Soft Story Retrofits for the Real World: Cantilevered Column Modifications for Increased Ductility and Redundancy

摘要

Existing "soft, weak, or open front" (SWOF) buildings present tactical challenges for engineers. Utilities and other obstructions often make moment-resisting frames difficult to install. The most viable retrofit system is often a cantilevered column and appropriate new foundation. Codes require designing cantilevered columns with very low R value (currently R=1.25, compared to R=3.5 for Ordinary Moment-Resisting Frames). The resulting larger base shear affects all components of the lateral force resisting system in the direction under consideration, leading to significant cost increases for those components. This paper describes cantilevered column systems using standard steel shapes with web cut-outs that modify local structural properties. These simple modifications would force yielding to occur away from the column base, and are expected to increase system ductility. The modified members could be easily replaced after yielding, thus acting as structural "fuses." Furthermore, the system provides redundancy and opportunities for easy connection of viscous dampers. It is hoped that the enhanced ductility and redundancy will justify a substantial increase in allowable R factor, with commensurate decreases in foundation and other component sizes and costs. The authors have designed cantilevered columns for about two dozen SWOF retrofits. In most of these projects a frame would have been impractical-if not impossible-to install. With increased R factor and associated lower base shear proposed in this paper, cantilevered columns could prove even more advantageous over custom or pre-manufactured moment-resisting frames even where frame installation is not complicated by existing obstructions.