A tendon system to re-center steel moment frames with weak stories

A wide open bottom story of a frame building is often expected by owners for use as a garage or shops. However, this leads to weak stories due to abrupt changes in lateral stiffness and often results in unexpected story collapse as observed in many previous earthquakes. To retrofit frame buildings that have experienced weak story damage, a tendon system is proposed in this study, which consists of a set of swaying columns and tendons. The swaying columns are used to uniformly redistribute the lateral deformation along the height, while the tendons provide extra lateral stiffness and renders the entire structural system a re-centering capability. To avoid unnecessary forces to swaying columns, pin-connections are used at the bottom. Tendons are placed over the entire story to gain large elastic displacements. Parametric analysis reveals that the swaying column, with a stiffness of about 0.9 times that of the weak story, and the tendons attached at the roof, with a stiffness of 0.04 times that of the weak story, can provide the optimal performance with a maximum residual story drift angle of less than 0.5%. Online hybrid tests were carried out, which demonstrated that uniformly distributed story drifts and acceptable residual deformation could be achieved by the proposed tendon system.