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Displacement-based design of concrete tilt-up walls Guan, Zhao

Abstract

Concrete tilt-up panels are commonly used in constructing lower-rising buildings in North America. Large window openings are sometimes required in the wall panels on the whole side of tilt-up buildings. Subject to severe earthquake, such a tilt-up panel is expected to experience inelastic deformation that has to be controlled. The relevant clause 21.7.1.2 is presented in the draft 2004 Canadian concrete code CSA Standard A23.3 regarding the displacement demand of concrete panel, which says: "Tilt-up Wall Panels shall be designed to the requirements of Clause 23 except that the requirements of Clause 21.7.2 shall apply to wall panels with openings when the maximum rotational demand on any part of the panel exceeds 0.02 radians." The aim of this research is to develop a simple method to estimate the inelastic displacement demand of tilt-up wall panels with openings accounting for the influence of a flexible metal roof. The tilt-up panel with openings is modeled as a static frame simply supported at the ground and the roof is modeled as a simply-supported beam at the tilt-up walls. The first mode of the wall is assumed to be dominant over all other modes, and the wall panel can be idealized as a single degree of freedom system with equivalent stiffness and concentrated mass. And also, the first mode of the roof is assumed to be dominant over all other modes, so that the roof can be idealized as a single degree of freedom system with equivalent stiffness and, concentrated mass. Hence, the lateral force resisting system in a typical tiltup building is modeled as an idealized 2 degree of freedom system. The properties of typical tilt-up panels and metal roofs commonly used in the lower mainland of British Columbia are investigated, and the possible range of stiffness ratios and mass ratios between wall panel and roof are presented. The range of the stiffness ratios and the mass ratios is used in this research. Program CANNY is used to perform simulations on computer with the idealized 2-DOF system. Total five earthquake records are used as excitations to the testing system. Three of them are modified to fit Vancouver acceleration spectrum of NBCC 2005, and two of them come from City of Los Angeles and Seattle. A l l of them are modified so that the earthquakes used in this research have a 2% probability of exceedance in 50 years. The deformation of metal deck diaphragm developed by physical tests is studied. The severe pinching of the testing deck after it yields is realized to be critical over all other nonlinear dynamic responses. Heavy Pinching creates a small area enclosed by each force-displacement cycle of deck, which indicates very limited energy dissipation by deck yield. So, pinching of metal deck is the most important factor considered in the selection of hysteresis model for the nonlinear metal roof. The simulation starts with linear elastic 2-DOF system. The yield strength of wall or roof used in the simulation of nonlinear 2-DOF system is determined by dividing the maximum force of wall or roof in the corresponding linear system by the force reduction factor (2 for concrete tilt-up panel). The simulations include 2 stages. During the first stage of test, the various factors that may affect the dynamic response of system, particularly the total elastoplastic displacement demand of tilt-up wall, are investigated. The parameters that were investigated include the force reduction factor of wall, the ductility of roof, the stiffness ratio of wall to roof, the mass ratio of roof to wall, the wall stiffness, the earthquake record. The equal displacement principal is verified for the current 2-DOF system by running simulations. After first stage of simulations, an important discovery is presented, which is the roof drift or the roof force is reduced proportional to the force reduction of tilt-up wall panels due to the yield of wall panels. A simple formula is developed to estimate the maximum inelastic wall displacement demand according to the above discovery and the equal displacement principal. In the second stage of simulations, the formula is confirmed using the inputs derived from practical wall and roof. The linear solution of the corresponding 2-DOF system is presented using spectral acceleration of Code NBCC and mode extraction. The total elastic roof displacement and roof drift relative to wall with the corresponding linear system are needed to estimate the inelastic wall displacement demand in the nonlinear system. The thesis ends with summaries and conclusions of the research and suggestion for further work.

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