A Steel Version of the Distributed Knee Brace (Dkb) System Retrofit Option for Soft Story Timber Buildings

Abstract eng:
Recent research efforts focusing on timber structures with soft story deficiencies by NEES (Network for Earthquake Engineering Simulation) funded by the National Science Foundation resulted in experimental verification of a number of design methodologies and retrofit solutions. Objectives of the research included evaluation of the underlying assumptions of the guidelines for seismic retrofit of multi-story timber buildings with weak or soft story deficiencies (FEMA P-807) and validation of current and prospective retrofit design options. As part of this research, a particular retrofit strategy titled the Distributed Knee Brace (DKB) system was proposed. The DKB system is categorized as an array of knee braces placed parallel to the soft story line of resistance that uses existing stud walls and floor joists reinforced with additional studs and strategically arranged knee braces. In a single line, the combination of two knee braces, a joist, and wall studs on each side creates lateral resistance out of existing gravity resisting elements. This retrofit strategy, through testing, demonstrated an ability to accommodate 6% drift while providing a highly redundant energy dissipation mechanism while capacity protecting upper stories. In the pilot research, the ductility of the system was derived from slipping/yielding of the nailed connections between the timber knee brace to floor joist and timber knee brace to wall stud connection. The current research is an independent continuation of the work on the DKB system. However, this research focuses on the behavior of the DKB system when nail yielding is replaced with a brace comprised of a combination of elastic compression bar buckling and small diameter steel rod yielding as the energy absorption/ductile mechanism. Replacing the timber nailed connections with the proposed steel rod alternative results in a more predictable solution that allows more flexibility in retrofit optimization while offering a more slender and a spatially less intrusive physical profile. To validate the steel brace version of the DKB system and confirm its viability, full scale, cyclic, pseudo-static physical testing was conducted. The tests confirmed that this system (similar to the previously tested DKB system) was able to support nearly full lateral load capacities beyond drifts of 6%. Furthermore, the steel version of the DKB system improved utilization of the existing timber walls resulting in a more optimum system performance.

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