Seismic Evaluation and Retrofit of Concentrically Braced Frames

Steel concentrically braced frames (CBFs) are a popular method of resisting lateral loads. Current AISC seismic design requirements for special concentrically braced frames (SCBFs) prescribe geometric limits to promote ductile yielding and buckling of the brace and use capacity design to size the adjacent non-yielding components. Newer design methods, in particular the balanced design procedure (BDP), adapt the AISC method to increase the ductility of the SCBF system by adding sequentially yielding mechanisms. However, older CBFs (NCBFs) may not meet the geometric, strength, or detailing requirements of SCBFs. The resulting seismic deficiencies can lead to substandard performance, which is concerning because many of these NCBFs are still in use today.;The objective of this study is to determine new retrofit strategies and methodologies to improve the seismic performance of NCBFs. The research includes four tests designed to investigate the performance of retrofitted NCBFs. These results were combined with prior tests conducted at the UW which simulated existing and retrofitted NCBFs. Using all of the data, new evaluation and retrofit methodologies were investigated based on the BDP. The evaluation of NCBFs seeks to: (1) identify seismic deficiencies withing the frame and (2) establish a hierarchy of yielding and failure that relates the deficiencies to the performance of the frame. The retrofit methodology aims to size the new components to promote secondary yield mechanisms (in addition to brace yielding) thereby maximizing the frame drift. Examples of applying the retrofit method to a suite of seismically deficient CBFs are provided.