Publication | 8.4.2011

Steel Frame Structure Performance in Blast Environments

The current practice in blast resistant design of building structures for conventional blast loads is, in general, to design the building envelope (infill and curtain walls) to resist the design blast load and dissipate energy through inelastic response while keeping inhabitants safe from the blast overpressures and debris. These façade components are usually anchored to the building structure at the perimeter beams or floors and roof diaphragms. The dynamic reactions of the façade components are transferred to the building lateral load resisting frames by the floor diaphragms. Ultimately, the lateral load resisting frames must transfer the dynamic reactions from the curtain and infill walls through the connections to the foundation while satisfying performance requirements. For conventional design blast loads, standard practice usually is to assume that the dynamic reactions, transferred to the building structural frame by the façade components, will produce only a moderate lateral response of the building frame. This is due to the relatively large natural period of the structure (compared to the façade components) and inertial mass. However, for larger than conventional threats, specially reinforced façade systems are required which may result in transfer of large lateral forces into the building frame structure. This may result in large force and deformation demands for the lateral load resisting frames.

The study presented herein is part of the “Steel Frame Structure Performance in Blast Environments” research program being carried out by Protection Engineering Consultants for the U.S. Department of State (DOS). The objective of this study is to investigate the implications of the blast resistant design of façade systems on the lateral response of steel frame structures under a specified design blast load environment. One of the guiding principles of this study was to make use of conventional structural analysis tools and methodologies that line up with established consensus criteria so that they can be widely usable and sustainable. The lateral response of different types of midrise typical steel frame structures, including moment frames and braced frames, was investigated through nonlinear dynamic analysis using the SAP2000 structural analysis software. Plastic hinge definitions and analysis procedures followed FEMA and UFC guidelines for nonlinear dynamic analysis of structures. The performance of these building frames was evaluated and compared globally and locally through assessment of overall frame response, and strength and plastic deformation demands frame members and connections.