Abstract
Base flexibility of structures changes and can increase the demands on structural elements during earthquake excitation. Such flexibility may come from the base connection, foundation, and soil under the foundation. This research evaluates the effects of column base rotational stiffness on the seismic demand of single storey frames with a range of periods using linear and nonlinear time history analysis. The base rotational stiffness ranges considered are based on previous studies considering foundation and baseplate flexibility. Linear and nonlinear spectral analyses show that increasing base flexibility generally increases frame lateral displacement and top moment of the column. Furthermore, moments at the top of the columns and the nonlinear base rotation may also increase with increasing base flexibility, especially for shorter period structures. Since many commonly used baseplate connections may be categorized as being semirigid, it is essential to design and model structures using realistic base rotational stiffness rather than simply use a fixed base assumption. The overall results also illustrate the range of increased seismic demand as a function of normalized rotational stiffness and structural period for consideration in design.
Highlights
The rotational stiffness at the base of a column affects the force and displacement demands on frame elements during an earthquake
Ruiz-Garcia and Kanvinde [3] showed that ideal pinned based connection leads to larger interstorey drift demands but smaller residual drift demands compared to the fixed base condition
Zareian and Kanvinde [4] found that increasing base flexibility results in the collapse mechanism with large deformations concentrated in a fewer storeys in 2, 4, 8, and 12-storey steel moment resisting frames
Summary
The rotational stiffness at the base of a column affects the force and displacement demands on frame elements during an earthquake. In real structures, there is foundation flexibility due to the soil, foundation, and base connection, all of which can change the rotational stiffness and violate this assumption leading to increased demands. Maan and Osman [1] modelled fiveand ten-storey buildings with different column base flexibility values. They showed that pinned to fully fixed column base cases bounded the responses for all frames. Aviram et al [2] showed that increasing base flexibility increased displacement demands and concentrated deformations in the first storey of a threestorey building. Zareian and Kanvinde [4] found that increasing base flexibility results in the collapse mechanism with large deformations concentrated in a fewer storeys in 2-, 4-, 8-, and 12-storey steel moment resisting frames
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