Contribution of lattice girders to the structural performance of precast solid concrete composite slabs

Abstract

Precast solid composite concrete flooring systems such as Omnia slabs have found widespread use throughout the world as an alternative to conventionally poured-in-place concrete roofing, due to the numerous benefits they offer. Despite their relative proliferation, when designing Omnia slabs to European standards structural engineers are still limited to using the same procedures as with conventionally reinforced slabs. This approach is overly conservative both at the construction stage, where it does not take into account the contribution of the concrete to the bending strength of the plank, and at the limit design stage, where it neglects the truss action of the lattice girders. The objective of this study was to study the effect of lattice girders on the structural performance of precast concrete composite slabs at the limit design stage, the key parameters being load deflection behaviour, crack patterns, and the capacity and failure mode at ultimate load conditions. Following a literature review, an experimental program was devised, featuring a series of Push-Out and Four-Point Bending flexural tests. Materials and interface conditions were kept as uniform as possible throughout the samples to ensure that the effect of the lattice girders on the structural response would be clearly discerned. The results suggest that lattice girders increase the horizontal shear resistance at the concrete-to-concrete interface, and provide a clear decrease in the deflections and crack widths experienced by a slab under flexural loads when compared to an equivalent monolithic slab with conventional reinforcement. The increase in ultimate load capacity was less distinct and may warrant further research, as do the results obtained from composite samples without lattice girders, which showed inferior performance compared to predictions from codes of practice.