An Analytical Study on Low Cycle Fatigue Effects in Buckling Restrained Braces

Abstract eng:
Dampers are used in structures to dissipate input energy caused by severe earthquakes. They can also reduce seismic demands on other structural elements. So dampers’ life is a significant parameter to prevent the structural damages. One of the failure modes observed in the steel dampers during earthquakes, is the damage caused by cumulative effects due to few cycles of high strain amplitudes. This undesirable effect is called Low-Cycle Fatigue (LCF). In buckling restrained braced frames (BRBFs), brace element acts as a hysteretic damper and dissipates high amount of input energy by plastic axial deformations. To ensure satisfactory operation of BRB with no failure during earthquake, it is necessary to investigate LCF effects. On the other hand smaller yielding length of BRB cores imposes higher strain amplitudes and makes the LCF failure more probable. In the present study, a criterion to control LCF failure in BRB has been selected based on fatigue life equations recommended in the previous experimental researches. A 7 story benchmark building with BRBs as the lateral resisting system is selected. Then the yielding length of BRB has been reduced to the minimum possible based on the fatigue criterion and the building has been redesigned. Fatigue analysis has been performed on both buildings to evaluate LCF capacity. In order to evaluate fatigue effects, time history analysis have been conducted on both buildings using the open source finite element platform, OpenSees (2005). Strain history demands in each yielding segments of the braces have been extracted and the cumulative damage index of Palmgren-Miner has been calculated to estimate LCF damage index. Results of this study show that despite the high cumulative plastic deformations the yielding segments experiences, both BRBs have proper fatigue damage indeces. Also it has been proved that developed criterion for controlling LCF failure is conservative for selected time histories and has appropriate safety factor for defining reduced core length of BRB.

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