Axial shortening of steel columns in buildings subjected to earthquakes
Steel members subject to axial compression and inelastic cyclic displacements, such as may occur during earthquake excitation, exhibit axial shortening due to material inelastic deformation irrespective of the occurrence of buckling. This column axial shortening can cause undesirable effects in the building, especially if it occurs to a different extent in different columns of a seismic-resisting system. This paper summarizes experimental and finite element studies to quantify the axial shortening of columns with known axial forces pushed to inelastic cyclic displacements. A flexural hinge model for a frame analysis program is developed and calibrated against that from experimental and analytical studies. Then, to quantify the effect of axial shortening on realistic moment and eccentrically-braced frames during earthquakes, inelastic dynamic time history analyses were conducted. While axial shortening of more than 7% of the column length was obtained during experimental testing, the axial shortening was always less than 1% of the column interstorey height in the steel frames studied. A method to estimate the axial shortening as a function of the expected inelasticity is developed. Finally, several new details are described in order to prevent detrimental effects due to axial shortening.
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Copyright (c) 2009 Gregory A. MacRae, Christopher R. Urmson, Warren R. Walpole, Peter Moss, Karissa Hyde, Charles Clifton
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