The designer of a composite floor has a number of basic and obvious decisions to make, including what type of beam to use and what type of decking. He or she also has some more detailed decisions, such as the choice of propped or unpropped construction, whether the steel beams will be restrained during construction, and how will the upper surfaces of the slabs be levelled during concreting?
All these decisions need to be well thought through, rather than assumptions made, and the more detailed decisions need to be communicated to those who will construct the floor so that site practice reflects design assumptions. There may also be specialists involved in the design, e.g. to establish the precise details of how the decking will be delivered and laid out. These individuals must also have a clear understanding of the primary designer’s assumptions.
This post looks at three aspects of composite construction; the provision of lateral restraint to the beams, whether the steelwork is propped and how the concrete is levelled.
Provision of Lateral Restraints to Steel Beams
For bracing against buckling, two requirements may be identified for all restraint systems:
1. Sufficient stiffness; to increase the buckling load of the restrained member to the desired level by limiting the buckling deformation.
2. Sufficient strength; to resist the loads transmitted as a result of restricting the buckling deformations.
Despite the importance of both stiffness and strength, many structural design codes provide only strength requirements, assuming that a member of such strength will possess adequate stiffness. EN1993-1-1 Clause 5.3.3(2)1 defines an equivalent stabilising force.
The elimination of lateral torsional buckling as a failure mode may enable a lighter section size to be used. However, a designer should recognise that profiled steel decking will not always provide this level of restraint. Two things need to be considered; firstly the in-plane stiffness of the decking itself, and secondly how it is fixed to the beam. If the ribs of the decking run parallel to the beam, then no restraint is provided at the construction stage, as the decking would merely act like a concertina were the beam flange to try and move sideways. If the ribs run perpendicularly then the decking may be assumed to have sufficient stiffness.
However, to exploit this stiffness it will need to be adequately fixed to the beam. When shear studs are present and they have been through-deck welded, as is common practice, the decking may be assumed to be adequately held in place. Otherwise, when the decking is fixed (using discrete connectors such as shot-fired pins) the designer must provide an adequate number of connectors, taking into account their individual resistances and the force needed to restrain the beam flange.
Propping of Composite Floors
The term ‘propping’ is used to describe the provision of temporary supports to the steelwork.
The benefit of propped construction is that it results in a greater part of the loading being applied to the (stiffer and stronger) composite section, be it beam or slab,and less to the steel element alone. Greater spans may then be achieved. For long span beams this can be particularly beneficial by reducing dead load deflections. Propping of decking (or ‘sheeting’ as it is termed in the Eurocodes) may enable secondary beams to be more widely spaced, and so the steelwork costs may be reduced (Figure 1). The disadvantage is that the construction process may be adversely affected.
What Should be Specified ?
A simple statement on a drawing such as: ‘beams and decking to be propped’ will not suffice. That statement might be read as implying the designer assumed continuous support during concreting, but clearly that can not be achieved in practice.
Propped construction essentially means that at the construction stage, the steel element (beam or decking) has a different structural form – typified by shorter spans and continuity over additional supports. Typically there would be a prop at mid-span, or possibly at third-span points. A proportion of the construction loading is transferred by the props to the floors below. When the concrete has attained sufficient strength, the props are removed, and the self-weight is resisted by the composite section. The steel element must be able to span between the props when subject to the construction loading.
When we come to consider a composite floor, rather than individual elements, the situation becomes a little more complex and one can envisage two scenarios (Figure 2). From a design point of view, the easier situation to consider is that shown in Fig. 2a, where the beams are propped but the decking is not. For a beam spanning between columns, the support stiffness provided by the columns will be similar to that provided by the temporary supports. In Fig. 2b the decking is propped and the beams are not. This option is complicated because the permanent supports to the decking (the beams) will deflect as load is applied, whereas the temporary supports will barely move (unless they are supported off the beams themselves).
Normally the outcomes of this ‘support incompatibility’ are not considered in the design of the elements, with loads distributed between supports assuming the supports have equal stiffness, rather than the stiffer temporary supports picking up a relatively higher proportion of the load. However, the ‘differential settlement’ that results from these different support stiffnesses should certainly be taken into account when considering the thickness of concrete to design for.
Good Practice on Site
Careful attention should be paid when propping decking on site because of the nature of the product (formed from thin steel). Timber bearers or steel runners should be used to span between individual props. They must be sufficiently stiff , and always extend the full width of the bay to avoid localised problems (Fig. 1). Typically they are 75-100mm wide to provide adequate bearing and avoid local damage to the decking. Props are normally placed at approx. 1m centres.
Levelling the Concrete Surface
Just as there are different ways in which a floor can be propped, there are different ways in which a concrete slab (its upper surface) can be levelled. To avoid overloading in the temporary condition it is essential that the designer’s assumptions are reflected in site practice. Failure to do this could see excessive self-weight of concrete applied to the structure. The two principal ways in which the upper surface of the concrete can be controlled during casting, are:
- Maintain a reasonably constant thickness of slab. As the beams and decking deflect under the wet weight of concrete, the upper surface of the concrete ‘defl ects’ to follow their form. This is achieved by setting tamping rails on the beam lines.
- The upper surface of the concrete is maintained level (meaning ‘fl at’). As the steelwork deflects, more concrete will be added to compensate, and the thickness of concrete will clearly not be uniform. Laser levelling may be used as a quick and cheap solution, possibly with power floating (Figure 3) to avoid the need for a screed.
The Concrete Society (2008) Composite concrete slabs on steel decking. Guidance on construction and associated design considerations (includes amendment No. 1, September 2013), Camberley, UK: The Concrete Society.
MCRMA/Steel Construction Institute (2009) Composite Slabs and Beams using Steel Decking. Best Practice for Design and Construction (Revised Edition) (P300), Ascot, Berkshire: SCI
Beal, A. N. (2011) ‘Floor slabs, lasers and levels’, Concrete, Camberley, UK: The Concrete Society
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