P-Delta effects and seismic design of steel structures

The objective of this research is to evaluate the influence of P-Delta effects on the lateral response of steel frame structures. P-Delta effects are defined as the amplification of shears and moments in structural members caused by vertical loads acting on a laterally displaced building frame. When columns are pushed into the inelastic range P-Delta effects may cause significant amplification in drift compared to elastic P-Delta analysis. Negative stiffness due to P-Delta effects and insufficient strength will amplify lateral deflections and may lead to incremental collapse during severe earthquakes.; A second-order distributed plasticity element based on the fiber model is used in the finite element formulation. Residual stresses, distributed yielding along the length and within the cross section, and geometric imperfections are explicitly modeled.; The behavior of columns subjected to only axial load is investigated for various slenderness ratios and geometric imperfections. The results of the column study indicate that the AISC-LRFD column curves are unconservative for weak-axis buckling. A parameter study is performed on beam-columns for developing interaction curves. A nonlinear form of interaction equation is proposed.; A parametric study is conducted on portal frames for quantifying the influence of P-Delta effects on the lateral response. The portal frame is used as a surrogate for multi-story single bay frames. The predictions for the elastic stiffness and mechanism slope match very well with finite element results. The study on overturning moment shows that columns on the compression side buckle in a side-sway inhibited mode with a K-factor less than one. When vertical loads are added, the K-factor may exceed unity and P-Delta effects due to the vertical loads will lead in all cases to a negative mechanism stiffness. If gravity moments are present in the columns, their effect is more important than that of the overturning moment, particularly for slender columns and high axial loads. They will cause a trilinear response since a plastic hinge in one column will form earlier than in the other. For slender columns with high gravity loads the intermittent stiffness may become negative, which could have a very detrimental effect on the dynamic response.