1.1 Composite beams and slabs
The design of structures for buildings and bridges is mainly concerned with the provision and support of loadâbearing horizontal surfaces. Except in some longâspan structures, these floors or decks are usually made of reinforced concrete, for no other material provides a better combination of low cost, high strength, and resistance to corrosion, abrasion and fire.
The economic span for a uniform reinforced concrete slab is little more than that at which its thickness becomes 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, ribs or walls than to thicken it. Where the beams or ribs 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 especially where the susceptibility of steel to loss of strength from fire is not a problem, as in most bridges, steel beams often become cheaper than concrete beams. It was at first 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 Tâbeam action that had long been used in concrete construction. The term âcomposite beamâ as used in this book refers to this type of structure.
The same term is in use 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 steelâframed shed and its cladding; but these are outside the scope of this book.
No income is received from money invested in construction of a multistorey building such as a large office block until the building is occupied. 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. The use of this formwork, known as profiled steel sheeting, began in North America (Fisher, 1970) and is now standard practice in Europe and elsewhere. These floors span in one direction only, and are known as composite slabs. Where the steel sheet is flat, so that twoâway spanning occurs, the structure is known as a composite plate. These occur in boxâgirder bridges.
Steel profiled sheeting and partialâthickness precast concrete slabs are known as structurally participating formwork. Cement or plastic profiled sheeting reinforced by fibres is sometimes used. Its contribution to the strength of the finished slab is normally 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 multistorey car parks, a moderate amount for office blocks, and most of all for public buildings and warehouses. Many methods have been developed for providing steelwork with fire protection.
Design against fire and the prediction of fire resistance is known as fire engineering (Wang et al., 2012). Several of the Eurocodes have a Part 1.2 devoted to it. Full encasement of steel beams, once common, is now more expensive than the use of lightweight nonâstructural materials. Concrete encasement of the web only, done before the beam is erected, is more common in continental Europe than in the UK, and is covered in EN 1994â1â1 (BSI, 2004). It enhances the buckling resistance of the member (Section 4.2.4) as well as provi...
