6.3.1 Progressive Collapse

ASCE-7 defines three ways to approach the structural design of buildings to mitigate damage due to progressive collapse. Each is described below with an emphasis on how the method is applied in the situation where explosive loads are the initiating cause of collapse.

1. Indirect Method: Consider incorporating general structural integrity measures throughout the process of structural system selection, layout of walls and columns, member proportioning, and detailing of connections to enhance overall structural robustness. In lieu of calculations demonstrating the effects of explosions on buildings, one may use an implicit design approach that incorporates measures to increase the overall robustness of the structure. These measures are discussed in the sub-sections below on structural systems, structural layout, and structural elements. This minimum standard is likely to be the primary method used for design of the type of buildings that are the focus of this primer.
2. Alternate-Load-Path Method: Localize response by designing the structure to carry loads by means of an alternate load path in the event of the loss of a primary load-bearing component. The alternate-loadpath method has been selected by agencies including the General Services Administration (GSA) as the preferred approach for preventing progressive collapse. This method provides a formal check of the capability of the structure to resist collapse following the removal of specific elements, such as a building column at the building perimeter. The method does not require characterization of the explosive threat. The structural engineer can usually perform the necessary analyses, with or without guidance from a protective design consultant. However, the analysis is likely to benefit from advice of the protective design consultant regarding element loss scenarios that should be considered in design.
3. Specific Local-Resistance Method: Explicitly design critical vertical loadbearing building components to resist the design-level explosive forces. Explosive loads for a defined threat may be explicitly considered in design by using nonlinear dynamic analysis methods. These are discussed below in the subsection on direct design methods with additional information in the subsection on structural elements. Blast-mitigating structural design or hardening generally focuses on the structural members on the lower floor levels that are closest to defined stationary exterior vehicle weapon threats.

Useful references are provided at the end of this section that directly relate to progressive collapse prevention.