Ferrite 0.02 bcc Fe3C Cementite 6.67 - Ferrite + Cementite laminar mixture Pearlite 0.80 (overall) - γ - Iron Austenite 2.0 (depends on temperature) fcc 3.3 The Structural Steels or ferrite – Pearlite Steels The iron-iron carbide portion of the phase diagram that is of interest to structural engineers is shown in Fig.8. The phase diagram is divided into two parts called “hypoeutectoid steels” (steels with carbon content to the left of eutectoid point [0.8% carbon]) and “hyper eutectoid steels” which have carbon content to the right of the eutectoid point. It is seen from the figure that iron containing very low percentage of carbon (0.002%) called very low carbon steels will have 100% ferrite microstructure (grains or crystals of ferrite with irregular boundaries) as shown in Fig. 9(a). Ferrite is soft and ductile with very low mechanical strength. The microstructure of 0.20% carbon steel is shown Fig. 9(b). This microstructure at ambient temperature has a mixture of what is known as ‘pearlite and ferrite’ as can be seen in Fig. 8. Hence we see that ordinary structural steels have a pearlite + ferrite microstructure. However, it is important to note that steel of 0.20% carbon ends up in pearlite + ferrite microstructure, only when it is cooled very slowly from higher temperature during manufacture. When the rate of cooling is faster, the normal pearlite + ferrite microstructure may not form, instead some other microstructure called bainite or martensite may result. We will consider how the microstructures of structural steel are formed by the slow cooling at 0.2% carbon. At about 9000C, this steel has austenite microstructure. This is shown as point ‘i’ in Fig. 8. When steel is slowly cooled, the transformation would start on reaching the point ‘j’. At this point, the alloy enters a two-phase field of ferrite and austenite. On reaching the point, ferrite starts nucleating around the grain boundaries of austenite as shown in Fig. 10(a). By slowly cooling to point 'k', the ferrite grains grow in size and diffusion of carbon takes place from ferrite regions into the austenite regions as shown in Fig. 10(b), since ferrite cannot retain carbon above 0.002% at room temperature. At this point it is seen that a network of ferrite crystals surrounds each austenite grain. On slow cooling to point ‘l’ the remaining austenite gets transformed into ‘pearlite’ as Version II 1 - 10 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS shown in Fig.10(c). Pearlite is a lamellar mixture of ferrite and cementite. The amount of ‘pearlite’ for a given carbon content is usually calculated using the lever rule assuming 0% carbon in ferrite as given below: Volume Carbonof%8.0 Carbonof% Pearliteoffraction = For example for microstructure of a 0.2% carbon steel would consist of a quarter of pearlite and three- quarters of ferrite. As explained earlier, ferrite is soft and ductile and pearlite is hard and it imparts mechanical strength to steel. The higher the carbon content, the higher would be the pearlite content and hence higher mechanical strength. Conversely, when the pearlite content increases, the ferrite content decreases and hence the ductility is reduced. The microstructure of ferrite-pearlite steels is given in Fig.9 (b). The white portion in Fig.9 (b) is ferrite and the black is pearlite. The constituents of the specimen are (pearlite + ferrite), but the phases are (ferrite + cementite). At the Eutectoid point where the carbon content is 0.8%, a fully lamellar pearlite structure is obtained as shown in Fig.9(c). Fig.9 Microstructures of steels (a)100% Ferrite in extra low carbon steel (b)Ferrite+Pearlite (c) 100% Pearlite in eutectoid steel (d)Pearlite+Cementite in hyper-eutectoid steel (Source: Thelning K.E,. Steel and its heat treatment, Butterworths, 1984.) Version II 1 - 11 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Note: Microstructures at (c) and (d) are not observed in structural steels. Austenite Ferrite nuclei (a) (b) Ferrite Austenite C PearliteFerrite (c) Fig.10 Different stages of formation of Pearlite We also see from Fig.9 (d) that in the case of hyper eutectoid steels (steels having carbon content more than 0.8%), a microstructure of (cementite + pearlite) is obtained. Ofcourse microstructures (c) and (d) are not observed in structural steels. As mentioned earlier, increase in carbon content is not the only way to obtain increased mechanical strength. We would see in the next section, the other methods of increasing the strength of steel. 3.4 Strengthening structural steels Cooling rate of steel from austenite region to room temperature produces different microstructures, which impart different mechanical properties. In the case of structural steels, the (pearlite + ferrite) microstructure is obtained after austenitising, by cooling it very slowly in a furnace. This process of slow cooling in a furnace is called ‘annealing’. As, mentioned in the earlier section, the formation of pearlite, which is responsible for mechanical strength, involves diffusion of carbon from ferrite to austenite. In the annealing process sufficient time is given for the carbon diffusion and other transformation processes to get completed. Hence by full annealing we get larger size pearlite crystals as shown in the cooling diagram in Fig.11. It is very important to note that the grain size of crystal is an important parameter in strengthening of steel. The yield strength of steel is related to grain size by the equation d k ff 0y += (1) where fy is the yield strength, f0 is the yield strength of very large isolated crystals (for mild steel this is taken as 5 N/mm2 ) and ‘k’ is a constant, which for mild steel is 38 N/mm3/2. From Eq.1 we see that decreasing the grain size could enhance the yield strength. We will see in the following section as to how this reduction of grain size could be controlled. The grain size has an influence both in the case of mechanical strength and the temperature range of the ductile-brittle transition (temperature at which steel Version II 1 - 12 I .exe HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 3.6 Inclusions and alloying elements in steel Steel contains impurities such as phosphorous and sulphur and they eventually form phosphides and sulphides which are harmful to the toughness of the steel. Hence it is desirable to keep these elements less than 0.05%. Phosphorous could be easily removed compared to sulphur. If manganese (Mn) is added to steel, it forms a less harmful manganese sulphide (MnS) rather than the harmful iron sulphide. Sometimes calcium, cerium, and other rare earth elements are added to the refined molten steel. They combine with sulphur to form less harmful elements. Steel treated this way has good toughness and such steels are used in special applications where toughness is the criteria. The addition of manganese also increases the under cooling before the start of the formation of ferrite+ pearlite. This gives fine-grained ferrite and more evenly divided pearlite. Since the atomic diameter of manganese is larger that the atomic diameter of iron, manganese exists as ‘substitutional solid solution’ in ferrite crystals, by displacing the smaller iron atoms as shown in Fig.13. This improves the strength of ferrite because the distortion of crystal lattice due to the presence of manganese blocks the mechanical movement of the crystal lattices. However, manganese content cannot be increased unduely, as it might become harmful. Increased manganese content increases the formation of martensite and hence hardness and raises its ductile to brittle transition temperature (temperature at which steel which is normally ductile becomes brittle). Because of these reasons, manganese is restricted to 1.5% by weight. Based on the manganese content, steels are classified as carbon-manganese steels (Mn>1%) and carbon steels (Mn<1%). In recent years, micro alloyed steels or high strength low alloy (HSLA) steels have been developed. They are basically carbon manganese steels in which small amounts of aluminium, vanadium, mobium or other elements are used to help control the grain size. -Manganese -Ferrite Fig.13 Substitutional solid solution of Manganese in Iron These steels are controlled rolled and/or controlled cooled to obtain fine grain size. They exhibit a best combination of strength and toughness and also are generally weldable without precautions such as preheating or post heating. Sometimes 0.5% molybdenum is added to refine the lamellar spacing in pearlite, and to make the pearlite evenly Version II 1 - 15 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS distributed. Today steel with still higher performance are being developed all over the world to meet the following specifications such as: (a) high strength with yield strength of 480 MPa and 690 MPa, (b) excellent weldability without any need for preheating, (c) extremely high toughness with charpy V notch values of 270 N-m @ 23oC compared with current bridge design requirement of 20 N.-m @ 23oC, and (d) corrosion resistance comparable to that of weathering steel. (The terminology used above has been discussed later in this chapter). The micro alloyed steels are more expensive than ordinary structural steels, however, their strength and performance outweighs the extra cost. Some typical steels with their composition range and properties and their relevant codes of practice, presently produced in India, are given in Tables 3 and 4. These steels are adequate in many structural applications but from the perspective of ductile response, the structural engineer in cautioned against using unfamiliar steel grades, without checking the producer supplied properties. Weldability of steel is closely related to the amount of carbon in steel. Weldability is also affected by the presence of other elements. The combined effect of carbon and other alloying elements on the weldability is given by “carbon equivalent value (Ceq)”, which is given by Ceq =%C + % Mn/6 + (% Cr + % Mo + % V)/5+(% Ni + % Cu)/15 The steel is considered to be weldable without preheating, if Ceq < 0.42%. However, if carbon is less than 0.12% then Ceq can be tolerated upto 0.45%. Table 3 Types of steel and their relevant IS standards Type of steel Relevant IS standards Structural steel 226(withdrawn),2062,3502,1977,961,8500 Steel for bars, rivets etc. 1148,1149,1570,2073,7388,4431,4432, 5517 Steel for tubes and pipes 1239,1914,1978 Table 4 Chemical composition of some typical structural steels Type of steel Designa- tion IS: code C S Mn P Si Cr Carbon equiva- lent Fe 410A 2062 0.23 .050 1.5 .050 - - SK 0.42 Fe 410B 2062 0.22 .045 1.5 .045 0.4 - SK 0.41 Standard structural steel Fe 410C 2062 0.20 .040 1.5 .040 0.4 - K 0.39 Fe 440 8500 0.20 .050 1.3 .050 .45 0.40 Fe540 8500 0.20 .045 1.6 .045 .45 0.44 Micro alloyed high strength steel Fe590 8500 0.22 .045 1.8 .045 .45 0.48 K- killed steel SK- Semi Killed steel (Explained in section 6.2) Version II 1 - 16 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 3.7 Thermo- Mechanically Control Process (TMCP) steels With increase in height, size and span in buildings, higher strength, longer section and heavier thickness are required for steel products to be applied. In the conventional method, increased strength is secured by increasing addition of alloying elements. However, such an addition adversely results in deterioration of weld crack resistance due to increase in carbon equivalent (Ceq ) and lowering of weld efficiency, due to the necessity to secure high pre-heating temperatures. To cope up with such requirements, Thermo-Mechanical Control Process (TMCP) steels with yield strength of 490 Mpa are being produced in countries like Japan. TMCP allows production of steel products having higher strength but carbon equivalent similar to those of conventional steels. Even for extra heavy sections, excellent weldabilty and stable strength can be achieved through application of TMCP. Even for thickness greater than 40 mm TMCP steels are finding wide applications. 4.0 STAINLESS STEELS In an iron-chromium alloy, when chromium content is increased to about 11%, the resulting material is generally classified as a stainless steel. This is because at this minimum level of chromium, a thin protective passive film forms spontaneously on steel, which acts as a barrier to protect the steel from corrosion. On further increase in chromium content, the passive film is strengthened and achieves the ability to repair itself, if it gets damaged in the corrosive environment. 'Ni' addition in stainless steel improves corrosion resistance in reducing environments such as sulphuric acid. It also changes the crystal structure from bcc to fcc thereby improving its ductility, toughness and weldability. 'Mo' increases pitting and crevice corrosion in chloride environments. Stainless steel is attractive to the architects despite its high cost, as it provides a combined effect of aesthetics, strength and durability. Now a days, stainless steel is used extensively in building construction. For example, the worlds’ tallest twin tower situated in Kuala Lampur, Malaysia used about 4000 tonnes of stainless steel made in India! Table 5 gives typical grades of stainless steel, which are used in building construction. Table 5 Stainless Steel grades and their usage Grade of stainless steel Usage 316 (18% Cr) Profiled roofing, cladding, gutters, facades and hand railings - in highly polluted environments 304 (18% Cr-(% Ni) Decorative elements in areas near coast line. Also for kitchen and sanitary wares - coastal and less polluted areas 430 (17% Cr) Roofing, gutters, decorative wall tiles, hallow structural sections non-polluted environments 409 (11% Cr) Painted roofing- non-polluted environments Stainless steels are available in variety of finishes and it enhances the aesthetics of the structure. On Life Cycle cost Analysis (LCA), stainless steel works out to be economical Version II 1 - 17 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS f ε εy fy 0.2% strain Uniform plastic Fracture 0.2% proof stress Elastic Non-uniform plastic Fig. 17 Stress strain curve for continuously yielding structural steels P Area=S L P Fig.18 Tensile test specimen before rupture However S is very difficult to evaluate compared to S0 and the nominal stress or the engineering stress is given by fn = P/ S0 . Similarly, the engineering strain is taken as the ratio of the change in length to original length. However the true strain is obtained when instantaneous strain is integrated over the whole of the elongation, given by ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ =∫= 0 L 0L t L L ln l dlε (2) By suitable manipulation it could be shown that )1(ff nnt ε+= (3) Version II 1 - 20 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS and similarly )1ln( nt εε += (4) where ft and fn are the true and nominal stresses respectively and εt and εn are the true and nominal strains respectively. 5.2 Hardness Hardness is regarded as the resistance of a material to indentations and scratching. This is generally determined by forcing an indentor on to the surface. The resultant deformation in steel is both elastic and plastic. There are several methods using which the hardness of a metal could be found out. They basically differ in the form of the indentor, which is used on to the surface. They are presented in Table 6. Table 6 Hardness testing methods and their indentors Hardness Testing Method Indentor (a) Brinell hardness Steel ball (b) Vickers hardness Square based diamond pyramids of 135 O included angle (c ) Rockwell hardness Diamond core with 120 O included angle Note: Rockwell hardness testing is not normally used for structural steels. In all the above cases, hardness number is related to the ratio of the applied load to the surface area of the indentation formed. The testing procedure involves forcing the indentor on to the surface at a particular road. On removal, the size of indentation is measured using a microscope. Based on the size of the indentation, hardness is worked out. For example, Brinell hardness (BHN) is given by the ratio of the applied load and spherical area of the indentation i.e. ⎥⎦ ⎤ ⎢⎣ ⎡ −− = 22 dDD)2/d( P BHN π (5) where P is the load, D is the ball diameter, d is the indent diameter. The Vickers test gives a similar hardness value (VHN) as given by 2 L P854.1 VHN = (6) where L is the diagonal length of the indent. Some typical values of hardness of some metals are presented in Table.7. Version II 1 - 21 I .exe HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Table 7 Hardness values of some metals Metal Brinell Hardness Number (BHN) Vickers Hardness Number (VHN) Copper (annealed) 49 53 Brass (annealed) 65 70 Steel 150-190 157-190 5.3 Effect of temperature on ductility and notch toughness At lower temperatures below 00C, the yield strength of steel is only marginally affected, while there is substantial reduction in ductility and toughness. The ultimate behaviour of steel progressively changes from ductile to brittle, reaching a lowest value of toughness at a threshold temperature called “Ductile-to-Brittle-Transition-Temperature” (DBTT) range. The transition temperature for structural steel is generally well below the room temperature. However, if it is near to the ambient temperature, due to the loss of ductility, engineering components may fail under service loading. This transition temperature is affected by metallurgical aspects such as grain size and also by the presence of notches. In certain instances, due to deviations in correct processing procedure when the DBTT of steel is above room temperature or the application temperature, serious failures have been observed in ships cruising through the Arctic sea, bridges in cold climates and cryogenic gas storage facilities. The charpy “V” notch test (also known as the notch-toughness test) is used to determine the DBTT. In this test, a falling pendulum hammer fitted with a striking edge as shown in Fig.19 breaks a standard notched specimen (Fig.20). In principle, the energy absorbed by the specimen during its failure translates into a loss of potential energy of the pendulum. Thus, a rough measure of this absorbed energy can be calculated from the difference between initial height (h1) of the pendulum when released and the maximum height (h2) it reaches on the far side after breaking the specimen. The variation of absorbed energy with respect to temperature is shown in Fig.21. Generally, structural engineering standards and codes [IS: 1757 (1988)] will allow the use of only those steels that exhibit a minimum energy absorption capability at a pre-determined temperature say 20 N-m at 23 ±5 oC. 5.4 Strain rate effect on yield strengths of steel Strain rate is another factor that affects the strength of steel. Typically, the tensile and yield strength increases at higher strain rates as shown in Fig.22 except that at higher temperatures the reverse is true. It may also be noted that increase in strain rate causes reduction in ductility. Consideration of this phenomenon is crucial for blast resistant design of steel structures in which very high strain rates are expected but of little practical significance in earthquake engineering applications wherein the strain rate is well within the range where fy does not change very much. Version II 1 - 22 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 6.0 THE MANUFACTURING PROCESS OF STRUCTURAL STEEL For design of structures, the structural engineer uses long and flat products. The long products include: angles; channels; joists/beams; bars and rods; cold twisted deformed (CTD) bars; thermo-mechanically treated (TMT) ribbed bars, while the flat products comprise: plates; hot rolled coils (HRC) or cold rolled coils (CRC)/sheets in as annealed or galvanised condition. The starting material for the finished products is as given below: • Blooms in case of larger diameter/cross-section long products • Billets in case of smaller diameter/cross-section long products • Slabs for hot rolled coils/sheets • Hot rolled coils in case of cold rolled coils/sheets • Hot/Cold rolled coils/sheets for cold formed sections 6.1 Electric Arc or Induction Furnace Route for Steel Making in Mini or Midi Steel Plants The production process depends upon whether the input material to the steel plant is steel scrap or the basic raw material i.e. iron ore. In case of former, the liquid steel is produced in Electric Arc Furnace (EAF) or Induction Furnace (IF) and cast into ingots or continuously cast into blooms/billets/slabs for further rolling into desired product. The steel mills employing this process route are generally called as mini or midi steel plants. Since liquid steel after melting contains impurities like sulphur and phosphorus beyond desirable limits and no refining is generally possible in induction furnace. The structural steel produced through this process is inferior in quality. Through refining in EAF, any desired quality (i.e. low levels of sulphur and phosphorus and of inclusion content) can be produced depending upon the intended application. Quality can be further improved by secondary refining in the ladle furnace, vacuum degassing unit or vacuum arc degassing (VAD) unit. 6.2 Iron Making and Basic Oxygen Steel Making in Integrated Steel Plants When the starting input material is iron ore, then the steel plant is generally called the integrated steel plant. In this case, firstly hot metal or liquid pig iron is produced in a vertical shaft furnace called the blast furnace (BF). Iron ore, coke (produced by carbonisation of coking coal) and limestone [Fig. 23(a)] in calculated proportion are charged at the top of the blast furnace. Coke serves two purposes in the BF(Fig.23(b)). Firstly it provides heat energy on combustion and secondly carbon for reduction of iron ore into iron. Limestone on decomposition at higher temperature provides lime, which combines with silica present in the iron ore to form slag. It also combines with sulphur in the coke and reduces its content in the liquid pig iron or hot metal collected at the bottom of the BF. The hot metal contains very high level of carbon content around 4%; silicon in the range of 0.5-1.2%; manganese around 0.5%; phosphorus in the range 0.03-0.12%; and somewhat higher level of sulphur around 0.05%. Iron with this kind of composition is Version II 1 - 25 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS highly brittle and cannot be used for any practical purposes. Hot metal is charged in to steel making vessel called LD converter or the Basic Oxygen Furnace (BOF). Open- (a) (h) (g) (f) (e) (d) (c) (b) Fig.23 Schematic diagram of manufacturing of structural steel sections from Iron ore(Source: Adams P.F., Krentz H.A. and Kulak G.L., “Limit state design in structural steel – SI Units”, Canadian Institute of Steel Construction (1979).) hearth process is also used in some plants, though it is gradually being phased out [Fig.23(c)]. Oxygen is blown into the liquid metal in a controlled manner, which reduces the carbon content and oxidises the impurities like silicon, manganese, and phosphorus. Lime is charged to slag off the oxidised impurities. Ferro Manganese (FeMn), Ferro Silicon (FeSi) and/or Aluminium (Al) are added in calculated amount to deoxidise the liquid steel, since oxygen present in steel will appear as oxide inclusions in the solid state, which are very harmful. Ferro alloy addition also helps to achieve the desired composition. Generally the structural steel contains: carbon in the range 0.10-0.25%; manganese in the range 0.4-1.2%; sulphur 0.025-0.050%; phosphorus 0.025-0.050% depending upon specification and end use. Some micro alloying elements can also be added to increase the strength level without affecting its weldability and impact toughness. Version II 1 - 26 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS If the oxygen content is brought down to less than 30 parts per million (PPM), the steel is called fully killed, whereas if the oxygen content is around 150 PPM, then the steel is called semi-killed. During continuous casting, only killed steel is used. However, both semi-killed and killed steels are cast in the form of ingots. The present trend is to go in for casting of steel through continuous casting, as it improves the quality, yield as well as the productivity. 6.3 Casting and Primary/Finish Rolling Liquid steel is cast into ingots [Fig.(23(d)], which after soaking at 1280-13000 C in the soaking pits [(Fig.23(e)] are rolled in the blooming and billet mill into blooms/billets [(Fig.23(f)] or in slabbing mill into slabs. The basic shapes such as ingots, cast slabs, bloom and billets are shown in Fig.24. The blooms are further heated in the reheating furnaces at 1250-12800 C and rolled into billets or to large structurals[(Fig.23(h)]. The slabs after heating to similar temperature are rolled into plates in the plate mill. Even though the chemical composition of steel dictates the mechanical properties, its final mechanical properties are strongly influenced by rolling practice, finishing temperature, and cooling rate and subsequent heat treatment. The slabs or blooms or the billets can directly be continuously cast from the liquid state and thereafter are subjected to further rolling after heating in the reheating furnaces. . Ingot slab bloom Billet Fig. 24 Basic shapes and their relative proportions Fig.25 Primary rolls for plates Version II 1 - 27 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS In brief, different aspects of steel as are important to a structural engineer, have been described. 10. REFERENCES 1. Graham W. Owens and Peter R. Knowles, “Steel designer’s manual”, ELBS Fifth Edition (1994). 2. Adams P.F., Krentz H.A. and Kulak G.L., “Limit state design in structural steel – SI Units”, Canadian Institute of Steel Construction (1979). 3. IS:2062 Steel for general structural purposes – Specification, Fourth Revision (1992). 4. IS:961 Specification for structural steel (High Tensile) , (1962). 5. Robert E. Reed Hill ,” Physical metallurgy principles”, Second Ed., EWP, New Delhi, (1985). 6. IS:1608 Method of tensile testing of steel products (Second Revision) (1995). 7. IS:1757 Method for charpy impact test (V-notch) on metallic material (1988) 8. IS: 8500, Structural steel - Micro alloyed (medium and high strength qualities) - Specification, First Revision (1991). Version II 1 - 30 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES 2 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES 1.0 INTRODUCTION Corrosion, fire protection and fatigue failure of steel structures are some of the main concerns of an engineer involved in the design and construction of structural steel work and these aspects do warrant extra attention. A review of international literature and the state of the art in constructional steelwork would reassure the designer that many aspects of corrosion, fire and fatigue behaviour of structural steel work, are no longer the major issues. For example, the steel construction industry has developed excellent protective coatings that would retain service life even after 20 years without any serious attention! Similarly the emergence of ‘fire engineering of steel structures’ as a specialised discipline has addressed many of the concerns regarding the structural steel work under fire. In India ‘Fire Resistant Steels (FRS)’ are available which are quite effective in steelwork subjected to elevated temperatures. They are also cost effective compared to mild steel! Similarly, fatigue behaviour of steel structural systems has been researched extensively in the past few decades and has been covered excellently in the published literature. Many countries have a separate code of practice, which deals exclusively with the fatigue resistance design of steel structural systems. Today, substantial information and guidelines are available to the designers so that these three aspects could be handled in a routine manner. In this chapter we will review aspects of corrosion, fire protection and fatigue behaviour of structural steelwork briefly and outline suitable prevention methods. 2.0 CORROSION OF STEEL There is a mindset among many Indian designers, that steel corrodes the most in India compared to other countries. This conception is very much untrue! No doubt, steel corrodes all over the world but the difference is, the problem is better tackled in the advanced countries. With the advent of new technologies of corrosion protection and better understanding of the material behaviour of steel, corrosion of steel no longer causes any undue worry for structural designers involved in structural steelwork. Nevertheless, a designer involved in structural steel work must be aware of the phenomena of corrosion and its prevention methods, both simple and detailed. 2.1 Corrosion mechanism as a miniature battery Every metal found in nature has a characteristic electric potential, based on its atomic structure and also the ease with which the metal can produce or absorb electrons. Those metals, which provide electrons more readily, are called anodes and those that absorb electrons are called cathodes. Anodes and cathodes are called electrodes and if they get connected in the presence of an electrolyte (a conducting medium), they form a battery as shown in Fig.1. © Copyright reserved Version II 2 - 1 I .exe CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES No material individually can be called as cathode or anode, as they can serve both the functions depending on the relative potential of the material to which they are connected. For example, steel is anodic in the presence of stainless steel or brass and cathodic in the presence of zinc or aluminium. From the mechanism shown in Fig. 1, we see that two bodies of different electric potential electrically connected together in the presence of an electrolyte, the anodic body provide electrons to the cathode (To remember easily: Anodes–Away; Cathodes-Collect). In this process the anode is gradually destroyed, in other words it corrodes. On the other hand, a body will not corrode until it is immersed in or wetted by an electrolytic solution and gets electrically connected to another body having a more positive electric potential. This is the main principle called “eliminate the electrolyte”, using which we device many of the corrosion prevention methods, in structural steel work. Fig.1 Mechanism of corrosion as a miniature battery A Metal Connection C Electrolyte 2.2 Corrosion of steel In the case of steel, when favourable condition for corrosion occurs, the ferrous ions go into solution from anodic areas. Electrons are then released from the anode and move through the cathode where they combine with water and oxygen to form hydroxyl ions. These react with the ferrous ions from anode to produce hydrated ferrous oxide, which further gets oxidised into ferric oxide, which is known as the ‘red rust’. Let us consider a portion of steel member, which is slightly rusted as shown in Fig. 2. The portion of the surface protected by the oxide film (rust) would be cathodic with respect to a portion, which is not so protected. Therefore, there will be a difference in electrical potential and hence the anode will corrode, forming rust on its surface. As rust builds up on one portion of the body, it becomes less anodic with respect to a previously rusted area. In this way they form and reform batteries and corrode the entire surface. Version II 2 - 2 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES Hydrogen embrittlement: This occurs mostly in fasteners and bolts. The atomic hydrogen may get absorbed into the surface of the fasteners. When tension is applied to these fasteners, hydrogen will tend to migrate to points of stress concentration. The pressure created by the hydrogen creates and/or extends a crack. The crack grows in subsequent stress cycles. Although hydrogen embrittlement is usually included in the discussion about corrosion, actually it is not really a corrosion phenomenon. C A Fig. 5 The mechanism of fretting corrosion 3.0 CORROSION PROTECTION TO STRUCTURAL STEEL ELEMENTS Taking care of the following points can provide satisfactory corrosion protection to most structural steel elements: • Avoiding of entrapment and accumulation of moisture and dirt in components and connections by suitable detailing as shown in Fig. 6 7 4 4 7 Fig.6 Simple orientation of members to avoid dirt and water entrapment • Avoiding contact with other materials such as bimetallic connections, as explained in the earlier section. • Detailing the structural steel work to enhance air movement and thereby keeping the surfaces dry as shown in Fig.7 • Providing suitable drain holes wherever possible to initiate easy draining of the entrapped water as shown in Fig. 8 Version II 2 - 5 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES • Providing suitable access to all the components of steel structures for periodic maintenance, cleaning and carrying out inspection and maintenance at regular intervals. • Providing coating applications to structural steel elements. Metallic coatings such as hot-dip galvanising, metal spray coatings, etc. are very effective forms of corrosion protection. Cleaning of the surfaces and applying suitable paints is the most commonly used and reliable method of corrosion protection. This is discussed in detail in the next section. 8 4 Fig.7 Detailing to enhance air movement between joints 8 4 Fig.8 Provision of drain holes wherever possible. 3.1 Surface preparation Before applying any protective coating to structural steel work, it is very essential that the surface must be free of dirt and other materials that would affect its adhesion. In this section we review the surface preparation methods which are commonly employed in structural steel work. Version II 2 - 6 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES Structural steel comes out of the mill with a mill scale on its surface. On weathering, water penetrates into the fissures of the mill scale and rusting of the steel surface occurs. The mill scale loses its adhesion and begins to shed. Mill scale therefore needs to be removed before any protection coatings are applied. The surface of steel may also contain dirt or other impurities during storage, transportation and handling. The various surface preparation methods are briefly explained below. Manual preparation: This is a very economical surface cleaning method but only 30% of the rust and scale may be removed. This is usually carried out with a wire brush. Mechanical preparation: This is carried out with power driven tools and up to 35% cleaning can be achieved. This method is quite fast and effective. Flame cleaning: In this process an Oxy-gas flame causes differential thermal expansion and removes mill scale more effectively. Acid pickling: This involves the immersion of steel in a bath of suitable acids to remove rust. Usually this is done before hot dip galvanising (explained in the next section). Blast cleaning: In this process, abrasive particles are projected at high speed on to the steel surface and cleaning is effected by abrasive action. The common blast cleaning method is the ‘sand blasting’. However in some states of India, sand blasting is not allowed due to some environmental reasons. 3.2 Preventive coatings The principal protective coatings applied to structural steel work are paints, metal coatings or combination of these two. Paints basically consist of a pigment, a binder and solvent. After the paint has been applied as a wet film, the solvent evaporates leaving the binder and the pigment on the surface. In codes of practices relating to corrosion protection, the thickness of the primer, the type of paints and the thickness of the paint in term of microns are specified depending upon the corrosive environment. The codes of practice also specify the frequency with which the change of paint is required. Metal coatings on structural steel work are almost either zinc or aluminium. Hot dip Zinc coatings known as “galvanising”, involves dipping of the steelwork into a bath of molten Zinc at a temperature of about 4500C. The work piece is first degreased and cleaned by pickling to enhance the wetting properties. Sometimes hot dip aluminising is also done. Alternatively, metal coating could also be applied using metal spraying. 3.3 Weathering steels To protect steel from corrosion, some countries produce steels which by themselves can resist corrosion. These steels are called as “weathering steels or Corten steels”. Weathering steels are high strength alloy weldable structural steels, which possess excellent weathering resistance in many non-polluted atmospheric conditions. They contain up to 3% of alloying elements such as chromium, copper, nickel, phosphorous, etc. On exposure to air, under suitable conditions, they form adherent protective oxide coatings. This acts as a protective film, which with time and appropriate conditions causes the corrosion rate to reduce until it is a low terminal level. Conventional coatings are, therefore, not usually necessary since the steel provides its own protection. Version II 2 - 7 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES Table 2 Corrosion protection treatment in External environment Shop applied treatments Option 1 Option 2 Option 3 Option 4 Option 5 Option 6 Surface preparation Blast clean Blast clean Blast clean Blast clean Grit blast Blast clean Pre fabrication primer Zinc phosphate epoxy 2 pack Zinc rich epoxy ----- 2 pack Zinc rich epoxy ---- Ethyl Zinc Silicate Post fabrication primer High build Zinc phosphate modified alkyd 2 pack Zinc rich epoxy Hot dip galvanise 2 pack Zinc rich epoxy Sprayed Zinc or Sprayed Aluminum Ethyl Zinc Silicate I ntermediate coat ---- High build Zinc phosphate 2 pack epoxy Micaceous iron oxide Sealer Chlorinated rubber alkyd Top coat ---- ---- ---- 2 pack epoxy Micaceous I ron oxide Sealer ---- Site applied treatments Surface preparation As necessary As necessary No site treatment As necessary No site treatment As necessary Primer Touch in Touch in ---- ---- ---- Touch in I ntermediate coat ---- Modified Alkyd Micaceous I ron Oxide ---- Touch in --- High build Micaceous I ron oxide chlorinated rubber Top coat High build Alkyd finish Modified Alkyd Micaceous I ron Oxide ---- High build chlorinated rubber ---- High build Micaceous I ron oxide chlorinated rubber Expected life in years Normal I nland 12 18 20 (+ -) 20 (+ -) 20 20+ Polluted I nland 10 15 12 (+ -) 18 (+ -) 15-20 20+ Normal coastal 10 12 20 (+ -) 20 (+ -) 20 20+ Polluted coastal 8 10 10 (+ -) 15 (+ -) 15-20 20+ Version II 2 - 10 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES In the case of concrete exposed to fire, it will start changing its colour to pink at about 2850C and will turn into deep red at about 5900C. Soon after that, concrete would turn into quartz aggregate and spalling would start. The degree of spalling is dependent upon the rate of temperature rise, moisture content and maximum temperature for each type of aggregate. Hence it seen that concrete exposed to fire beyond say 6000C, may undergo an irreversible degradation in mechanical strength unlike steel where much of its original strength is regained. The above points underline the advantage of steel in terms of economy even in the case of fire. 4.1 Fire loads and fire rating of steel structures The term ‘fire load’ in a compartment of a structure is the maximum heat that can be theoretically generated by the combustible items and contents of the structure. The fire load could be measured as the weight of the combustible material multiplied by the calorific value per unit weight. Fire load is conveniently expressed in terms of the floor space as MJ/m2 or Mcal/m 2. More often it would be expressed in terms of equivalent quantity of wood and expressed as Kg wood / m2 (1 Kg wood = 18MJ). The commonly encountered fire loads are presented in Table 3. The values are just an indication of the amount of fire load and the values may change from one environment to the other and also from country to country. Table 3 Fire load on steel structures Examples of fire load in various structures Type of steel structure Kg wood / m 2 School 15 Hospital 20 Hotel 25 Office 35 Departmental store 35 Textile mill show room >200 The fire rating of steel structures are expressed in units of time ½, 1, 2, 3 and 4 hours etc. The specified time neither represents the time duration of the real fire nor the time required for the occupants to escape. The time parameters are basically a convenient way of comparative grading of buildings with respect to fire safety. Basically they represent the endurance of structural steel elements under standard laboratory conditions. Fig. 9 represents the performance of protected and unprotected steel in a laboratory condition of fire. The rate of heating of the unprotected steel is obviously quite high as compared to the fire-protected steel. We shall see in the following sections that these two types of fire behaviour of steel structure give rise to two different philosophies of fire design. The time equivalence of fire resistance for steel structures or the fire rating could be calculated as feq CWQMinutesT =)( (1) Version II 2 - 11 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES where Qf is the fire load MJ/m 2 which is dependent on the amount of combustible material, ‘W’ is the ventilation factor relating to the area and height and width of doors and windows and ‘C’ is a coefficient related to the thermal properties of the walls, floors and ceiling. As an illustration, the “W” value for a building with large openings could be chosen as 1.5 and for highly insulating materials “C” value could be chosen as 0.09. 4.2 Mechanical properties of steel at elevated temperatures We need to know about the mechanical properties of steel at elevated temperatures in the case of fire resistant design of structural steel work. Hence in this section we review the important mechanical aspects of steel at elevated temperatures. The variations of the non-dimensional modulus of elasticity, yield strength and coefficient of thermal expansion with respect to temperature are shown in Fig. 10. The corresponding equations are given below. The variation of modulus of elasticity ratio E with respect to the corresponding value at 200C, with respect to temperature T is given by 1000 500 0 0C 30 60 90 Furnace temperature Unprotected steel Fire protected steel Time (Minutes) Fig. 9 Rate of heating of structural steel work CTCfor T T CE TE E 00 0 _ 6000 1100 ln2000 0.1 )20( )( << ⎥⎦ ⎤ ⎢⎣ ⎡ +== (2) C 0 1000TC 0 600for 5.53T ) 1000 T 0.1(690 <<− − = The yield stress of steel remains unchanged up to a temperature of about 2150C and then loses its strength gradually. The yield stress ratio (with respect to yield stress at 20 _ f 0C) vs. temperature T relation is given by Version II 2 - 12 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES There are two methods of assessing whether or not a bare steel member requires fire protection. The first is the load ratio method which compares the ‘design temperature’ i.e. maximum temperature experienced by the member in the required fire resistance time, and the ‘limiting temperatures’, which is the temperature at which the member fails. High Hp / A Value Low Hp / A Value Fig.11 The section factor concept Hp =2D+4B-2tHp =2D+2B Hp =2D+3B-2t tD Hp =2D+B B Fig. 12 Some typical values of HP of fire protected steel sections The limiting temperatures for various structural members are available in the relevant codes of practice. The load ratio may be defined as: Version II 2 - 15 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES Load applied at the fire limit state Load ratio = --------------------------------------------------------------------- Load causing the member to fail under normal conditions If the load ratio is less than 1, then no fire protection is required. In the second method, which is applicable to beams, the moment capacity at the required fire resistance time is compared with the applied moment. When the moment capacity under fire exceeds the applied moment, no fire protection is necessary 4.5 Methods of fire protection Fire protection methods are basically dependent on the fire load, fire rating and the type of structural members. The commonly used fire protection methods are briefly enumerated below. Spray protection: The thickness of spray protection depends on the fire rating required and size of the job. This is a relatively low cost system and could be applied rapidly. However due to its undulating finish, it is usually preferred in surfaces, which are hidden from the view. Board protection: This is effective but an expensive method. Board protection is generally used on columns or exposed beams. In general no preparation of steel is necessary prior to applying the protection. Intumescent coating: These coatings expand and form an insulating layer around the member when the fire breaks out. This type of fire protection is useful in visible steelwork with moderate fire protection requirements. This method does not increase the overall dimensions of the member. Certain thick and expensive intumescent coatings will give about 2-hour fire protection. But these type of coatings require blast cleaned surface and a priming coat. Concrete encasement: This used to be the traditional fire proofing method but is not employed in structures built presently. The composite action of the steel and concrete can provide higher load resistance in addition to high fire resistance. However this method results in increases dead weight loading compared to a protected steel frame. Moreover, carbonation of concrete aids in encouraging corrosion of steel and the presence of concrete effectively hides the steel in distress until it is too late. 5.0 FATIGUE OF STEEL STRUCTURES A component or structure, which is designed to carry a single monotonically increasing application of static load, may fracture and fail if the same load or even smaller load is, applied cyclically a large number of times. For example a thin rod bent back and forth beyond yielding fails after a few cycles of such repeated bending. This is termed as the ‘fatigue failure’. Examples of structures, prone to fatigue failure, are bridges, cranes, offshore structures and slender towers, etc., which are subjected to cyclic loading. The fatigue failure is due to progressive propagation of flaws in steel under cyclic loading. This is partially enhanced by the stress concentration at the tip of such flaw or crack. As we can see from Fig. 13, the presence of a hole in a plate or simply the presence of a notch in the plate has created stress concentrations at the points ‘m’ and ‘n’. Version II 2 - 16 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES The stress at these points could be three or more times the average applied stress. These stress concentrations may occur in the material due to some discontinuities in the material itself. These stress concentrations are not serious when a ductile material like steel is subjected to a static load, as the stresses redistribute themselves to other adjacent elements within the structure. d m n σ σ Hole m n Notch Stress concentration σ >σ Fig. 13 Stress concentrations in the presence of notches and holes σ σ d m n fy = Yield stress Net section fully plastified at failure σu σu Fig. 14 Stress pattern at the point of static failure At the time of static failure, the average stress across the entire cross section would be the yield stress as shown in Fig.14. However when the load is repeatedly applied or the load Version II 2 - 17 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES load (R = 1). Different curves for different values of fatigue life ‘N’ can be drawn through point ‘C’ representing the fatigue strength for various numbers of cycles. The vertical distance between any point on the ‘N’ curve and the 450 line OC through the origin represents the stress range. As discussed earlier, the stress range is the important parameter in the fatigue resistant design. Higher the stress range a component is subjected to, lower would be its fatigue life and lower the stress range, higher would be the fatigue life. 5.2 Fatigue resistant design of structural steel work It is seen from practical experiences that most of the fatigue failures are due to improper detailing rather than an inadequate design of the member for strength. Let us consider a lap joint using fillet weld as shown in Fig. 19. From the schematic stress diagram it is seen that the fillet weld toe becomes a point of stress concentration. As a result, if the joint is subjected to cyclic loads, the weld toe experiences a variation of larger stress range compared to the parent member. Hence, a crack may be initiated at the weld toe where there is stress concentration. This stress concentration can be eliminated by using a butt welded joint, ground flush with the plate surface. Fig.18 Modified Goodman diagram for fatigue resistant design of steel structures It becomes very important to avoid any local structural discontinuities and notches by good design and this is the most effective means of increasing fatigue life. Where a structure is subjected to fatigue, it is important that welded joints are considered carefully. Indeed, weld defects and poor weld details are the major contributors of fatigue failures. The fatigue performance of a joint can be enhanced by the use of techniques such as proper weld geometry, improvements in welding methods and better weld quality control using non-destructive testing (NDT) methods. The following general points are important for the design of a welded structure with respect of fatigue strength: (a) use butt welds instead of fillet welds (b) use double sided welds instead of single sided fillet welds (c) pay attention to the detailing which may cause stress concentration and (d) in very important details subjected to high cyclic stresses use any non-destructive testing (NDT) method to ensure defect free details. From the point of Version II 2 - 20 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES view of fatigue design, the codes of practice classify various structural joints and details depending upon their vulnerability to fatigue cracks. For example, IS: 1024 classifies the detailing in the structural steel work in seven classes viz., A, B, C, D, E, F and G depending upon their vulnerability to stress concentrations. A typical detailing classified as ‘E’ is shown in Fig. 20. This class ‘E’ applies to members fabricated with full cruciform butt welds. Similarly, the class ‘F’ is applicable for members with ‘ T’ type full penetration butt welds, members connected by transverse load – carrying fillet welds and members with stud shear connectors in composite sections. Such a typical detailing is shown in Fig. 21. The IS: 1024 (1968) provides allowable stress tables for all the classifications from A-G for different stress ratios of R = Fmin/Fmax and different life (number of cycles N). Using these tables the allowable stress for a given life time may be linearly interpolated and the life time for a given allowable stress could be logarithmically interpolated. The accuracy of any fatigue life calculation is highly dependent on a good understanding of the expected loading sequence during the whole life of a structure. Once a global load pattern has been developed, then a more detailed inspection of particular area of a structure where the effects of loading may be more important called the ‘hot spot stresses’ which are basically the areas of stress concentrations. σσ σ Fillet Weld σ> Point of stress concentration Schematic stress diagram Fig. 19 Stress concentration at the weld toe Load is transmitted directly through the central plate Class E Stress refers to this member y x Fig. 20 Class ‘E’ detailing according to IS: 1024 (1968) Version II 2 - 21 CORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS OF STEEL STRUCTURES Direction of applied stress Weldement Fig. 21 Class ‘F’ detailing according to IS: 1024 (1968) 6.0 SUMMARY In this chapter the three important aspects of structural steel work viz. the corrosion, fire protection, fatigue behaviour have been reviewed. Aspects of corrosion, its mechanism and means of protection of structural steel work have been discussed briefly. It was shown that the risk to structural steel work by corrosion could be effectively handled using the presently available technology. Aspects of fire resistant design of steel structures were also reviewed. Finally the fatigue failure of structural steel work and the importance of detailing in its prevention have been discussed. 7.0 FURTHER READING 1. Adams P.F., Krentz H.A. “Limit State Design in Structural Steel – SI Units”, Canadian Institute of Steel Construction (1979). 2. Doran D.K., “Construction Materials Reference Book”, Butterworth Heinemann (1995). 3. Graham W. Owens and Peter R. Knowles, “Steel Designer’s Manual”, ELBS fifth Edition (1994). 4. Jack C. McCormac, “Structural Steel Design”, Harper & Row Publishers, NY (1981). 5. John H. Bickford , “ An introduction to the design and behaviour of bolted joints”,(Second Edition), Marcel Dekker Inc., NY,(1990) 6. Radaj D, “Design and analysis of fatigue resistant welded structures”, Abington Publishing, (1990). 7. IS: 1024 – 1968, Code of Practice for use of welding in bridges and structures subjected to dynamic loading, Bureau of Indian Standards. Version II 2 - 22 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY it to some promoter, who used other people’s money… But the engineer himself looks back at the unending stream of goodness which flows from his success with satisfaction that few other professions may know. And the verdict of his fellow professionals is all the accolade he wants.” Thus, Hoover described the professional role and responsibilities of the Engineer succinctly. But what is meant by the “professional” role? The word “profess” has religious connotations and probably has its origin in 17th century England [2]. Monks professed their vows and were generally well educated. It is from this group of religious men, erudite University educators were drawn. (“Professors” were those who professed.) Hence the term “professional” was associated with a high degree of education and societal responsibility. In today’s context, all professions require extensive specialised education of significant intellectual content the practitioner to provide a recognisable service to the community a certification (usually by government or by a chartered body). Professions are organised into professional societies, which police themselves. People engaged in these professions are independent of external influences and cannot be coerced by their clients, employers or governments to carry out unethical instructions. Controlling governments yield power to the professionals or their Societies. (For example, in many western countries, only a physician can write prescriptions; only a registered engineer/architect can approve the plans of a structure/building). In return, the professions take on a very responsible position, vis-à-vis the public. Generally, the professional should not hurt anyone unless it is required, (e.g. a dentist!). To eliminate conflicts developing between the roles of the professional and of the citizen, every profession has a Code of Ethics developed by the professionals themselves. For example, the Code Of Ethics, developed by the American Society of Civil Engineers, is based on three fundamental principles requiring Engineers to (i) use their skills to benefit mankind (ii) be honest and fair, and faithfully serve others; and (iii) improve the competence and prestige of the profession. 3.0 DURABILITY AND LIFE CYCLE COST ISSUES Traditionally the professional Structural Engineer had invariably played a vital role in the design of constructed facilities, often, in close association with other professionals like Architects and others in related disciplines. As a designer, he is responsible for the complete process from the conceptual stages to the finished structure. Increasingly, the Society expects him to assume responsibility for the durability of the product. In other words, the responsibility of a professional Structural Engineer in the 21st century will not be confined merely to the immediate economic and environmental impact of his design decisions; society expects him to make rational and responsible choices by considering the life cycle costs and the long-term environmental effects on the community In the following pages, we will highlight the enhanced role of the Professional Engineer in the 21st century and explore how the two design criteria are interlinked. Version II 3 - 3 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY 3.1 The Infrastructure Crisis The Construction Industry, with all its imperfections and limitations, is rightly perceived as the provider of the Nation’s infrastructure. Clearly, it is of paramount importance to train and educate those who create and manage it, in order to ensure the economic and environmental survival of the world. While the world has witnessed some fantastic advances in Science and Technology in recent years, many of these achievements have been made at an outrageous price, plunging the world into a number of crises, which have impacted directly on the construction industry. The global effect of these dramatic changes in the world in the last 50 years can be collectively termed the “infrastructure crisis”, which has to be encountered and managed by the construction industry. Three-fourths of the world’s population live in the (non-industrialised) developing world like India. Uncontrolled population growth, (particularly in the developing world), and evolutionary industrialisation have resulted in global urbanisation. The world population has grown from 5 bn in the late 1980's to 6 bn in 2000 and is now estimated to grow to 8 bn by 2036 and to over 9 bn. by 2050. (The population of India is now just over 1 bn). More than 95% of this increase will take place in the developing parts of the world, India included. For the first time in history, more than half the world population will live around the cities. It is estimated that there are more than 120 cities with over a million people, the majority in the developing world, thus accelerating urban decay in cities which can least afford repeated remedial action. The magnitude of the problem in the Indian context is illustrated next by considering the “housing sector”. Over the next 40 years, India is set to overtake China as the most populous country in the world. The present urban population is estimated to be 330 million, equalling the total population of the country 50 years ago. The urban population that was merely 14% of the total number of citizens 50 years ago now amounts to 33% and is set to grow to 50% by 2025. With economic liberalisation and expected enhanced growth, the rate of urbanisation in India in coming decades is likely to increase. Despite the best of efforts of well-intentioned people in Government and aid agencies, the Nation has not been unable to cope up with the ever-increasing need for shelter for every citizen. India needs some 200 million houses to accommodate all its citizens, whereas we have only167 million houses, of various types [3]. Half of these houses had mud, grass and straw walls and more than a third had grass, straw and thatch roofs. The need for upgrading the housing stock and the magnitude of the task are obvious, particularly in the context of expected urban growth. It has been estimated that over 50% of the land in urban areas is second-hand. Much land is adversely affected by foundations from demolished buildings, which previously stored harmful chemicals, petroleum products etc. Old foundations must be viewed as contaminants and it is important to prevent land contamination by sub-structures. Another major source of concern is water pollution. Many rivers and streams in India are not in their natural state, mainly because of industrial pollution and irresponsible drainage Version II 3 - 4 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY of sewage into them. The adverse effects of high pollution levels in our water resources are already painfully evident in India. It is now widely recognised that much of the recent economic progress in the Western world has been at the expense of the environment and the effects of this environmental degradation are being felt globally, for instance in the form of climate change, ozone depletion, deforestation and acid rain. It is necessary therefore to assess and improve the environmental performance in all economic sectors including construction. Global warming caused by the emission of Greenhouse Gases (i.e. CO2) into the atmosphere puts increased energy into the climate system, resulting in increases in the number and intensity of storms, rapid climatic changes, and larger, more damaging and extreme weather events. As the effects of greenhouse gases in the atmosphere take 30 years to show, the current changes in the world weather (rise in sea levels, global warming, larger deserts, severe draughts and storms) relate to emissions up to the year 1970. The effect of current pollution levels will not be evident until 2030; the present century will, therefore, be a century of disaster management. Besides the large-scale deaths and devastation of the environment that follow from these disasters, the greatest effect will be the destruction of the infrastructure and therefore its impact on the Construction Industry [1]. 3.2 The “Durability Crisis” Issues of durability have always been subjects of debates among Engineers. Is it better to spend (say) 40% more initially, in order that the life of a structure could be doubled? What is better value to the client? Spend less initially or opt for a longer life? Total neglect of durability considerations in all the infrastructure projects undertaken so far combined with primitive construction practices still prevailing in India have resulted in what can only be termed a “durability crisis”. It is now well established that degradation of all structures has become very common in almost all the cities in India and this is particularly true of buildings and structures made of reinforced/prestressed concrete. The great tragedy is that there have been no efforts to address this issue by the present generation of Developers, Engineers, Architects and other design professionals. As a consequence, major problems have been allowed to accumulate for future generations of owners and taxpayers to face. This is not to say that other parts of the world are free from this “durability crisis”. For example, the present total construction expenditure in the UK is 56 million British Pounds, of which 50% is spent in repairs and rehabilitation of recently completed structures. As an example, the Midlands Link Motorway around the city of Birmingham cost around 28 million British Pounds to construct; this motorway needed repairs and rehabilitation within 20 years of its completion. Between 1972 and 1989, a further 45 million British Pounds were spent in repairs. It is now estimated that another 125 million Pounds will be required in the next 15years. In Europe, the annual repair cost is estimated as 1.4 billion ECU; in the U.S. the cost of rehabilitating half a million bridges (mostly concrete) is estimated to be $100 billion. It must be noted that in many countries in the West, life cycle costing is now a mandatory requirement in the planning process. For example, International Surface Transport Efficiency Act of the US (1991) mandates Version II 3 - 5 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY Except in a few special structures like tower cranes and transmission towers, it is rare to build a structure entirely in steel. Frequently the optimal solution is obtained by employing concrete elements compositely with structural steel, especially in multi- storeyed buildings and bridges. These methods ensure significant cost benefits to the developers (or owners of property) as well as to the community. Composite structural forms have been extensively developed in the western world to maximise the respective benefits of using structural steel and concrete in combination, but this technology is largely ignored in India, despite its obvious benefits. The sizes of composite beams and columns will be appreciably smaller and lighter than that of the corresponding reinforced or prestressed sections for resisting the same load. A direct economy in the tonnage of steel and indirect economies due to a decrease in construction depths of the floors and reduced foundation costs will, therefore, be achieved. Generally, improvements in strengths of the order of 30% can be expected by mobilising the composite action. An independent study carried out by the Central Building Research Institute (CBRI) Roorkee demonstrated that there are substantial cost savings to be achieved by the use of Composite Construction [6]. 3.5 Life Cycle Costs ASTM E917-83 (1983) describes the standard practices for evaluating LCC of buildings and building systems. The motivation for the LCC is that on any investment decision, all costs arising from the decision, both immediate and in future are potentially important. The recent development of fast track methods in construction in the western industrialised world triggered the wide spread implementation of Life Cycle Cost study, which would ensure enhanced productivity and efficient utilisation of the capital. Construction projects completed on the basis of lower initial cost alone have often proved to be far more expensive in the long run, besides causing damage to the environment and bringing poorer return on the investment. Thus the durability of structure and its life cycle cost are closely inter-linked. Enhanced durability invariably reduces or eliminates the construction-related adverse environmental impact on the community. As pointed out already, Indian Engineers seldom give any serious consideration to vital factors like durability and lifetime costs. Environmental safety and inconvenience to the community do not seem to be given even a cursory thought. As a result, the owners (and taxpayers) do not get the most rational choice arrived at by taking into account all aspects of the design challenge. The result is that - both in the short term as well as on a long- term basis -. the construction costs in India are among the highest in the world, (despite the labour costs being very low, compared to the West) while the Construction Industry continues to pollute the environment and cause long-term damage to it. At this stage it is appropriate to define the Life Cycle Cost of a Structure, made up of several components listed below [7]. 1. Initial Cost • Actual “Cash” Cost of the project Version II 3 - 8 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY • Cost of the Investment locked-up without Returns (“The Time Cost”) • Cost penalty to the community by traffic delays and detours; Losses suffered by local Business (“Hidden Penalty Cost”) • Cost of damage to the Environment due to Pollution (“The Environment Cost”) 2. Periodic Maintenance Cost, including energy cost 3. Cost of dismantling the structure, at the end of its life 4. Less the salvage value of the construction products. All these values are evaluated in life cycles, present value terms or Annual Value terms. A more comprehensive LCC analysis may include adjustment for taxes, adjustment of financing cost etc. The basic aspects of lifetime costs are discussed in some detail in the following paragraphs: 3.5.1 Initial Cost (a) Actual “Cash” Cost: Many government departments report the “cash cost” as the Cost of the project. This is both wrong and misleading. Frequently, many reputed Designers also report that the cash costs of Reinforced or Prestressed concrete alternatives are cheaper than Steel Options. This is because the Steel Intensive Options considered by them are based on outdated design and construction practices. For example, they do NOT employ the relatively new “Steel-concrete composite” construction, or Limit State methods of Design, possibly because Indian Codes have not kept pace with developments in technology! A recently published CBRI study has demonstrated that likely cash savings by using Steel-Intensive Designs, compared with concrete-intensive option for multi-storey buildings will be at least 3% - 16%. The experience in Europe, particularly in the United Kingdom, bears this out. (Over 90% of the new buildings in the London area are built of Steel-Concrete Composite Construction; over 60% of all bridges throughout Great Britain are built of Steel-concrete composite construction.) It is difficult to believe that the necessary expertise is unavailable in India, as the technology is not complex. (b) Cost of the Investment Lock-Up without Returns (“The Time Cost”): Ignoring the “time cost” has been a cultural weakness in India and needs to be overcome if we are to take our place in the community of Nations. It must be recognised that time does cost money. The time taken for concrete-intensive construction would be 2-3 times it takes for steel-intensive alternative. Locking up the capital – without any return – by choosing the former results in a loss to the owner of at least 12% - 15% per year of delay. A recent study reported that even for a modest project like a flyover costing Rs. 10 crores, [See Fig. 1] the loss under this head amounts to Rs. 1.00 crore of taxpayer’s money, i.e. 10% of the total cost. (c) Cost penalty to the Community due to inappropriate construction planning (e.g. Traffic Delays and Detours; Losses suffered by local Businesses etc. collectively termed - Version II 3 - 9 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY “Hidden Penalty Cost”): A prestressed concrete fly-over built in Chennai was chosen as a case study. This construction, which lasted 15 months, had resulted in all local road users and residents having to take a detour of 2 km for each trip resulting in a needless extra expenditure by the community of one crore of rupees in a project costing approximately Rs. 10 crores! This was spent in burning petrol bought by using the valuable foreign exchange. Is this a wise use of foreign exchange? Fig. 1 Flyover construction – The Indian way Version II 3 - 10 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY one fly-over, to be around Rs 40,00,000, to Rs. 75,00,000. (There are 15 fly-overs currently being built in Chennai!!) A third “penalty” is the time spent by busy executives in the traffic jams and hold-ups, caused by this construction work. There is no evidence to suggest that the Community had given its informed consent for the colossal sums being spent on their behalf. Hiding “the hidden penalty cost” (particularly in metropolitan cities) would cause long-term damage to the attractiveness of the city as a place to invest. As the public become aware that technologies do exist to create infrastructure with minimum negative impact or inconvenience to public they will increasingly demand utilisation of such technologies. (d) Cost Of Damage to The Environment Due To Pollution (“The Environment Cost”): As a direct result of pollution by particulate material (construction dust, movement of heavy construction equipment etc), the penalties paid by the hundreds of residents in the locality, by way of health-care costs must be adding to staggering sums. It is, of course, impossible even to guess this figure unless a detailed study is undertaken. Many road users are horrified to observe that the highway is closed up for prolonged periods merely for storing construction materials and junk, contributing to substantial dirtying of the environment. [See Fig. 3] Traffic hold-ups due to the construction cause added air pollution in the neighbourhood. It is clear that Professional Engineers are unlikely to be admired for irresponsibly causing pollution in the neighbouring environment for prolonged periods. (e) Total Initial Cost: From the foregoing it is clear that the total likely savings by adopting steel-concrete composite construction to the taxpayer when a building, fly-over or bridge is completed, (compared with concrete-intensive construction) will be at least 30% - and more. Substantial reduction in pollution levels, environmental damage and traffic hold-ups will be added bonuses. 3.5.2 Periodic Maintenance Cost Periodic and preventive maintenance undoubtedly contributes to the longevity of the structure. Unfortunately this is the most neglected activity in India. Economising on periodic maintenance will invariably result in much enhanced expenditure at a later date. The problem is compounded by several myths that seem to prevail among Engineers and Architects. Some of these that affect the periodic and timely maintenance of structures are discussed below: (a) Myth No. 1: Concrete lasts forever without maintenance: The reality is that there is no magical ingredient in concrete to do it! Concrete is subject to deterioration by the same environmental factors as steel (viz. Chloride contamination, alkali silicate reactions, sulphate attack etc.) In addition, we have problems due to poor site control, insufficient concrete cover, ineffective drainage, insufficient cement content, shrinkage, creep etc. Version II 3 - 13 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY (b) Myth No. 2: Concrete bridges outlast steel bridges: The reality is that there is no credible statistical evidence that concrete bridges outlast steel bridges. Many steel bridges with over 100 years of service life are still performing well. In contrast, the first major prestressed concrete bridge in the USA (Walnut Lane Bridge in Philadelphia) had to be replaced by a steel bridge after a service life of about 40 years. The deterioration rates of 57000 bridges listed in Federal Highway Administration analysed by Lehigh University showed no correlation with the material of construction. The only factors they could identify are (1) age, (irrespective of the material of construction) and (2) the intensity of daily traffic. Studies by the OECD (Organisation for Economic Co-operation and Development) reveal that steel bridges are expected to last much longer than prestressed concrete bridges. Indeed in Belgium and Japan they found that steel bridges outlast prestressed concrete bridges by 15 – 26 years. (c) Myth No. 3: Concrete bridges last forever without maintenance: Some people believe that once in place, reinforced and prestressed concrete bridges last forever and that steel bridges are slowly corroding away. The perception is that concrete is an inert material, less vulnerable to the environment than structural steel. The fact is that Concrete deterioration is a subject, which is widely researched but not so widely discussed. Appearances can be deceiving – at least in the case of Concrete Bridges; according to US Govt. Strategic Highway Program, a bridge deck or sub-structure that appears sound, may actually be deteriorating from inside out. Steel is easily repairable at almost any stage of corrosion and over the years has shown a remarkable tolerance to lack of maintenance. (d) Myth No. 4: Structural steel can not be adequately protected from corrosion: The reality is that there are high performance coatings available to day, which provide long term protection for EXPOSED Structural Steel at an economic price. Frequently, interior Steelwork does NOT require any paint or other protective coatings. For Steelwork to corrode we need the presence of both water and air SIMULTANEOUSLY, or exposure to aggressive conditions. These conditions do NOT exist in most buildings and in many inland structures and bridges. There is certainly no evidence to suggest that many landmark structures like the London Bridge, Eiffel Tower, Empire State Building, Sears tower and many other Steel intensive structures are corroding away! (e) Myth No. 5: A steel structure is less safe in a fire than other types of structures: The reality is that Steel Structures are no less safe than other structures. The properties of all materials are degraded when exposed to fire. The modulus of elasticity of concrete is permanently reduced; the cross section of timber is consumed. The modulus of elasticity of steel is however, not permanently reduced and recovers once the member cools down. Steel structures can be economically fire protected to meet all Version II 3 - 14 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY building code requirements. Steel structures can be rehabilitated after a fire at a modest cost. (f) Myth No. 6: Maintenance of Concrete intensive structures is significantly cheaper than that of Steel intensive Structures: Concrete–intensive construction is not as simple as is usually imagined. Nor are concrete-intensive structures cheap, to maintain (contrary to popular myth). Structural steel forgives human errors or lapses in maintenance, but not concrete! It is vital that Engineers pay attention to vital maintenance issues and not be carried away by myths. The incidence of major concrete repairs to bridges is significantly greater than many realise and time cost of such repairs, when all costs including traffic delay costs are taken into account is very significant. It has been shown in published literature that the probability of significant repair work to be carried out within each 20 year life of concrete bridges is as high as 0.185. The direct cost of such repairs is insignificant (around 0.1 to 0.5% of the initial cost in present value) whereas the indirect cost due to the traffic delay during such longer repairs in concrete structures has been estimated to be at least 25%. Further, it is well known that repair and retrofit of steel members is a lot simpler than reinforced concrete members. 3.5.3 Cost of Dismantling the Structure at the End of its Life Many structures in the urban environment are being demolished due to deterioration beyond repair, due to land values escalating very high, rendering the building economically unviable, or due to their inability to meet the modern functional requirements. At this stage a cost is incurred to dismantle or demolish the structure. It is well known that the cost of dismantling the steel structure is well below that of reinforced concrete structures. 3.5.4 Salvage Value of the Construction Products Some of the products salvaged from the structure dismantled are of some economic value. This value is subtracted from the total cost after adjusting for the time of salvage. It is well known the cost of material recovered from steel intensive construction is almost equal to the original cost of the structure, although this value is to be reckoned at a later time in the life cycle, whereas in concrete intensive construction, substantial additional cost is incurred in disposing off the material. The recycling of demolished material from urban construction has become a major problem in urban areas, leading to intensive research on the subject. 3.5.5 Uncertainties in Life Cycle Costing The effort to evaluate the life cycle cost is fraught with many difficulties listed below. • Non-availability of reliable and consistent cost data Version II 3 - 15 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY subject to restrictions by building specifications on height, floor area etc. Constraints include ability to withstand loads from human activities and from natural forces like wind and earthquakes. As pointed above, a system consists of many subsystems, i.e. components of the system. For example, in a building, major subsystems are structural framing, foundations, cladding, non-structural walls and plumbing. Each of these subsystems consists of several interrelated components. In the case of structural framing, the components include columns, beams, bracing, connections etc. The richness and variety of structural systems can be appreciated by the available building structural types that range from massive building blocks to shell structures, from structures above or below ground or in water, to structures in outer space. Examples of a few steel-framed structures are shown in Fig. 4. 4.1 Goals Before starting the design of a system, the designer should establish the goals for the system. These specify what the system is to accomplish and how it will affect the environment and other systems or vice versa. Goals are generally made in statements of specific design objectives such as purpose, time and cost limitation, environmental constraints etc., which would enable the generation of initial and alternative designs. The goals for a system design applied to a subsystem serve the same purpose as for a system. They indicate the required function of the subsystem and how it affects and is affected by other subsystems. 4.2 Objectives Having set down the goals, the designer defines the system objectives. These objectives are similar to goals but explain in detail the requirements that the system must satisfy to attain the goals. Some of the essential objectives of any project relate to health, safety and welfare requirements of the occupants, which are generally defined in local building codes or building regulations. Other special objectives include minimisation of initial costs, life-cycle costs, construction time etc. At least one criterion (e.g. Fire resistance) must be associated with each objective. A criterion is a range of values within which the performance of the system must lie (e.g. Two hours fire rating is needed). The criterion serves as a guide in the evaluation of alternative systems to the project. 4.3 Constraints and standards Constraints are restrictions on the values of design variables, which may or may not be under the control of the designer. For example, an I-beam section of 200 mm depth may Version II 3 - 18 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY be desirable, but not available. There are also various legal and building code requirements. A minimum of one standard must be associated with each constraint. 4.4 Codes and Specifications A structural engineer is guided in his design efforts by the relevant codes and specifications. Although the word ‘codes’ and ‘specification’ are normally used interchangeably, there is a distinction between them. A detailed set of rules and suggestions prepared by an interested party is called an engineering specification. On the other hand, Codes are frequently formulated by a group of professionals with a view to their adoption by the profession as a whole. These are revised at regular intervals based on new developments in materials, research, construction techniques etc. Though codes offer general guidance to a certain extent, they do not provide answers to all the problems that arise in practice. Mere adherence to codes and specifications will curb all initiatives and innovative designs. 4.5 Construction and other costs Construction cost and time are usually dominant design concerns. If the construction cost exceeds the budget, the completion of the project may be in jeopardy. Minimisation of the life cycle cost of a system would result in the most desirable solution. 5.0 DESIGN REQUIREMENTS The principal design requirements of a structure are set out already under Introduction. The primary structural safety requirement is met by ensuring that the structure has an acceptably low risk of failure during its design life. Another important requirement is that the structure must be sufficiently stiff to ensure that excessive deflection and vibrations do not affect the in-service performance of the structure. The requirement of harmony within the structure is affected by the relationships between the different subsystems of the main system, the architectural subsystem, the mechanical and electrical subsystems, and the functional subsystems required by the use of the structure. Finally, the system should be in harmony with its environment, and should not react unfavourably with either the community or its physical surroundings. Conceptual design refers to the task of choosing a suitable system. (As an example architect is generally concerned with the building layout, limits and parameters). In modern construction practices, a multidisciplinary team of architect, structural designer and service engineer together evolve the conceptual design. A typical organisational chart for a multidiscipline design team is seen in Fig. 5, which shows the inter-relationship between the various design professionals. The structural engineer is charged with the task of ensuring that the structure will resist and transfer the forces and loads acting on it with adequate safety, while supporting other Version II 3 - 19 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY subsystems and making due allowance for the requirements of serviceability, economy, harmony and constructibility. The iterative process of achieving such a design is shown in Fig. 6. Since several simplifying approximations are made in the preliminary design, it is necessary to re-check the design. The loads are recalculated more precisely and the structure is reanalysed. The performance of the structure is then re-evaluated with respect to the structural requirements, and any changes in the member and joint sizes are made [See Fig. 7]. An Engineer or Architect DESIGN PROFESSIONAL THE PROJECT MANAGER Lead disciplines Principal disciplines Struct. Engg Electrical Engg. Architecture Mech. Engg Civil Engg. Support disciplines G eo te ch . E n g g S u rv ey in g S p a ce p la n n in g L a n d S ca p in g S ch ed u li n g , E st im a ti n g U rb a n p la n n in g Fig. 5 Example of multi - discipline project organisation An Engineer or ArchitectLEAD DESIGN TEAM LEADER Design Team Management Owner’s representative An Engineer or Architect Version II 3 - 20 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY 6. Installation of ventilating/ heating plant, lifts, water supply, power etc. 7. Corrosion protection required 8. Fire protection required 9. Operating and maintenance costs 3 – pin portal Knee brace Flat Fig: 8 Single Bay, Single-storey Structures Aesthetic considerations are important in many cases and the choice of design may not always be based on cost alone. The weight saving may be offset by the higher cost of the stronger material or the higher cost of fabrication/construction of complicated systems. Often no one solution for a given structure is prominent or obvious to the exclusion of all Fig.9 Beam and column construction Version II 3 - 23 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY other alternatives. As an example, we can illustrate several choices available to the designer for a single bay, single storey structure [See Fig. 8]. An example of beam and column system frequently used is illustrated in Fig. 9. Cable stayed structures are frequently employed in long span bridges and buildings and are shown in Fig. 10. In the following chapters, the analysis and design of steel elements are discussed in depth, followed by the analysis and design of selection of structural elements. (a) (b) Fig. 10 Cable-stayed structures 6.0 CONCLUDING REMARKS The paper discussed the role of a Structural Engineer in designing constructed facilities in the 21st century. The relevant environmental factors which affect his work, the durability and infrastructure crises, which face the Industry, are all discussed in detail. The importance of life cycle costing and the rational selection of appropriate materials for construction are discussed in depth. A strong case is made for taking account of the durability and environmental considerations in the design process. They make a vital contribution to the life cycle cost of a structure. The paper concludes with a description of the structural design process in the everyday life of a structural engineer. Version II 3 - 24 ROLE OF STRUCTURAL ENGINEER IN THE 21st CENTURY 7.0 REFERENCES 1. Swamy, R.N. (2000) “Educating Engineers”, The Structural Engineer, London, Volume 78 (17) 2. Vesiland, A.L., (Duke University, USA): Private Communication 3. Suresh. V., “Rural and Urban Housing : Opportunities for use of steel”, Keynote address at the Seminar on Steel in Building Construction, (Ministry of Steel and INSDAG), September 2000 4. Owens. G. and Wood. A. (1998): “World-wide use of Steel in Construction”, Journal of Constructional Steel Research, Volume 46 (1-3). 5. INSDAG and STUP CONSULTANTS (2000): “ Life Cycle Cost of Viaduct Structures of Elevated Light Rail Transit System Project”, INSDAG, Calcutta. 6. Naithani, K.C., Gupta, V.K., Sarvendra Kumar, Mittal, M.K., Ghosh, S.K., Karmakar. D. and Maini P.K. (1999). “Cost Economics Study of Composite Construction and its Comparison with traditional R.C. Construction, CECR. 7. Narayanan. R. and Kalyanaraman, V (2000) : Durability and Life Cycle Costs”, National Seminar on Global Standards on Quality Assurance and Reliability in Structures, Association of Consulting Engineers (India), Coimbatore Centre. 8. McGuire W. (1968), “Steel Structures”, Prentice Hall International Inc., London. 9. Merrit F.S. (1983), “Standard Handbook for Civil Engineers”, (3rd Edition) McGraw Hill Book Company, New York. 10. Dowling P.J., Knowles P.R. and Owens G.W., (1988), “Structural Steel Design”, The Steel Construction Institute, Butterworths, London. Version II 3 - 25 the regulations allow. Frequently the professions are allowed to regulate themselves; in these a cases the Regulations or Codes of Practices are evolved by consultation and consensus within the profession. 3.0 ALLOWABLE STRESS DESIGN (ASD) With the development of linear elastic theories in the 19th century the stress-strain behaviour of new materials like wrought iron & mild steel could be accurately represented. These theories enabled indeterminate structures to be analysed and the distribution of bending and shear stresses to be computed correctly. The first attainment of yield stress of steel was generally taken to be the onset of failure. The limitations due to non-linearity and buckling were neglected. The basic form of calculations took the form of verifying that the stresses caused by the characteristic loads must be less than an “allowable stress”, which was a fraction of the yield stress. Thus the allowable stress may be defined in terms of a “factor of safety" which represented a margin for overload and other unknown factors which could be tolerated by the structure. The allowable stress is thus directly related to yield stress by the following expression: safetyofFactor stressYield stressAllowable = In general, each member in a structure is checked for a number of different combinations of loading. The value of factor of safety in most cases is taken to be around 1.67. Many loads vary with time and these should be allowed for. It is unnecessarily severe to consider the effects of all loads acting simultaneously with their full design value, while maintaining the same factor of safety or safety factor. Using the same factor of safety or safety factor when loads act in combination would result in uneconomic designs. A typical example of a set of load combinations is given below, which accounts for the fact that the dead load, live load and wind load are all unlikely to act on the structure simultaneously at their maximum values: (Stress due to dead load + live load) < allowable stress (Stress due to dead load + wind load) < allowable stress (Stress due to dead load + live load + wind) < 1.33 times allowable stress. In practice there are severe limitations to this approach. These are the consequences of material non-linearity, non-linear behaviour of elements in the post-buckled state and the ability of the steel components to tolerate high theoretical elastic stresses by yielding locally and redistributing the loads. Moreover the elastic theory does not readily allow for redistribution of loads from one member to another in a statically indeterminate structures. 4 - 3 4.0 LIMIT STATE DESIGN An improved design philosophy to make allowances for the shortcomings in the “Allowable Stress Design” was developed in the late 1970’s and has been extensively incorporated in design standards and codes formulated in all the developed countries. Although there are many variations between practices adopted in different countries the basic concept is broadly similar. The probability of operating conditions not reaching failure conditions forms the basis of “Limit States Design” adopted in all countries. “Limit States" are the various conditions in which a structure would be considered to have failed to fulfil the purpose for which it was built. In general two limit states are considered at the design stage and these are listed in Table 1. Table 1: Limit States Limit State of Strength Serviceability Limit State Strength (yield, buckling) Stability against overturning and sway Fracture due to fatigue Plastic collapse Brittle Fracture Deflection Vibration Fatigue checks (including reparable damage due to fatigue) Corrosion Fire “Limit State of Strength” are: loss of equilibrium of the structure and loss of stability of the structure. “Serviceability Limit State" refers to the limits on acceptable performance of the structure. Not all these limits can be covered by structural calculations. For example, corrosion is covered by specifying forms of protection (like painting) and brittle fracture is covered by material specifications, which ensure that steel is sufficiently ductile. 5.0 PARTIAL SAFETY FACTOR The major innovation in the new codes is the introduction of the partial safety factor format. A typical format is described below: In general calculations take the form of verifying that S * ≤ R* 4 - 4 where S* is the calculated factored load effect on the element (like bending moment, shear force etc) and R* is the calculated factored resistance of the element being checked, and is a function of the nominal value of the material yield strength. S * is a function of the combined effects of factored dead, live and wind loads. (Other loads – if applicable, are also considered) In accordance with the above concepts, the safety format used in Limit State Codes is based on probable maximum load and probable minimum strengths, so that a consistent level of safety is achieved. Thus, the design requirements are expressed as follows: Sd ≤ Rd where Sd = Design value of internal forces and moments caused by the design Loads, Fd Fd = γf * Characteristic Loads. γf = a load factor which is determined on probabilistic basis Rd = Characteristic Value of Resistance γm where γm = a material factor, which is also determined on a ‘probabilistic basis’ It should be noted that γf makes allowance for possible deviation of loads and the reduced possibility of all loads acting together. On the other hand γm allows for uncertainties of element behaviour and possible strength reduction due to manufacturing tolerances and imperfections in the material. Collapse is not the only possible failure mode. Excessive deflection, excessive vibration, fracture etc. also contribute to Limit States. Fatigue is an important design criterion for bridges, crane girders etc. (These are generally assessed under serviceability Limit States) Thus the following limit states may be identified for design purposes: • Ultimate Limit State is related to the maximum design load capacity under extreme conditions. The partial load factors are chosen to reflect the probability of extreme conditions, when loads act alone or in combination. • Serviceability Limit State is related to the criteria governing normal use. Unfactored loads are used to check the adequacy of the structure. • Fatigue Limit State is important where distress to the structure by repeated loading is a possibility. The above limit states are provided in terms of partial factors reflects the severity of the risks. 4 - 5 d) The resistance effect shall be greater than or equal to the destabilizing effect. Combination of imposed and dead loads should be such as to cause most severe effect on overall stability. 7.0 LIMIT STATE OF SERVICEABILITY As stated in IS: 800, Serviceability Limit State is related to the criteria, governing normal use. Serviceability limit state is limit state beyond which service criteria, specified below, are no longer met: a) Deflection Limit b) Vibration Limit c) Durability Consideration d) Fire Resistance Load factor, γf, of value equal to unity are used for all loads leading to Serviceability Limit States to check the adequacy of the structure under serviceability limit states, unless specified otherwise. The deflection under serviceability loads of a building or a building component should be such that, they do not impair the strength of the structure or components or cause damage to finishing. Deflections are to be checked for the most adverse but realistic combination of service loads and their arrangement, by elastic analysis, using a load factors as per Table 3. Table 4 gives recommended limits of deflections for certain structural members and systems. As per IS: 800, suitable provisions in the design are required to be made for the dynamic effects of live loads, impact loads and vibration due to machinery operating loads. In severe cases possibility of resonance, fatigue or unacceptable vibrations shall be investigated. Unusually flexible structures (generally the height to effective width of lateral load resistance system exceeding 5:1) need to be investigated for lateral vibration under dynamic wind loads. Structures subjected to large number of cycles of loading shall be designed against fatigue failure as discussed in Chapter 2. Durability or Corrosion resistance of a structure is generally, under conditions relevant to their intended life as are listed below: a) The environment b) The degree of exposure c) The shape of the member and the structural detail d) The protective measure e) Ease of maintenance Fire resistance of a steel member is a function of its mass, its geometry, the actions to which it is subjected, its structural support condition, fire protection measures adopted and the fire to which it is exposed. Design provisions to resist fire are briefly discussed in Chapter 2. 4 - 8 Table 4: Partial safety factors [According to IS: 800 (2007)] Type of Building Deflection Design Load Member Supporting Maximum Deflection Live load/Wind load Purlins and Girts Purlins and Girts Elastic cladding Brittle cladding Span / 150 Span / 180 Live load Simple span Elastic cladding Span / 240 Live load Simple span Brittle cladding Span / 300 Live load Cantilever span Elastic cladding Span / 120 Live load Cantilever span Brittle cladding Span / 150 Profiled Metal Sheeting Span / 180 Live load or Wind load Rafter supporting Plastered Sheeting Span / 240 Crane load (Manual operation) Gantry Crane Span / 500 Crane load (Electric operation up to 50 t) Gantry Crane Span / 750 V er ti ca l Crane load (Electric operation over 50 t) Gantry Crane Span / 1000 No cranes Column Elastic cladding Height / 150 No cranes Column Masonry/Brittle cladding Height / 240 Crane(absolute) Span / 400 Crane + wind Gantry (lateral) Relative displacement between rails 10 mm Column/frame Gantry (Elastic cladding; pendent operated) Height / 200 In d u st ri al b u il d in g L at er al Crane+ wind Column/frame Gantry (Brittle cladding; cab operated) Height / 400 Live load Floor & Roof Elements not susceptible to cracking Span / 300 Live load Floor & Roof Elements susceptible to cracking Span / 360 Live load Elements not susceptible to cracking Span / 150 V er ti ca l Live load Cantilever Elements susceptible to cracking Span / 180 Elastic cladding Wind Building Brittle cladding Height / 300 Height / 500 O th er B u il d in g s L at er al Wind Inter storey drift --- Storey height / 300 4 - 9 8.0 CONCLUDING REMARKS This chapter reviews the provisions of safety, consequent on uncertainties in loading and material properties. The partial load factors employed in design to take into account these variations are discussed and illustrated. 9.0 REFERENCES 1. Owens G.W., Knowles P.R : "Steel Designers Manual", The Steel Construction Institute, Ascot, England, 1994 2. British Standards Institution : "BS 5950, Part-1 Structural use of steelwork in building", British Standards Institution, London, 1985 3. IS: 800 (2007), General Construction in Steel – Code of Practice, Bureau of Indian Standards, New Delhi, 2007. 4 - 10 DESIGN OF TENSION MEMBERS 5 DESIGN OF TENSION MEMBERS 1.0 INTRODUCTION Tension members are linear members in which axial forces act so as to elongate (stretch) the member. A rope, for example, is a tension member. Tension members carry loads most efficiently, since the entire cross section is subjected to uniform stress. Unlike compression members, they do not fail by buckling (see chapter on compression members). Ties of trusses [Fig 1(a)], suspenders of cable stayed and suspension bridges [Fig.1 (b)], suspenders of buildings systems hung from a central core [Fig.1(c)] (such buildings are used in earthquake prone zones as a way of minimising inertia forces on the structure), and sag rods of roof purlins [Fig 1(d)] are other examples of tension members. Stay cables Stayed bridge Suspenders Suspension Bridge (b) Cable Supported Bridges (a) Roof Truss Tie Rafter Suspenders (c) Suspended Building (e) Braced Frame Fig. 1 Tension Members in Structures X bracings Top chord (d) Roof Purlin System Sag rod Purlin Tension members are also encountered as bracings used for the lateral load resistance. In X type bracings [Fig.1 (e)] the member which is under tension, due to lateral load acting in one direction, undergoes compressive force, when the direction of the lateral load is changed and vice versa. Hence, such members may have to be designed to resist tensile and compressive forces. © Copyright reserved Version II 5-1 DESIGN OF TENSION MEMBERS Version II 5-2 The tension members can have a variety of cross sections. The single angle and double angle sections [Fig 2(a)] are used in light roof trusses as in industrial buildings. The tension members in bridge trusses are made of channels or I sections, acting individually or built-up [Figs. 2(c) and 2(d)]. The circular rods [Fig.2 (d)] are used in bracings designed to resist loads in tension only. They buckle at very low compression and are not considered effective. Steel wire ropes [Fig.2 (e)] are used as suspenders in the cable suspended bridges and as main stays in the cable-stayed bridges. (a) (c) (d) (e) Fig. 2 Cross Sections of Tension Members (b) 2.0 BEHAVIOUR OF TENSION MEMBERS Since axially loaded tension members are subjected to uniform tensile stress, their load deformation behaviour (Fig.3) is similar to the corresponding basic material stress strain behaviour. Mild steel members (IS: 2062) exhibit an elastic range (a-b) ending at yielding (b). This is followed by yield plateau (b-c). In the Yield Plateau the load remains constant as the elongation increases to nearly ten times the yield strain. Under further stretching the material shows a smaller increase in tension with elongation (c-d), compared to the elastic range. This range is referred to as the strain hardening range. After reaching the ultimate load (d), the loading decreases as the elongation increases (d- e) until rupture (e). High strength steel tension members do not exhibit a well-defined yield point and a yield plateau (Fig.3). The 0.2% offset load, T, as shown in Fig. 3 is usually taken as the yield point in such cases. T Fig. 3 Load – Elongation of Tension Members δ a b c d bs= 0.2% DESIGN OF TENSION MEMBERS 2.1 Design strength of tension members Although steel tension members can sustain loads up to the ultimate load without failure, the elongation of the members at this load would be nearly 10-15% of the original length and the structure supported by the member would become unserviceable. Hence, in the design of tension members, the yield load is usually taken as the limiting load. The corresponding design strength in member under axial tension is given by (1)/ 0mgydg AfT γ= Where, fy is the yield strength of the material (in MPa), Ag is the gross area of cross section and γm0 is the partial safety factor for failure in tension by yielding. The value of γm0 according to IS: 800 is 1.10. 2.2 Plates under Tension Frequently plates under tension have bolt holes. The tensile stress in a plate at the cross section of a hole is not uniformly distributed in the elastic range, but exhibits stress concentration adjacent to the hole [Fig 4 (a)]. The ratio of the maximum elastic stress adjacent to the hole to the average stress on the net cross section is referred to as the Stress Concentration Factor. This factor is in the range of 2 to 3, depending upon the ratio of the diameter of the hole to the width of the plate normal to the direction of stress. fy fy fu (d) Ultimate (b) Elasto-Plastic (c) Plastic (a) Elastic In statically loaded tension members with a hole, the point adjacent to the hole reaches yield stress, fy, first. On further loading, the stress at that point remains constant at the yield stress and the section plastifies progressively away from the hole [Fig.4 (b)], until the entire net section at the hole reaches the yield stress, fy, [Fig. 4(c)]. Finally, the rupture (tension failure) of the member occurs when the entire net cross section reaches the ultimate stress, fu, [Fig. 4(d)]. Since only a small length of the member adjacent to the smallest cross section at the holes would stretch a lot at the ultimate stress, and the overall member elongation need not be large, as long as the stresses in the gross section is below the yield stress. Hence, the design strength as governed by net cross-section at the hole, Tdn, is given by Fig. 4 Stress Distribution at a Hole in a Plate under Tension )2(1mnudn /Af0.9T γ= where, fu is the ultimate stress of the material, An is the net area of the cross section after deductions for the hole [Fig.4(b)] and γm1 is the partial safety factor against ultimate Version II 5-3 DESIGN OF TENSION MEMBERS Kulak and Wu (1997) have reported, based on an experimental study, the results on the tensile strength of single and double angle members. Summary of their findings is: • The effect of the gusset thickness, and hence the out of plane stiffness of the end connection, on the ultimate tensile strength is not significant. • The thickness of the angle has no significant influence on the member strength. • The effect of shear lag, and hence the strength reduction, is higher when the ratio of the area of the outstanding leg to the total area of cross-section increases. • When the length of the connection (the number of bolts in end connections) increases, the tensile strength increases up to 4 bolts and the effect of further increase in the number of bolts, on the tensile strength of the member is not significant. This is due to the connection restraint to member bending caused by the end eccentric connection. • Even double angles connected on opposite sides of a gusset plate experience the effect of shear lag. Based on the test results, Kulak and Wu (1997) found that the shear lag due to connection through one leg only causes at the ultimate stage the stress in the outstanding leg to be closer only to yield stress even though the stress at the net section of the connected leg may have reached ultimate stress. They have suggested an equation for evaluating the tensile strength of angles connected by one leg, which accounts for various factors that significantly influence the strength. In order to simplify calculations, this formula has suggested that the stress in the outstanding leg be limited to fy (the yield stress) and the connected sections having holes to be limited to fu (the ultimate stress). The design tensile strength, Td, should be the minimum of the following: Strength as governed by tearing at net section: Tdn = 0.9Anc fu / γm1+ β Ago fy / γm0 (7a) Where, fy and fu are the yield and ultimate stress of the material, respectively. Anc and Ao, are the net area of the connected leg and the gross area of the outstanding leg, respectively. The partial safety factors γm0 = 1.10 and γm1 = 1.25. β, accounts for the end fastener restraint effect and is given by, β = 1.4 – 0.076 (w/t) (f /f ) (b /Lc ) ≤ (fu.γmo / fy.γm1) and β 0.7 ≥y u s where w and bs are as shown in Fig 8 Lc = Length of the end connection, i.e., distance between the outermost bolts in the end joint measured along the length direction or length of the weld along the length direction and t = thickness of the leg Alternatively, the rupture strength of net section may be taken as Tdn = α An fu /γm1 Version II 5-6 DESIGN OF TENSION MEMBERS where α = 0.6 for one or two bolts, 0.7 for three bolts and 0.8 for four or more bolts along the length in the end connection or equivalent weld length An = net area of the total cross section w1 w bs=w+w1-t w Fig 8 Angles with End Connection Strength as governed by yielding of gross section: Tdg = Ag fy /γm0 (7 b) Where, Ag is the gross area of the angle section. Strength as governed by block shear failure: A tension member may fail along end connection due to block shear as shown in Fig. 9. The corresponding design strength can be evaluated using the following equations. If the centroid of bolt pattern is not located between the heel of the angle and the centreline of the connected leg, the connection shall be checked for block shear strength given by Block shear plane Fig. 9 Block Shear Failure Tdb = ( Avg fy /( 3 γm0) + 0.9Atn fu /γm1 ) or Tdb = (0.9Avn fu /( 3 γm1) + Atg fy /γm0 ) (7c) where, Avg and Avn = minimum gross and net area in shear along a line of transmitted force, respectively, and Atg and Atn = minimum gross and net area in tension from the hole to the toe of the angle, perpendicular to the line of force, respectively. The design strength of an angle loaded in tension through a connection in one leg is given by the smallest of the values obtained from Eqn. 7(a) to 7(c). These equations are valid for both single angle and double angles in tension, irrespective of whether they are on the Version II 5-7 DESIGN OF TENSION MEMBERS same side or opposite sides of the gusset. A sample design of angle tension member is given in worked example 2. The efficiency,η, of an angle tension member is calculated as given below: )8()//( 0mygd fAT γη = Depending upon the type of end connection and the configuration of the built-up member, the efficiency may vary between 0.85 and 1.0. The higher value of efficiency is obtained in the case of double angles on the opposite sides of the gusset connected at the ends by welding and the lower value is usual in the bolted single angle tension members. In the case of threaded members the efficiency is around 0.85. In order to increase the efficiency of the outstanding leg in single angles and to decrease the length of the end connections, some times a short length angle at the ends are connected to the gusset and the outstanding leg of the main angle directly, as shown in Fig. 10. Such angles are referred to as lug angles. The design of such end connections is discussed in the chapter on connections. Fig. 10 Tension Member with Lug lug angle 3.0 DESIGN OF TENSION MEMBERS In the design of a tension member, the design tensile force is given and the type of member and the size of the member have to be arrived at. The type of member is usually dictated by the location where the member is used. In the case of roof trusses, for example, angles or pipes are commonly used. Depending upon the span of the truss, the location of the member in the truss and the force in the member either single angle or double angles may be used in roof trusses. Single angle is common in the web members of a roof truss and the double angles are common in rafter and tie members of a roof truss. Plate tension members are used to suspend pipes and building floors. Rods are also used as suspenders and as sag rods of roof purlins. Steel wires are used as suspender cables in bridges and buildings. Pipes are used in roof trusses on aesthetic considerations, in spite of fabrication difficulty and the higher cost of such tubular trusses. Built-up members made of angles, channels and plates are used as heavy tension members, encountered in bridge trusses. Version II 5-8