Calculation of Long-term Deflections in Concrete Slabs Using Finite Element Methods

The deflection of concrete slabs involves numerous factors including the geometric properties, concrete properties and load history. Long-term deflections are the result of a decrease in stiffness overtime due to concrete shrinkage and creep effects. Finite element programs are useful to analyse the behaviour of concrete slabs most notably two way slab systems. However, they lack the current functionality to accurately model long term deflections due to the complexity of factors involved. Currently there is no industry accepted method to calculate long-term deflections in concrete slabs using finite element software. AS 3600 Concrete Structures makes no mention and provides no guidelines for the analysis of concrete slab deflections using finite element software. Without industry guidelines there is an increased risk of structures failing to meet the required serviceability limit state using finite element methods. This study has explored the factors that contribute to the time-delayed increase in concrete slab deflections known as long-term deflections. RAM Concept is a finite element program where the Load Factors, Cracked Element Analysis and RAM Concept's Effective Curvature Ratio (ECR) methods have been used to estimate the concrete slabs long-term deflections. The finite element results have been compared to the previously tested one-way (Gilbert, 2008) and two-way (Gilbert and Guo, 2002) concrete slabs that had been subjected to sustained loads.