Sacha Roger Laffely
Steel concentrically braced frame (CBFs) are widely used as a laterally-resisting system in the case of seismic actions as they are generally easier and faster to erect, and in most cases, they cost less than moment-resisting frame system. However, special care must be made in the design of gusset plate connections as over the years, a great deal of steel structure has emphasized their importance through different types of failure. In this master project, the failure of a gusset plate connection from a steel concentrically braced frame is analyzed with finite element modelling. This CBFs was part of two-story parking garage structure where most of the braces oriented in the east-west direction had fractured at their gusset plate connections after the 2011 Tohoku Earthquake in Japan. With finite element analysis, the cause of the buckling of the gusset plate failure mechanism was explained. As a matter of fact, the concentration of stresses and inelastic deformation during cyclic loading, particularly when the brace was in compression, led to fracture in the gusset plate. The fracture initiation process was then predicted with the Cyclic Void Growth Model. Therefore, four retrofit schemes are proposed to rectify the poor behavior of the gusset plate. They are evaluated based on a static pushover analysis and the results have shown several issues that might arise depending on the modifications. It has been shown, that one of the retrofit scheme where the gusset plates are design using elliptical clearance theory provided the best results. The simulation showed that the brace could buckle in compression without having premature failure in the gusset plate as the brace end rotation capacity was high enough. This study shows the importance of a well thought connection design in a concentrically braced frame system and how can different retrofit concepts alter or improve their behavior. It is relevant to note that not every concept and ideas will provide the desired impact and consequences.
2018
