Are existing span-to-depth rules conservative for flat slabs?

Abstract

A series of parametric studies is presented to show the influence on deflection of factors including age at striking and peak construction load. The parametric studies suggest that the peak construction load usually governs long-term deflections in slabs struck at 2 days or later. Eurocode 2 is shown to significantly underestimate long-term deflections in slabs under permanent loading if the 28 day concrete tensile strength is used as intended since deflections are usually governed by cracking during construction. This is reflected in existing span-to-depth rules in EC2 and BS 8110 that do not realistically take construction loading into account. Span-to-depth rules in EC2 and BS 8110 are compared for flat slabs and it is shown that EC2 permits significantly thinner slabs. It is shown that deflections can be excessive in slabs dimensioned to the EC2 rules. Consequently, it is proposed that a modified concrete strength is used in the EC2 formula for span-to-depth ratios to allow for cracking during construction. It is concluded that there is scope for reducing slab thickness below BS 8110 guidelines for concrete cylinder strengths greater than 25–30 MPa.