Progressive-failure analysis of steel building structures under abnormal loads

Engineered structures are designed to resist all expected loadings without failure. However, structural failures do occasionally occur due to inadequate design and construction, especially for extreme and abnormal loads. This thesis concerns the progressive collapse of structures due to abnormal loading events, and develops a method of advanced analysis for predicting the progressive collapse behaviour of building structures in the plastic limit state.; A new procedure for progressive-failure analysis is developed that computes structural responses accounting for geometric/material nonlinearities and axial/flexural/shear deformations. The nonlinear behaviour of materials is discussed in detail. Combined-stress failure states and stiffness degradation models are proposed to simulate plastic deformation of structural members. Elliptic force-deformation relationships are employed to model the nonlinear material behaviour of members, and corresponding model parameters are determined from published experimental data.; Having the proposed nonlinear model, a generic member stiffness matrix is derived taking into account elastic-plastic bending, shearing and axial deformations. A modified moment distribution method is employed to obtain the stiffness coefficients. A computer-based incremental-load nonlinear analysis procedure is developed that progressively updates member stiffness using reduction factors that simulate degraded stiffness behaviour. Studies are conducted to demonstrate the effectiveness of the proposed method in predicting the progressive failure of structures under abnormal loading.; A general model of a compound element is proposed to consider the influence of semirigid connections on the progressive failure of steel frameworks. The stiffness degradation of semirigid connections is modeled by a moment-rotation relationship with four parameters. The stiffness degradation of a compound element resulting from the combined influence of member plasticity and nonlinear connection behaviour is modeled by a moment-rotation relationship with three parameters. The results from the proposed analysis method involving semirigid connection behaviour are compared with the results of other methods.; The proposed progressive-failure analysis method is threat-independent, in the sense that it is initially assumed that some type of short-duration abnormal loading has caused local damage represented by the removal of one (or more) critical member(s). The degree of damage to connections due to member-end disengagement is accounted for by a so-called health index factor. Three types of localized damage modes are investigated to identify different damage scenarios for the structure. Account can be taken of the connection damage that occurs when members disengage from the structure. Account is taken of any debris loading that occurs when disengaged structural components fall onto lower parts of the structure. The associated dynamic effect is taken into account for the quasi-static analysis by utilizing an impact amplification factor. Any progressive collapse occurring thereafter involves a series of failure events associated with topological changes.; The progressive-failure analysis procedure is based on the alternate-load-path method suggested in the design and analysis guidelines of the General Services of Administration (GSA, 2003) and the Department of Defense (DoD, 2005). The residual load carrying capacity of the damaged framework is analyzed by incrementally applying prevailing long-term loads and impact debris loads. The deterioration of structural strength is progressively traced to the state at which either global stability is reached or progressive collapse to ground level occurs for part or all of the structure. The analysis procedure is extensively illustrated for several planar steel moment frames, including rigid and semirigid frames designed with and without consideration of seismic loading. The results obtained demonstrate...