The principal structural elements of a typical multi-storey building are the floors, beams, columns and foundations.
The principal structural elements of a typical multi-storey building comprise floors, beams and columns. A wide variety of alternative forms and arrangements can be used in multi-storey steel framed structures.
The principal structural elements of a typical multi-storey building
Floor slabs: Several different types of slab can be used, in either composite or non composite form.
A number of different floor slab types can be used in association with a steel frame. The floor slab usually spans one way; it is either simply supported or continuous. Most slab types can be designed to act compositely with the supporting beams if adequate shear connection is provided
Composite floors consist of a concrete topping cast onto metal decking.
Composite floor slabs use metal decking, which spans between secondary steel beams usually spaced at between 2.5 and 3m centres. Concrete, which may be either lightweight or normal weight, is then poured onto the decking, usually by pumping, to make up the composite system. Metal decking acts both as permanent formwork for the concrete, eliminating the need to provide props, and as tensile reinforcement for the slab. Steel bars are included in the slab to prevent cracking and to provide reinforcement in the event of degradation of the decking in a fire.
Alternative arrangements of primary and secondary beams can be adopted for an optimum deck span of 3m. and a typical system is illustrated.
There are many types of steel decking available, but perhaps the most commonly used is the re-entrant profile type which provides a flat soffit and facilitates fixings for services and ceilings.
Fig.1: Primary and secondary steel beams
Composite floor slabs have become popular for multi-storey buildings when rapid construction is required.
This form of slab construction is particularly popular for multi-storey buildings when rapid construction is required. Some of the advantages of the composite system are:
Steel decking acts as a permanent shuttering which can eliminate the need for slab reinforcement and propping of the construction while the concrete develops strength. This leads to simple, rapid construction.
- Composite action reduces the overall depth of structure. It provides up to 2 hours fire resistance without additional fire protection and 4 hours with added thickness or extra surface protection.
- It is a light, adaptable system that can be cut to awkward shapes and can easily be drilled or cut out for additional service requirements.
- The overall weight of this system is low, particularly if lightweight concrete topping is used, reducing frame loadings and foundation costs.
- The demands on cranage are low as many sheets of steel decking may be lifted at a time, and then laid out by hand; the concrete topping may be placed by pump.
- Precast concrete units are also often used with a steel frame, offering long spans and quick installation.
- Precast concrete floors area heavier form of construction than comparable composite metal deck floors, but offer the following advantages:
- Fewer floor beams since precast floor units typically span 6 to 8m.
- Shallow floor construction can be obtained by supporting precast floor units on shelf angles.
- Fast construction because there is no propping and no time is needed for curing and the development of concrete strength.
Precast concrete floor systems
Composite action may be difficult with precast floors, and construction requires a crane.
The disadvantages of precast concrete floors are:
- Composite action not readily achieved without a structural floor screed.
- Heavy floor units are difficult to erect, requiring the use of a crane which may have implications on the construction programme.
Slimdek® is an engineered floor solution developed to offer a cost-effective, service-integrated, minimal depth floor for use in multi-storey steel framed buildings with grids up to 9 metres x 9 metres.
Slimdek extends the range of cost-effective steel options for modern buildings. Ease of planning and servicing, combined with a reduction in building height, gives significant cost and speed of construction benefits.
Slimdek solutions can be designed to incorporate the latest technology in energy efficient services principles.
Alternative floor framing systems
Composite beams mobilise the strength of the floor slab to improve the strength of the steel beam.
Composite action is achieved by welding studs to the beam flange to form a continuous shear connection. The slab then acts as the compression flange for the beam. When used with composite deck flooring, the studs are welded through the decking onto the flange of the beams below to form a connection between steel beam and concrete slab.
Maximum beam span is about 15m and beam depth can be estimated as span/25 (see figure 1)
Weight comparisons for different beam layouts indicate the use of long span main beams.
Figure 1 shows the effect of varying the arrangements of primary and secondary beams. The weight of steelwork per square metre for each arrangement is plotted against span of primary beam. This comparison has been based on a limiting criterion of span/360 for superimposed load deflection required by BS 5950 and incorporates an allowance for trimmers and connections.
The comparison highlights a number of interesting features relating to this type of floor system and floor grids in general:
- Long spanning secondary systems result in heavier floor steel than using long span primary beams. For example, a 9m by 15m structural bay with 15m secondary beams uses 20 percent more steel than a 15m by 9m bay with 15m primary beams.
- For the longer spans the effect of setting more stringent deflection criteria (say a flat limit of 25mm instead of span/360) is to increase the weight of steel required significantly (between 5 and 10 percent).
In practice long span secondary beams are often used.
Weight of structural steel for composite floors
The above graph should be used with caution because:
- They are based on idealised grids
- Although the long span primary system is some 20 percent lighter than the equivalent long span secondary system, it contains as much as 40 percent more labour content in fabrication and erection.
- Although an allowance is made for items such as connections and trimming round openings these are difficult to define. In practice, the irregularities encountered in a real building may have an overriding influence.
Section sizes of main and secondary beams is more consistent if the more lightly loaded secondary beams have longer spans
The weight of columns is largely independent of beam span.
The graphs also exclude the influence of the columns on the weight of the floor steel. Studies on composite floors have shown that the average weight of column on a particular floor is largely independent of the span of the floor provided. Of course, the columns must not be so closely spaced that the column size is not related to the load it carries. This being the case, to find the combined weight of both floors and vertical steel in a building, the effect of the columns can be added to the floors.
The figure below shows the weight per metre of typical column (forming part of a 9m by 6m grid) for a building with increasing height. To obtain an estimate of the overall tonnage of structural steel in a building, the weight per square metre for the floors can be added and multiplied by the gross floor area.
Weight of structural steel for columns
Longer spans are possible using composite steel floor trusses, but fabrication costs are higher.
Use of composite steel floor trusses as primary beams in the structural floor system permits much longer spans than would be possible with conventional universal beams. The use of steel trusses for flooring systems is common for multi-storey buildings in North America, but seldom used in Britain. Although they are considerably lighter than the equivalent universal beam section, the cost of fabrication is very much greater, as is the cost of fire proofing the truss members. For maximum economy, trusses should be fabricated using simple welded lap joints. Maximum span for floor trusses is about 30m and truss depth can be estimated as span/15.
Typical composite truss details
Truss forms allow integration of services.
The openings between the diagonal members should be designed to accept service ducts, and if a larger opening is required, a Vierendeel panel can be incorporated, typically at the centre of the span.