NEESR-SG International Hybrid Simulation of Tomorrow's Braced Frame Systems

Braced frames are commonly used as seismic resisting systems. A collaborative research program at the University of Washington (UW), University of California Berkeley (UCB), University of Minnesota (UM), and the National Earthquake Engineering Research Center (NCREE) in Taipei, Taiwan has investigated design methods to improve their seismic capacity and reduce the damage. The tests results from over 40 full-scale experiments have formed the basis of a new balanced-design method that balances coveted yielding mechanisms throughout the system with undesirable failure modes to increase system ductility. The culmination of this research effort was a pair of first-of-its-kind 3D experiments at the Multi-Axial Subassemblage Testing (MAST) laboratory at the University of Minnesota. The tests were large-scale two-story, one-bay by one-bay, steel, concentrically braced frame system and represented the latest advances in braced frame design and detailing (Figure 1). Both conventional (buckling) braces and advanced, buckling-restrained systems were tested. The tests investigated the latest design advances but also assessed the effects of out-of-plane deformation on the behavior of braced frames; which to date has been an uncertain aspect of their performance. Additional multi-story braced frame tests were performed at the NCREE Laboratory in Taiwan and the UC Berkeley NEES Laboratory to evaluate different brace configurations, different design strategies, and improvements to the system and the design process. In their totality, the results represent an important, unique and critical contribution toward improving the understanding of these multi-story systems.

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Figure 1 Full-Scale Three-Dimensional Braced Frame Test Specimens Ready for Testing in MAST Laboratory: (a) SCBF specimen; (b) BRBF specimen.

The experimental results have also advanced nonlinear modeling techniques for braced frames, since the experiments have been used to calibrate and validate the computer models. Of critical importance is the capability to capture inelastic deformations of the brace, including large out-of-plane displacements, which result from brace buckling as shown in Fig. 2a. These deformations place significant secondary inelastic deformation demands on beams, columns, and connections, which significantly affect the seismic performance as shown in Fig 2b and c. Prior to this research, computer models commonly used in practice seldom captured the full range of braced frame behavior. However, both continuum modeling approaches and macro-element models, such as line elements or concentrated springs, have

been investigated. Analytical models verified and developed to improve the ability to predict the seismic performance of the system. In particular, the macro-element models provide a significant yet simple advancement for a wide range of practical applications. Force-based fiber-type beam-column elements are used to model the braces, beams and columns. A primary contribution is the gusset plate model. Typical nonlinear and linear braced frame models assume perfectly pinned end for the brace; the test results show that the gusset provides significant rotational restraint but is not rigid and this behavior must be simulated to accurately predict the response of the frame. A simple yet effective connection model was developed. The proposed model provides accurate simulation of global behavior, while retaining simplicity and providing reasonable predictions for many local behaviors. For collapse assessment, a model was implemented in the software program to simulate brace fracture. Dynamic analyses have been conducted using the comprehensive modeling approach to assess the vulnerability of low-rise and mid-rise brace frames. The results indicate that the seismic reduction factor depends on the building size, and not just the type of lateral resisting system as is currently implemented in the code.

a) Typical Multi-Story Frame b) Brace Buckling Deformation

c) Local Yielding of Gusset d) Local Yielding of Column and Cracking of Gusset Plate

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Figure 2 Typical Braced Frame Behaviors