Speaker
Description
Conventional design practice for reinforced concrete (RC) floor slabs employs linear elastic analysis, with nonlinear and time-dependent behaviors addressed through stiffness reduction factors applied post-analysis. Creep and shrinkage — two of the most significant time-dependent concrete properties — are conventionally accounted for at the Serviceability Limit State (SLS) through effective elastic modulus approaches, creep coefficients, and shrinkage curvature superposition. Leading design standards, including Eurocode 2, ACI 318M, and AASHTO LRFD, confine creep and shrinkage provisions predominantly to SLS verifications and stop short of providing clear provisions for their incorporation into Ultimate Limit State (ULS) design, leaving a notable gap in codified guidance. This gap is particularly consequential for large plan area RC floor slabs, where restrained shrinkage and creep-induced strains accumulate over extended lengths, generating internal stresses that can meaningfully alter moment and shear distributions at the ultimate limit state — consequently influencing reinforcement demand and its distribution across the slab.
To address this gap, a two-stage computational framework is proposed. In the first stage, nonlinear finite element analyses of suspended flat slabs are conducted in RAM Concept, explicitly modeling creep and shrinkage to establish reference internal stress and force resultant distributions. In the second stage, the same configurations are analyzed using linear elastic analysis, with creep and shrinkage represented as equivalent temperature loads incorporating internal restraint factors. Results from both stages are compared across a range of slab geometries and support conditions, spanning from full shear wall to column-only systems. From this comparison, a restraint factor R is calibrated such that the equivalent temperature load analysis reproduces the nonlinear moment and shear distributions to an acceptable degree of accuracy. The proposed factor R enables engineers to account for ULS implications of creep and shrinkage within a conventional linear elastic workflow, without recourse to computationally intensive nonlinear analysis.