Design of Fixed-Base Hollow Structural Section Columns Subjected to Large Seismic Drift

During an earthquake, gravity columns in a one-story building must support vertical gravity loads while undergoing large lateral drifts associated with deflections of the vertical seismic force resisting system and deflections of the roof diaphragm. Many of these columns are tube shapes detailed with bases that are cast into the concrete slab or have some amount of fixity, thus creating moments as the top of the column is subjected to lateral drift. While provisions in current building codes require the consideration of lateral drift in conjunction with gravity loads, there is little guidance about how to do this in design. In this paper, three methods for seismic design of gravity columns in one story buildings are discussed: 1) detailing for a pinned base, 2) creating a stable plastic hinge at the base of the column, and 3) designing the column to remain elastic. Slenderness limits for square tube columns are proposed to facilitate the formation of plastic hinges (i.e., option 2). A design procedure is proposed for keeping the tube columns elastic (i.e., option 3) using the interaction equation for axial compression and flexure. It is found that the elastic design procedure is characterized by a simple ratio. An example is presented, demonstrating all three options.

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