Effect of Slab Stiffness on EBF Link Rotation Demands and Implications for Link Ultra Low-Cycle Fatigue Susceptibility

Abstract

This study aims to quantify slab contributions in eccentrically braced frames (EBFs) by investigating the dynamic performance of two 3-story EBF configurations with and without concrete slabs: one configuration having long (flexural yielding) links and the other having shorter (shear yielding) links. Particular effort is spent investigating the relative accumulation of ultra low-cycle fatigue damage within the link regions during dynamic loading. All slab and major frame geometries are modeled using shell elements. Linear springs and nodal constraints model the discrete slab-to-beam interactions. Recorded earthquake ground accelerations, scaled to design-level intensities, load the EBFs. Ultra low-cycle fatigue damage is investigated using a calibrated micro-mechanics based ductile fracture model. Results indicate reduced system-level frame demands (inter-story drift, residual drift, and link rotation) due to increased system stiffness from the concrete slabs. Reductions in these demands were larger for EBFs with long (flexural yielding) links as compared to the short (shear yielding) links. Even with reduced rotation demands, EBF models with slabs sustained similar link fatigue damage, as compared with the bare-steel models, due to increased plastic strain demands presumably from a neutral axis shift.