Deflections are inevitable!

No matter how hard you try to stop it, deflections are inevitable! 

Structural elements under load will deflect. Safe and practical design of composite floor systems aims to limit deflection to an acceptable minimum, and to do this designers need to be able to recognise when and how deflections are likely to occur in both the construction stage and the final composite stage.

 

Calculations can be complex as overall deformation is a combination of that of the slab and of both primary and secondary beams and can be in excess of 40mm relative to datum. Connected beams of differing sizes and spans will have varying stiffnesses and therefore deflections.

 

Construction stage

Construction stage design of decking must take into account the weight of the wet concrete, reinforcement and an allowance for temporary construction load in accordance with BS5950 Part 4 or BS EN 1991-1-6. Where this load is likely to be exceeded, the SMD Technical Team should be consulted.

Exceptional concentrated loads may be caused by the weight of heaped, wet concrete if best practice is not followed. It is possible for the decking to bear the weight of stored materials provided these loads are applied in accordance with the guidance in the Concrete Society’s Technical Report TR75 (Composite Concrete Slabs using Steel Decking).

Loadings from plant and materials that may be applied to the composite slab during the remainder of the construction phase of the project may be more onerous than the design loadings for the intended building use.

In accordance with UK National Annex to BS EN 1994-1-1 and BS5950-4, the deflection of the deck at construction stage is limited to the lesser of Span/180 or 20mm. This can be increased to be the lesser of Span/130 or 30mm when the effects of ponding are taken into account, ie the deck deflection is expected to be greater than slab depth/10 and the additional weight as a result is considered in the calculations. Beams should also be assessed for stiffness and resistance to construction loads, so that any deflections do not lead to significant additional concrete loads from ponding.

It is important that these figures are considered in both the specification of the required slab surface tolerance and the concrete placement method employed by the main contractor. Method of concreting on metal decking must form part of the project Health & Safety hazard identification and risk assessment. Guidance on recommended pouring methods and surface/flatness tolerance is available in the Concrete Society’s Technical Report TR75 and in the SCI Advisory Desk Note AD344: Levelling Techniques for Composite Floors.

Constraints which dictate tighter control of deck deflection at construction stage, for example where limits are determined by floor finishes, or if a directly trafficked slab finish is required, should be considered at commencement of the design stage. These deflection limits should normally be achieved by designing a sufficiently stiff steel frame incorporating appropriately sized and spaced members. The more robust frame required in these circumstances will increase project cost. If this solution is not acceptable, the structural engineer may consider specifying temporary propping to spans that are actually within the safe load/span criteria. The composite beam design must allow for the additional loading that will occur when the temporary propping is removed.

 

Composite stage

Composite stage deflection limits of beams and slabs are usually governed by serviceability considerations. In this form of construction, in which beams are often relatively shallow but long-spanning, load deflection and dynamic impacts are more likely to be critical.

Combined dead and imposed loads should be considered to ensure that deflections of a building’s floor structure will not be unacceptable to occupants and will not result in damage to brittle finishes. Loading from both imposed floor finishes and external cladding should be incorporated into the design of edge beams to ensure that any deformation does not affect the performance of the cladding.

In addition to this, floors in dance studios and gymnasia or in any applications in which they are likely to be subjected to vibration must be assessed for dynamic sensitivity to design for appropriate deflection limits.

SCI Publication P300 gives guidance on design of composite slabs, beams and decking in all stages and also provides advice on construction loading. Table 5.1 in the P300 gives suggested deflection limits for composite beams based on various loading combinations and serviceability requirements.

Pre-cambering can be used to mitigate beam deflections under dead load but should be limited to avoid hindering laying of the metal decking.

 

Guidance

Advice on the effects of loading and frame deflections on our decking profiles can be obtained from SMD’s Technical Department and additional information is provided in our TGN Online. Our Deflection Best Practice Data Sheet DATA/10 is available from the SMD Technical Reference Library.

Shaping the Skyline Magazine

Request a copy of our ‘Shaping the Skyline’ magazine which includes this article along with many others!

  • This field is for validation purposes and should be left unchanged.

Other Related Topics

Programme, what programme