Transverse reinforcement in the form of mesh or additional loose bars is required in composite beam design to transfer the longitudinal shear force from the shear connectors (typically studs) into the effective width of the slab. Traditionally, light mesh reinforcement has been used throughout the slab, as a ‘deemed to satisfy’ approach, although BS 5950-3.1 and EN 1994-1-1 give explicit guidance than can result in a requirement for additional reinforcement.
This updated Advisory Desk Note focuses on Eurocode design, although the principles also apply to BS 5950 design so the guidance given may be readily adapted.
The requirements for transverse reinforcement in EN1994-1-1 are based on the premise that the longitudinal shear resistance of the slab must be greater than the resistance of the shear connectors (i.e. the longitudinal shear force that can be transferred to the slab). Thereby the ability to achieve 6 mm slip at failure is maintained, because failure of the connectors will always be more critical. However, and this was the origin of AD 241, the number of shear connectors found in many composite beams is greater than the number needed to achieve the required beam resistance. Often, the design of composite beams is governed by serviceability limits, and they are not designed to achieve their full bending resistance. In such cases the studs provided are needed in order to satisfy the rules for minimum degree of shear connection, which are associated with limiting slip at the steel to concrete interface. So in terms of beam resistance alone, fewer studs could be used, and therefore less transverse reinforcement.
AD 241 therefore proposed applying a reduction factor to the longitudinal shear force that was a function of the applied moment divided by the moment resistance.
As noted above, a big change since AD 241 was originally written has been the publication of P405. Covering composite beams with both transverse and parallel decking, and considering a wider range of variables than EN1994-1-1, it provides new rules for minimum degree of shear connection. In many cases the number of studs needed on a beam has dramatically reduced compared to the EN1994-1-1 provisions. It is worth noting that one of the variables considered is the beam utilisation in bending, with minimum degree of connection now varying according to:
Applying the original guidance given in AD 241 alongside the guidance in P405 could therefore result in a certain amount of double counting. Moreover, when beams are designed in accordance with P405 it is unlikely that the ‘old problem’ of being unable to accommodate sufficient transverse reinforcement to provide a resistance in excess of that of the shear connectors will remain.
By applying the rules in P405 (which appear in numerous design software packages) there is no need for AD 241.
Contact: Eleftherios Aggelopoulos
Tel: 01344 636555