Effect of Slab Modeling Assumptions in Bidirectional Seismic Analysis of Low‐Rise RC Frames: Comparing ACI 318, Eurocode 8, and Other Design Codes

Earthquakes affect buildings in both horizontal directions, generating complex structural interactions. Although bidirectional effects on reinforced concrete (RC) frames have been studied, the influence of slab representation in numerical models under bidirectional excitations remains underexplored. This gap has practical implications: A review of 11 design codes and standards reveals no consensus on stiffness reduction factors for approximating nonlinear slab behavior with elastic models. Provisions vary from fixed values to analysis‐dependent parameters, while some standards offer no guidance at all. Addressing this gap, this study benchmarks four code‐based elastic slab models against a nonlinear slab model under bidirectional seismic excitations. Three RC frame buildings were analyzed using 3D OpenSeesPy models with five slab representations: nonlinear (reference), no slab, and elastic slabs with gross, 25%, and 50% effective stiffness. Each model underwent bidirectional nonlinear dynamic analyses using 22 ground motion pairs, recording interstory drifts, peak floor accelerations (PFAs), and column forces. The results provide practical guidance: Gross property models capture PFA more accurately, while 25% and 50% effective stiffness better represent column forces and drifts. Models excluding slabs perform poorly, especially in irregular buildings, indicating that this common practice in some countries should be reconsidered. Future research directions include extending this study to mid‐ and high‐rise structures, conducting frame‐slab experiments to validate nonlinear slab modeling approaches, and evaluating record‐to‐record variability under bidirectional excitations.