UC San Diego

During an earthquake, steel braced frame columns are potentially subjected to high axial forces combined with inelastic rotation demand resulting from lateral story drift. Little design guidance is available concerning the reliability of steel columns under this level of combined loading. To evaluate the performance of wide-flange columns under high axial load and drift demand, specimens have been subjected to laboratory and analytical investigation. Nine W14 wide-flange columns were tested at different levels of axial force demand (35% to 75% of column yield strength) combined with lateral story drift demand of up to 10%. Since a specified loading sequence does not exist in building codes, one was developed based on the results of nonlinear earthquake time-history analysis of 3-story and 7-story buckling-restrained braced frame models. The first step in the loading sequence consisted of imposing simulated gravity load. Then in- phase, increasing amplitude cyclic axial load and story drift were applied. Experimental results showed that flange local buckling was the dominant buckling mode. Specimens achieved interstory drift capacities of 0.07 rad. to 0.09 rad., in part, due to the delay in flange local buckling resulting from the stabilizing effect provided by the stocky column web of the W14 specimens. The finite element analysis program Abaqus was used to model the column specimens and to perform a parametric study investigating the effect of flange and web local buckling, lateral-torsional buckling, and axial load. Analysis and experimental results were observed to be well correlated. The behavior of deep columns (W18 and W24) with higher web slenderness than the tested W14 sections was also investigated. Models of deep column sections showed significant strength degradation due to the interaction of flange and web local buckling. Testing and finite element analysis indicated that the plastic rotation capacities currently predicted by Seismic Rehabilitation of Existing Buildings (ASCE 41) are very conservative for axial load ratios above 0.5. At axial load ratios below 0.5 the plastic rotation capacity of some finite element models was less than that predicted by ASCE 41. Using the database of finite element analysis results and regression analysis, nonlinear models were developed to more accurately predict rotation capacity of steel wide-flange columns. These models consider the interaction of flange and web local buckling, lateral- torsional buckling, and axial load