(livro) structural steel design and construction

(Parte 2 de 9)

Structural members delivered to site should be lifted directly and fixed into position, avoiding storage on site. Steel stairs, erected along with the frame, give immediate access for quicker and safer erection.

Metal decking may be lifted in bundles and no further craning is required as it is laid by hand and fixed by welded studs. This gives both a working and safety platform against accidents.

Secondary beams should be placed close enough to suit the deck, so that temporary propping can be avoided, and the deck could be concreted immediately.

1.1.3 Steel-concrete Composite Design Considerable benefit is gained by composition of the slab with the steel beam with possible weight savings of up to 30%. An effective width of a slab is assumed to carry the compressive stresses leaving virtually the whole of the steel beam in tension creating a T -beam effect. Interaction between the slab and the beam is generated by 'through deck" stud welding on to the beam flange.

1.1.4 Deflection and Cambering Where the floors are unpropped, the deflection due to wet construction requires consideration to avoid the problem of ponding.

Dead load deflections exceeding 15-20mm can be easily offset by cambering which is best achieved by cold rolling the beam. This is a specialized operation but not only is the camber permanent because of stress re-distribution due to controlled yielding. These will depend upon a number of factors and the advice of the specialist should be sought.

Because cambering can add 10-20% to the basic steel cost, this should be compared with the cost of a deeper and stiffer beam section provided that the increase in building height does not compromise additional cladding costs.

1.1.5 Fire Resistance In the event of fire the metal deck unit would cease to function effectively due to loss of strength. However, additional strength can be provided by the added wire mesh for up to one hour fire rating.

For higher period of fire resistance or for exceptionally high imposed loads, heavier reinforcement in the form of bars placed within the deck troughs can be used. Up to 4 hours fire rating can be obtained using this method based upon fire engineering calculations with the deck units serving only as a permanent formwork.

For beams and columns, fire resistance may be provided by lightweight systems, which are quick to apply and economic. Normally cement based sprays are applied to beams, and boards around columns. For tall buildings, steel columns may be encased or circular steel columns infilled with high-strength concrete to enhance resistance against compression and fire.

They are normally located in buildings to accommodate lift shafts and staircases.

In steel structures, it is common to have triangulated vertical truss to provide bracing (see Fig. 1.2a). Unlike concrete structures where all the joints are naturally continuous, the most direct way of making connections between steel members is to hinge one member to the other. For a very stiff structure, shear wall or core wall is often used (Figure 1.2b ). The efficiency of a building to resist lateral forces depends on the location and the types of the bracing systems employed, and the presence or otherwise of shear walls and cores around lift shafts and stairwells.

Shear l (a)

(b)

Figure 1.2 Commom bracing systems (a) Vertical truss system (b) Shear Wall

1.2.4 Braced Versus Unbraced Frames Building frame systems can be separated into vertical loadresistance and horizontal load-resistance systems. The main function of a bracing system is to resist lateral forces. In some cases the vertical load-resistance system also has some capability to resist horizontal forces. It is necessary, therefore, to identify the sources of resistance and to compare their behaviour with respect to the horizontal actions. However, this identification is not that obvious since the bracing is integrated within the structure. Some assumptions need to be made in order to define the two structures for the purpose of comparison.

Figures 1.3 and 1.4 represent the structures that are easy to define, within one system, two sub-assemblies identifying the bracing system and the system to be braced. For the structure shown in Figure 1.3, there is a clear separation of functions in which the gravity loads are resist by the hinged subassembly (Frame B) and the horizontal load loads are resisted by the braced assembly (Frame A). In contrast, for the structure in Fig. 1.4, since the second sub-assembly (Frame B) is able to resist horizontal actions as well as vertical actions, it is necessary to assume that practically all the horizontal actions are carried by the first sub- assembly (Frame A) in order to define this system as braced.

According to Eurocode 3 (EC3, 1992) a frame may be classified as braced if its sway resistance is supplied by a bracing system in which its response to lateral loads is sufficiently stiff for it to be acceptably accurate to assume all horizontal loads are resisted by the bracing system. The frame can be classified as braced if the bracing system reduces its horizontal displacement by at least 80 percent.

1.2.5 Sway Versus Non-Sway Frames A frame can be classified as non-sway if its response to in- plane horizontal forces is sufficiently stiff for it to be acceptable to neglect any additional internal forces or moments arising from horizontal displacements of the frame. In the design of multi-storey building frame, it is convenient to isolate the columns from the frame and treat the stability of columns and the stability of frames as independent problems. For a column in a braced frame it is assumed the columns are restricted at their ends from horizontal displacements and therefore are only subjected to end moments and axial loads as transferred from the frame. It is then assumed that the frame, possibly by means of a bracing system, satisfies global stability checks and that the global stability of the frame does not affect the column behaviour. This gives the commonly assumed non- sway frame. The design of columns in non-sway frame follows the conventional beam-column capacity check approach, and the column effective length may be evaluated based on the column end restraint conditions.

Another reason for defining "sway" and "non-sway frames" is the need to adopt conventional analysis in which all the internal forces are computed on the basis of the undeformed geometry of the structure. This assumption is valid if second-order effects are negligible. When there is an interaction between overall frame stability and column stability, it is not possible to isolate the column. The column and the frame have to act interactively in a "sway" mode. The design of sway frames has to consider the frame subassemblage or the structure as a whole.

British Code: BS5950: Part 1(1990) provides a procedure to distinguish between sway and non-sway frames as follows: 1) Apply a set of notional horizontal loads to the frame.

These notional forces are to be taken as 0.5% of the factored dead plus imposed loads and are applied in isolation, i.e., without the simultaneous application of actual vertical or horizontal loading. 2) Perform a first-order linear elastic analysis and evaluate the individual relative sway deflection d for each storey. 3) If the actual frame is uncladed, the frame may be considered to be non-sway if the inter-storey deflection satisfies the following limit:

h o ~ 4000 where h =storey height (1.1) for every storey.

4) If the actual frame is claded but the anal¥sis is carried out on the bare frame, then i!J recognition of the fact that the cladding will substantially reduce deflections, the condition is reflected and the frame may be considered to be non-sway if h 5 ~ 2000 where h = storey height (1.2) for every storey. 5) All frames not complying with the criteria in Eqs. (1.1) or (1.2) are considered to be sway frames.

Eurocode 3 (1992) also provides some guidelines to distinguish between sway and non-sway frames. It states that a frame may be classified as non-sway for a given load case if P I P ~ 10 for that load case where P is the elastic critical' buckling value for sway P is the design value ofthe total vertical load. When the system buckling load factor is ten times more than the design load factor, the frame is said to be stiff enough to resist lateral load, and it is unlikely to be sensitive to side sway deflections.

1.2.6 Classification of Multi-storey Buildings The selection of appropriate structural systems for tall buildings must satisfy both the strength and stiffness requirements. The structural system must be adequate to resist lateral and gravity loads that cause horizontal shear deformation and overturning deformation. Other important issues that must be considered in planning the structural schemes and layout are the requirements for architectural details, building services, vertical transportation, and fire safety, among others. The efficiency of a structural system is measured in term of their ability to resist higher lateral load which increases with the height of the frame. A building can be considered as tall when the effect of lateral loads is reflected in the design. Lateral deflections of tall building should be limited to prevent damage to both structural and non-structural elements. The accelerations at the top of the building during frequent windstorms should be kept within acceptable limits to minimise discomfort to the occupants.

Figure 1.5 shows a chart which defines, in general, the limits to which a particular framing system can be used efficiently for multi-storey building projects. The various structural systems in Fig. 2.5 can be broadly classified into two main types: (1) medium-height buildings with shear-type deformation predominant and (2) high-rise cantilever structures such as framed tubes, diagonal tubes and braced trusses. This classification of system forms is based primarily on their relative effectiveness in resisting lateral loads.

I. .: ,n.

+ Frame A FrameB

Figure 1.3 Frames split into two subassemblies

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I. ...

Frame A Frame B

Figure 1.4 Mixed frames split into two subassemblies

1.3 FLOOR SYSTEMS

1.3.1 Design Consideration Floor systems in tall buildings generally do not differ substantially from those in low-rise buildings. However, the following aspects need to be considered in design: 1. Weight to be minimised. 2. Self-supporting during construction. 3. Mechanical services to be integrated in the floor zone. 4. Adequate fire resistance. 5. Buildability. 6. Long spanning capability. 7. Adequate floor diaphragm

Modern office buildings require large floor span in order to create greater space flexibility for the accommodation of greater variety of tenant floor plans. For building design, it is necessary to reduce the weight of the floors so as to reduce the size of columns and foundations and thus permit the use of larger space. Floors are required to resist vertical loads and they are usually supported by secondary beams. The spacing of the supporting beams must be compatible with the resistance of the floor slabs.

The floor systems can be made buildable using prefabricated or precast elements of steel and reinforced concrete in various combinations. Floor slabs can be precast concrete slab or composite slabs with metal decking. Typical precast slabs are 4 m to 7 m, thus avoiding the need of secondary beams. For composite slabs, metal deck spans ranging from 2m to 7 m may be used depending on the depth and shape of the deck profile. However, the permissible spans for steel decking are influenced by the method of construction, in particular it depends on whether temporary propping is provided. Propping is best avoided as the speed of construction is otherwise diminished for the construction of tall buildings.

Floor beans systems must have adequate stiffness to avoid large deflections which could lead to damage of plaster and slab finishers. Where the deflection limit is too severe, pre-cambering with an appropriate initial deformation equal and opposite to that due to the permanent loads can be employed to offset part of the deflection.

Sometimes openings in the webs of beams are required to permit passage of horizontal services, such as pipes (for water and gas), cables (for electricity, tele-and electroniccommunication), and ducts (air-conditioning), etc. Various long span flooring systems in Sections 3.4 offer solutions to integrate building service into the structural depth leading to potential saving in weight and cladding cost.

1.3.2 Composite Floor Systems Composite floor systems typically involve structural steel beams, joists, girders, or trusses linked via shear connectors with a concrete floor slab to form an effective T-beam flexural member resisting primarily gravity loads. The versatility of the system results from the inherent strength of the concrete floor component in compression and the tensile strength of the steel member. The main advantages of combining the use of steel and concrete materials for building construction are:

Steel and concrete may be arranged to produce ideal combination of strength, with concrete efficient in compression and steel in tension. Composite flooring is lighter in weight than pure concrete slab. The construction time is reduced since casting of additional floors may proceed without having to wait for the previously cast floors to gain strength. The steel decking system provides positive-moment reinforcement for the composite floor and requires only small amount of reinforcement to control cracking and for fire resistance. The construction of composite floor does not require highly skilled labour. The steel decking acts as a permanent form work. Composite beams and slabs can accommodate raceways for electrification, communication, and air distribution system. The slab serves as a ceiling surface to provide easy attachment of a suspended ceiling. The composite floor system produces a rigid horizontal diaphragm, providing stability to the overall building system while distributing wind and seismic shears to the lateral load resisting systems. Concrete provides thermal protection to steel at elevated temperature. Composite slabs of two hours fire rating can be easily achieved for most building requirements.

The floor slab may be constructed by the following methods: a flat-soffit reinforced concrete slab (Fig. 1.6a), precast concrete planks with cast in-situ concrete topping (Fig. 1.6b), precast concrete slab with in-situ grouting at the joints (Fig. 1.6c), and a metal steel deck tops with concrete, either composite or non-composite (Fig. 1.6d).

The composite action of the metal deck results from side embossments incorporated into the steel sheet profile.

1.3.3 Composite Beams and Girders

Steel and concrete composite beams may be formed by shear connectors connecting the concrete floor to the top flange of the steel member. Concrete encasement will provide fire resistance to the steel member. Alternatively, direct sprayed-on cementitious and board type fireproofing materials may be used economically to replace the concrete insulation on the steel members. The most common arrangement found in composite floor systems is a rolled or built-up steel beam connected to a formed steel deck and concrete slab (Fig. 1.6d). The metal deck typically spans unsupported between steel members while also providing a working platform for concreting work.

(Parte 2 de 9)