R = 100? Toward codification of controlled rocking steel braced frames

In the move toward performance-based seismic design, controlled rocking steel braced frames show great potential because of their ability to avoid structural damage during large earthquakes. This paper describes performance objectives for these systems that are consistent with current codes, and it examines what hysteretic properties are needed to achieve these objectives by discussing the behavior of single-degree-of-freedom (SDOF) systems. Based on a series of 10, 164 SDOF analyses, it is shown that low-rise controlled rocking steel braced frames could be designed with slightly larger force reduction factors (R) than are currently codified for ductile concentrically braced frames, while taller frames could be designed only to avoid uplift under code-specified wind loads. However, this requires the frame members to be capacity designed with due consideration of higher mode effects, which cannot be properly assessed by applying an overstrength factor to the design lateral force distribution. These conclusions are validated with case studies of a two-story and a 12-story controlled rocking steel braced frame.