Behaviour of stainless steel reduced beam section under cyclic loading

Abstract

Stainless steel (SS) in the structural application is noticeably growing in priority due to its considerable strain hardening along with the highly ductile characteristic which is required for sustaining extreme loads like earthquake. The application of stainless steel in construction industries is also increasing for its environmental and mechanical advantages such as corrosion resistance, heat resistance, pleasant appearance and low maintenance cost. Stainless steel specially, the austenitic and duplex grade exhibit higher ductility compared to that of carbon steel (CS). The percentage elongation values at fracture for stainless steel are between 40–60% and values of ultimate-to-yield strength ratios are typically around 1.5–2.0. Several design codes are currently available for SS such as SEI/ASCE-8-02, EN 1993-1-4, AS/NZS 4673. However, due to insufficient studies on the behavior of SS members under cyclic loading, the benefits of SS are not incorporated in the codes. Most of the cases SS members are assumed to be of behavior like carbon steel (CS) members as they are readily available in the codes. After Northridge and Kobe earthquakes, use of the Reduced Beam Section (RBS) has been introduced in steel frames which allows stable yielding of the beam column joint by moving the plastic hinge region in the beam a short distance from the column face. The behavior of RBS of CS is well established in different codes and can be used for performance-based analysis. However, study on the performance of RBS of SS is very limited. In this study the moment rotation behavior of SS beam under cyclic loading were observed and based on that backbone curve has been proposed. Using ABAQUS software finite element models (FEM) are developed for beam column joints with reduced beam section. Initially the developed model was validated with experimental results and later used to observed the behaviour of SS reduced beam sections under cyclic loading. Two grades of SS were considered in the study such as: Austenitic and Duplex, and facing standard loading protocol described in FEMA 356 was used as cyclic load. As strength of duplex grade was higher than austenitic grade SS, the area of hysteresis loops (M-θ) of duplex SS joints with RBS were found much higher than that of austenitic SS RBS. However, after normalizing the hysteresis loops by yield stress (M/My-θ/θy), the area of austenitic SS joints became higher compare to duplex SS joints. From the hysteresis loops, generalized back bone curves of SS reduced bean sections were developed and different parameters of the backbone curves were compared with FEMA 356 recommended values. The residual strength ratios of austenitic SS RBC joints varies from 0.5 to 1.25 which is much higher than FEMA recommended value of 0.2. For duplex SS this ratio varies from 0.3-0.9. The ratio of ultimate moment capacity to yield moment for RBS of austenitic and duplex grade SS were also higher than FEMA 356 maximum recommended values. This phenomenon is more prominent for duplex SS. The strain hardening slopes of the backbone curves varies between 6% to 13% for SS joints whereas FEMA 356 proposed this values within 0-6%. Slope of the degradation part of the backbone curves was also found milder for both grades of SS reduced beam sections.