Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. Our job is to make sure we follow the plans and building codes, execute our framing skill with precision, to provide a level of quality 2nd to none.
As early as 1970, the structural engineering and building safety community recognized that a large number of two-, three- and four-story wood frame buildings designed with the first floor used either for parking or commercial space were built with readily identifiable structural system deficiencies, referred to as a “soft story”. Thus, many multi-story woodframe buildings are susceptible to collapse at the first story during earthquakes. The majority of these older multi-story wood frame buildings have large openings and few partition walls at the ground level. This open space condition results in the earthquake resistance of the first story being significantly lower than the upper stories.
As part of the five-university multi-industry, U.S. National Science Foundation – funded NEES-Soft project, a performance-based retrofit method has been developed for these types of buildings. This paper presents the first generation of this method and resulting retrofit design using an engineered wood technology that is just being introduced in the United States. Cross laminated timber (CLT) panels are used to strengthen and stiffen the soft stories of the building in order to achieve the performance level desired by the stakeholders under a specified seismic intensity. The performance-based seismic retrofit (PBSR) method is summarized and focus in the paper is placed on the retrofit achieving a target drift 50% of the time under a prescribed seismic intensity. The numerical model is developed based on full-scale experiments, which will also be presented, and performance is validated using a state-of-the-art nonlinear time history analysis model.
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