Manufactured, Pre-Engineered Moment Resisting Frames Used in Soft-Story Building Retrofits of Light-Framed Construction

Several cities throughout California's high seismic regions have expressed concern over the large number of existing soft-story buildings and the risk they pose. Many city building departments are actively developing guidelines for retrofit of these structures because of their documented risk of collapse during a major seismic event. Officials have the difficult task of enacting mandatory retrofit requirements that will mitigate the risks, while still being flexible enough so building owners can economically implement the requirements. A common solution to address the potential risk of collapse at the first floor (soft-story level) of an existing building is to add a lateral-force resisting element in the direction(s) where the deficiency exists. The system can vary based on the level of desired performance the owner and designer have agreed upon or the code standard that is being used which will determine strength, stiffness and ductility requirements of the system. While this added lateral-resisting element could be a shearwall or a braced frame, steel moment resisting frames are a preferred retrofit strategy in light-framed, soft-story structures since they do not impact the open space at the first level. Two common types of open spaces that create a line of deficiency in the building, thus defining the first story as "soft" are tuck-under parking and retail space. These types of soft-story structures often have residential occupancies in the stories above. The moment frame solution allows the building owner to maintain the open space for which the use is intended while providing the necessary lateral-force resistance to mitigate the soft-story problem.