Composite Structures

Composite beams and slabs

The design of structures for buildings and bridges is mainly concerned with the provision and support of load-bearing horizontal surfaces. Except in long-span bridges, these floors or decks are usually made of reinforced concrete, for no other material has a better combination of low cost, high strength, and resistance to corrosion, abrasion, and fire.
The economical span for a reinforced concrete slab is little more than that at which its thickness becomes just sufficient to resist the point loads to which it may be subjected or, in buildings, to provide the sound insulation required. For spans of more than a few metres it is cheaper to support the slab on beams or walls than to thicken it. When the beams are also of concrete, the monolithic nature of the construction makes it possible for a substantial breadth of slab to act as the top flange of the beam that supports it.
At spans of more than about 10 m, and particularly where the susceptibility of steel to damage by fire is not a problem, as for example in bridges and multi-storey car parks, steel beams become cheaper than concrete beams. It used to be customary to design the steelwork to carry the whole weight of the concrete slab and its loading; but by about 1950 the development of shear connectors had made it practicable to connect the slab to the beam, and so to obtain the T-beam action that had long been used in concrete construction. The term ‘composite beam’ as used in this book refers to this type of structure.
The same term is used for beams in which prestressed and in-situ concrete act together, and there are many other examples of composite action in structures, such as between brick walls and  beams supporting them, or between a steel-framed shed and its cladding; but these are outside the scope of this book.
No income is received from money invested in the construction of a multi-storey building such as a large office block until the building is occupied. For a construction time of two years, this loss of income from capital may be 10% of the total cost of the building; that is, about one-third of the cost of the structure. The construction time is strongly influenced by the time taken to construct a typical floor of the building, and here structural steel has an advantage over in-situ concrete.
Even more time can be saved if the floor slabs are cast on permanent steel formwork that acts first as a working platform, and then as bottom reinforcement for the slab. This formwork, known as profiled steel sheeting, has long been used in tall buildings in North America.(1) Its use is now standard practice in most regions where the sheeting is readily available, such as Europe, Australasia and Japan. These floors span in one direction only, and are known as composite slabs.,where the steel sheet is flat, so that two-way spanning occurs, the structure known as a composite plate. These occur in box-girder bridges, and are covered in Chapter 9 (Volume 2).
Profiled sheeting and partial-thickness precast concrete slabs are known as structurally participating formwork. Fibre-reinforced plastic or cement sheeting, sometimes used in bridges, is referred to as structurally nonparticipating, because once the concrete slab has hardened, the strength of the sheeting is ignored in design.

The degree of fire protection that must be provided is another factor that influences the choice between concrete, composite and steel structures, and here concrete has an advantage. Little or no fire protection is required for open multi-storey car parks, a moderate amount for office blocks, and most of all for warehouses and public buildings. Many methods have been developed for providing steelwork with fire protection. (2) Design against fire and the prediction of resistance to fire is known as fire engineering. There are relevant codes of practice, including a draft European code for composite structures. (3) Full or partial encasement in concrete is an economical method for steel columns, since the casing makes the columns much stronger. Full encasement of steel beams, once common, is now more expensive than the use of lightweight non-structural materials.
It is used for some bridge beams (Volume 2). Concrete encasement of the web only, cast before the beam is erected, is more common in continental Europe than in the UK. It enhances the buckling resistance of the member (Section 3.52), as well as providing fire protection.
The choice between steel, concrete, and composite construction for a particular structure thus depends on many factors that are outside the scope of this book. Composite construction is particularly competitive for medium or long span structures where a concrete slab or deck is needed for other reasons, where there is a premium on rapid construction, and where a low or medium level of fire protection to steelwork is sufficient.

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