Thermo-elasto-plastic behavior of biaxially loaded steel beam-columns inducing those from World Trade Center towers

An experimental and theoretical investigation into the behavior of biaxially loaded steel beam-columns is conducted including the effects of high temperature. Systems of materially nonlinear differential equations of beam-column equilibrium are first formulated for both ambient and high temperature conditions. An iterative finite integral procedure is formulated and programmed to solve the governing differential equations. To check the validity of the theoretically predicted behavior and strength of the beam-columns, a series of forty-two laboratory tests are conducted at both ambient and high temperature up to 950°F. Upon achieving a good agreement between the predicted beam-column behavior and that observed in the experiments, the theory developed is then applied to predicting the behavior and strength of typical beam-columns which were used in the 110-story World Trade Center (WTC) buildings. Thermo-elasto-plastic stiffness degradation and load-moment interaction curves are generated for typical WTC beam-columns which existed in the impacted area during 9/11 attacks.