Performance evaluation of innovative steel braced frames

Conventional concentrically braced steel frames are not capable of redistributing the large unbalanced vertical forces caused by brace buckling. This presents many design challenges to engineers. To retain the advantages of providing efficient stiffness and strength to limit inter-story drifts, structural engineers are developing new concentrically braced steel frame configurations. This dissertation focuses on seismic performance evaluation of the suspended zipper braced frame configuration.; The suspended zipper braced frame configuration is similar to the inverted-V braced frame, except that a vertical structural element, the zipper column, is added at the beam mid-span points from the second to the top story of the frame. In the event of severe earthquake shaking, the lower-story braces will buckle and create unbalanced vertical forces at mid span of the beams. The zipper columns will mobilize the beams and the braces above the story where buckling occurs to resists the unbalanced vertical forces. Such action will force the entire system to be engaged to resist the earthquake loads, hence preventing concentration of inelastic action in one story.; Seismic performance evaluation of the suspended zipper braced frame is conducted in two phases. Hybrid and analytical models of the suspended zipper braced frame are developed and validated in the first phase. A probabilistic seismic performance evaluation method is developed and used in the second phase to evaluate the seismic risk of the suspended zipper braced frame.; The hysteresis response of the inverted V-braced sub-assembly is first examined using a quasi-static cyclic test. The results of the quasi-static test are used to calibrate an analytical model capable of modeling the hysteresis behavior of a steel hollow structural section (HSS) that buckles out of plane. With the calibrated analytical model, the response of the suspended zipper braced frame under static and dynamic loading is studied analytically using the Open System for Earthquake Engineering Simulation (OpenSees) framework. The results of these system-level analytical simulations demonstrate the intended force redistribution in the system is indeed occurring. Hybrid simulation tests are conducted to validate the analytical model of the suspended zipper braced frame. These hybrid simulation tests, for the first time, combine the nonlinear analytical elements with physical elements into a hybrid model using OpenSees. The excellent agreement between the analytical and hybrid simulation results shows that the developed models can be used to evaluate the seismic performance of complex structural systems, such as the suspended zipper braced frame.; Probabilistic evaluation of seismic performance of the suspended zipper braced frame is conducted using the validated analytical and hybrid models with a newly developed performance methodology based on the Pacific Earthquake Engineering Research Center (PEER) probabilistic seismic performance-based evaluation framework. This method efficiently combines the seismic hazard, model response, structural and nonstructural damage and repair cost uncertainties to evaluate the expected seismic risk of structures under scenario earthquakes. The method for seismic performance evaluation is computerized and a comparison between different bracing systems is analyzed on a test bed building. The results of such probability-based performance evaluation provide the information needed to demonstrate the advantage of using the suspended zipper braced frame.