UBC Theses and Dissertations
Nonlinear analysis of irregularities/discontinuities in high-rise concrete shear wall buildings Mahmoodi, Maryam
State-of-the-art nonlinear analysis was used to investigate three different types of irregularities or discontinuities in high-rise concrete shear wall buildings. The objective was to develop knowledge that will assist practicing engineers who design buildings. Overhanging wall discontinuity due to the wall above being longer than the wall below creates significant amplification of concrete compression strains immediately below the overhang. While the strains are highly nonlinear, the results of the current study were used to develop simple amplification factors for estimating the nonlinear strain increases from the results of linear finite element analysis, which can be done by practicing engineers. A simple safe limit for the maximum compression strain in a wall below an overhang determined from linear analysis is 0.001 in order to limit the nonlinear vertical compression strain in the zone below the overhang to 0.004. A discontinuity in lateral stiffness of building occurs at grade level where the concrete diaphragms connect tower walls to foundation walls or at the top of podium levels. In design practice, these diaphragms are usually modelled as linear elastic members, and the choice of effective stiffness significantly influences how much force will go into the backstay force path. The effect of membrane forces on the flexibility of concrete diaphragms was investigated and a range of simplified models was presented. The nonlinear models provide a more accurate estimate of the diaphragm stiffness, while the simple upper and lower-bound (constant) stiffness models are much easier to use in practice. The influence of out-of-plane bending of the diaphragms on reducing membrane stiffness of the diaphragms was also investigated. Sloped-column Irregularity is a new type of irregularity defined in the 2020 National Building Code of Canada for the seismic design of buildings as an outcome of the current study. Nonlinear time history analysis was used to investigate how the differential horizontal movement at the top and bottom of sloped columns causes vertical accelerations of the building mass. A simplified procedure was developed to account for the possible range of member stiffnesses and to account for vertical ground motions in a simplified way.
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