Non-invasive passive energy dissipation systems for the seismic design and retrofit of steel structures

A passive axial elasto-plastic device is introduced locally near beam-to-column joints of steel moment resisting frames to enhance the energy dissipating capacity during earthquakes. The geometry, stiffness and slip or yield load are chosen to maximize the energy dissipated during a monotonic push. A typical 6-story moment resisting frame is first retrofitted with haunch type connections, with no slipping allowed. The building is then retrofitted with the same geometric configuration of the haunch but with an increased stiffness. Finally a third retrofit strategy consisting of replacing the haunch with energy dissipating devices is implemented. Nonlinear time-history analyses where the fracture of welds is modeled with a strength degrading element are performed under different intensities of seismic ground motions. Results indicate that an increased haunch stiffness is more effective in protecting the weld fractures under large seismic loading. The energy dissipating haunch reduced significantly the response of the structure while still protecting the welded connections.