Collapse Assessment of Controlled Rocking Steel Braced Frames With Different Base Rocking Joint Parameters

Abstract eng:
Previous studies have shown that designing controlled rocking steel braced frames (CRSBFs) using a response modification factor of R = 8 is sufficient to prevent collapse of structures during earthquakes higher than the design level. However, it has also been suggested that CRSBFs could be designed using higher response modification factors (R), and that additional rocking joints could be used to mitigate higher mode force demands. This paper examines how the selection of R and the specification of additional rocking joints above the base influence the collapse risk of CRSBFs, as assessed using incremental dynamic analysis (IDA) and the FEMA P695 methodology. Six designs are considered, in which different base rocking joints are designed for one six-storey frame using different R values and post-tensioning arrangements. An additional six scenarios are considered, in which the same base rocking joints are used in combination with a second rocking joint above the second storey. A suite of 44 ground motions is selected and scaled until collapse occurs in at least 50% of the cases, and collapse fragility functions are generated using the truncated IDA curves. In all twelve cases, the probability of collapse is less than 10% during a 2% in 50 year event. However, the results show that R and the posttensioning arrangement have a large influence on the collapse risk, while adding rocking joints above the base has a less significant influence.

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