||Moderate-Ductility Dual Systems and Reserve Capacity|
||2010 - 2012|
Overview and Objectives: Empirical evidence indicates that steel braced frames possess appreciable reserve capacity – in the form of gravity framing and gusset plate connections. These partially restrained connection elements form a “reserve” moment frame system that can prevent sidesway collapse even when the primary lateral force resisting system (LFRS) is significantly damaged due to brace fracture. When required, reserve capacity can be enhanced without significant expense. As summarized by Hines et al. (2009), collapse performance of CBF systems that possess limited ductility appears to be impacted less by a system’s strength than by its reserve capacity. It is therefore proposed to reconsider the design of braced frames in low and moderate seismic regions as moderate-ductility dual systems.
Such dual system behavior can be viewed from two different perspectives:
- A stiff primary braced frame with a moment frame reserve system to prevent collapse in the event of brace failure.
- A flexible moment frame stiffened by a sacrificial braced frame designed to withstand wind loads and to provide service-level drift control.
This system is intended to offer an alternative to designers who wish to address reserve capacity directly and reduce design forces on the primary lateral system.
Work Description: Development of prototype designs for dual systems in 3-, 6-, and 9-story configurations similar to the configurations developed by Hines et al. (2009). The first set of configurations will reflect in a literal sense the R = 5 design criteria proposed above. A second set of configurations will attempt to provide improvements to this design criteria, such as providing ductile eccentric braced frame details for the first story only, for the sake of comparing the standard and improved performance.
Deliverables: Design criteria for a dual system frame designed to resist moderate seismic loads. Development of cost-effective reserve system details.
Benefit: New design approaches and system configurations may provide designers with opportunities both to ensure better seismic performance and to reduce cost. The philosophy behind such systems should enable designers to judge where added cost will most benefit system performance.
Milestones: A series of one-story models have been completed to establish benchmarks for comparison of modeling software packages and have been compared and calibrated with experimental results of a moment frame tested to collapse at State University of New York in Buffalo (Lignos 2008).
(image courtesy of Dr. Hines)