3 May 23

Pouring Composite Slabs

Deflections and loadings

When pouring a composite slab on metal decking, the process in which it is created will cause the deck to deflect under the weight of the concrete and additional construction loads (e.g. personnel working on the deck).


There are some important points which should be considered before specifying and undertaking a composite slab pour, these are;

  • It is not possible to achieve a defined datum level when constructing concrete floor slabs when installed on metal decking (MCRMA Technical Paper 13).
  • Composite slabs should be poured to a constant thickness to avoid excessive deck deflections and any subsequent ponding of the concrete.
  • When considering the deflection of the slab, compound deflection from the secondary and primary beams must also be considered.
  • The beam Designer and/or Project Engineer should advise on the expected frame deflection.
  • During the pour, concrete must not be dropped from height or heaped on the decking to avoid localised damage or overloading of the metal deck.
  • Grout loss is to be expected and any occurrence of this is not detrimental to the structural integrity of the slab.
  • After curing, surface regularity should be measured parallel to the supporting beams, to ensure compliance with the BS 8204 Flatness Designation, typically SR3 for suspended floors.

Our latest video demonstrates deflections during the construction phase of a steel frame building with metal deck and concrete slabs, and how they can impact on design.

Our Concrete Team can cater for your contracts concrete requirements, contact us for more information.


For more information on this topic, see TGN Section 12.8 to 12.12 specifically relating to this topic

For In-depth Technical Guidance Notes on a range of topics, refer to TGN Online