A Model for Progressive Collapse of Conventional Framed Buildings

Abstract

Increased importance has been placed on the design and analysis of structures to account for potential terrorist attacks. Designers must account for many options that include various levels of mitigation and protection measures to ensure the safety of the building and its occupants. One critical ingredient to the analysis is an understanding of the structural performance of structures subjected to blast loading especially when considering issues of collapse. The approach presented in this paper provides an efficient alternative to dynamic analysis of a moment-framed building subject to blast induced damage. First, framing members (girders and columns) are designed to withstand typical dead and live loads. Factored design loads are calculated using a first order (load path) static analysis of the frame. Structural steel, reinforced concrete, reinforced/unreinforced masonry, and wood members are designed in accordance to the applicable building code requirements and design specifications. After structural damage due to blast loads is predicted, progressive collapse of the building is modeled as a series of events. Events include local plastic hinge formation, member failure from overstress, local buckling, lateral torsion buckling, or axial buckling, and global buckling or instability. Element section yield and failure loads are evaluated for axial, strong and weak bending and torsion using the applicable codes (striped of load and resistance factors). Stiffness at hinges is assumed to be bilinear. Global buckling is evaluated by a generalized Eigenvalue approach and instability by simple Eigenvalues. The approach uses a course finite element model of the building that includes beams, columns, walls and floor slabs. The approach can determine if progressive collapse will propagate to the entire building or if a stable condition can be achieved. An optional model that includes the debris loading of removed structural elements is also included.