Seismic Response of Post-Tensioned Steel Rocking Concentrically Braced Frames With Various Aspect Ratios

Abstract eng:
Post-tensioned, steel, rocking concentrically braced frames (also known as selfcentering concentrically braced frames or SC-CBFs) are a new type of seismic lateral force resisting system that reduces or eliminates the damage and residual drift often associated with conventional concentrically-braced frames (CBFs) under design level earthquakes. The SC-CBF is made up of a CBF whose column bases are not anchored to the foundation (they are free to uplift), post-tensioning steel that runs vertically over the height of the SC-CBF to prestress the SC-CBF to the foundation and some sort of energy dissipation device. The SCCBF eliminates the damage to the CBF members under design level earthquakes by limiting the base overturning moment that can develop and then keeping the members nominally elastic. The maximum base overturning moment is limited by allowing the SC-CBF to rock on its foundation and yielding of the post-tensioning steel. The residual drift is reduced or eliminated under design level earthquakes by a prestressing force from the post-tensioning steel running vertically over the height of the SC-CBF. The prestressing force provides a restoring overturning moment that returns the SC-CBF to its original undisplaced state after the earthquake. One of the key aspects and assumptions of the SC-CBF system is that the member forces are controlled by limiting the base overturning moment that can develop. Generally, base overturning moment is considered to be a first mode response, but as the aspect ratio increases this assumption becomes less valid. This means that the member forces may not be as well controlled as thought by limiting the base overturning moment. This paper studies the seismic response of six SC-CBFs across a range of aspect ratios through nonlinear time-history analysis and shows how the performance of the SC-CBF changes with aspect ratio.

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