Keywords: progressive collapse, static analysis, dynamic analysis, material nonlinearity, geometric nonlinearity.

Currently, there is public interest on the issue of progressive collapse of building structures. This paper explores important aspects of progressive collapse investigations, of interest to structural engineers, that are being studied by investigators or presented in certain building codes. These issues include: 1) analysis methods, 2) structure-load modelling issues, 3) level of protection provided to a structure, 4) modelling of joints, and 5) consideration of the effects of high strain rates on mechanical properties. Nonlinear dynamic analysis provides the most reliable simulation of progressive collapse, but it can be computationally demanding for large structural systems. On the other hand, linear elastic analysis may not be suitable for all structures. Furthermore, the desired level of protection and building type may not warrant the computational cost. The suggested levels of protection against progressive collapse range from ensuring general structural integrity to hardening of the structure to prevent collapse.

The paper discusses progressive collapse resistant design guidelines available in two of the design codes in the United States, namely, the Department of Defense (DoD) and General Services Administration (GSA) design guides. The design of philosophy in the DoD guidelines is based on the desired level of protection and requires categorization of the structure in one of four levels of protection. GSA design guide shares many similar provisions with the DoD guidelines but appears to emphasize linear static analysis for assessment of progressive collapse potential. Other than the DoD and GSA guidelines, building codes in the U.S. provide limited guidance for mitigation of progressive collapse.

This paper also discusses structural analysis methods, load combinations, and the assumptions involved in each of the suggested analysis method. Understanding the assumptions assists in identifying the limitations of each method and whether it is appropriate for the desired level of protection. The concept of notional column removal associated with the Alternate Path (AP) method is then discussed and recommended column removal methodologies is presented. The ultimate goal of progressive investigation with AP method is to assess the potential damage associated with the loss of certain load carrying members. The permissible damage limits in DoD and GSA guidelines are compared in this paper. An important differences between the two design guides in that the permissible area damage due to loss of primary structural components in the DoD guide is less than 50% of the permissible area damage in the GSA design guide.

Important modelling choices involved in nonlinear dynamic analysis for progressive collapse investigations are discussed. Some of the common choices for finite elements, material models, and integration schemes for reinforced concrete and steel structures are presented. The strength and deformation failure criteria used in some of the models in this paper are based on the DoD guidelines.

General observations on structural behaviour for systems modelled according to the guidelines presented in this paper are discussed. The most notable observations include: 1) importance of nonlinear dynamic analysis for modelling of inertia effects as well as material and geometric nonlinearities, 2) differences between response diagrams for deep and shallow reinforced concrete beams in progressive collapse simulations, and 3) differences in response between selected steel connections in progressive collapse simulations.

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