Effective design measures against soft storey development in Buckling Restrained Braced Frames

Zsarnóczay, Ádám and Vigh, László Gergely (2017) Effective design measures against soft storey development in Buckling Restrained Braced Frames. In: 16th World Conference on Earthquake Engineering, 16WCEE 2017, 2017.01.09-13, Santiago Chile. (In Press)

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Abstract

Buckling Restrained Braced Frames (BRBF) are concentrically braced steel frames with Buckling Restrained Braces (BRB) in their diagonals. BRBs are special displacement dependent anti-seismic devices. Buckling of the internal steel core of a BRB is prevented by the continuous lateral support of its casing. This configuration ensures balanced hysteretic behavior and large energy dissipation capacity. The advantageous dissipative properties of BRBs can only be utilized with an appropriate design procedure. The objective of the presented paper is to draw attention to the high probability of soft story formation in concentrically braced BRBF under seismic excitation beyond the design level of the frame. Although BRBF had been shown to have sufficiently high performance, a significant reduction in collapse probability could be achieved if premature failure due to soft stories was prevented by appropriate design measures. The issue of soft story formation is tackled using three BRBF archetypes. Structural design is performed with several versions of the BRBF design procedure proposed by the authors for European application. Two dimensional finite element models are created for each scenario (i.e. archetype + design) with the OpenSees finite element code. A custom BRB element developed by the authors and calibrated to experimental results is used to improve the description of nonlinear cyclic hardening behavior of BRBs. An extended version of the probabilistic seismic performance assessment framework presented in FEMA P695 is used for this study. Performance for each scenario is evaluated with nonlinear dynamic analyses in a multi-stripe framework and described by fragility curves after taking several sources of uncertainty into consideration. Results of a sensitivity analyses on hundreds of versions of a two-story BRBF suggest that soft story development can be effectively mitigated by either strengthening the columns of the braced frame or shifting the distribution of lateral stiffness along the height of the BRBF from a modal towards a triangular shape. Detailed probabilistic seismic performance assessment on a few selected structures confirms the advantages of the modifications. These results also highlight that increased interstory drifts shall be expected below the design seismic intensity level due to shifting the BRBF stiffness distribution.

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