Multipliers for deflections in reinforced concrete flat slabs

Ideally, slab deflections should be calculated using non-linear cracked section analysis. In practice, slabs are often designed using elastic finite element analysis and deflections are estimated using a reduced elastic modulus for the concrete. This paper examines the problem of determining an equivalent elastic modulus that accounts for the increase in deflection due to cracking, shrinkage and creep. A method is proposed for estimating long-term deflections in flat slabs by increasing elastic deflections with multipliers derived from analysis of equivalent beam strips. Deflections in flat slabs are usually governed by cracking during construction in which case deflection multipliers are almost independent of the permanent design load under which EC2 requires deflections to be calculated. In this case, it is shown that cracking and shrinkage increase long-term deflection multipliers for flat slabs by a factor of approximately two.