Retrofits for Steel Joist Roofs Exposed to Blast Loads

Abstract

Retrofit methods are being developed to make conventional structures more resistant to the effects of explosive attacks by terrorists. Past efforts have concentrated on retrofitting columns and exterior walls. However, the structural roof system must also be evaluated and possibly upgraded to be consistent with the protection level provided by the retrofitted walls and columns. An experimental/analytical program has been performed to determine the response of a lightweight steel roof system to blast loads. The experimental results have validated the analysis methodology. A method of designing retrofits has been developed and evaluated against experimental data; this method will be extended to other roof systems. Background and Overview Efforts are being undertaken to improve the safety of occupants of buildings when those buildings are located near an explosive attack by terrorists. The roofs of structures can fail and become a hazard to building occupants. This is particularly true for buildings with a small number of stories, where the roof of the structure is close to the ground and the pressure levels from the detonation would be high enough to cause damage. The U.S. Department of Defense, Office of Special Technology, Technical Support Working Group (TSWG), and the U. S. Army Corps of Engineers (USACE) have sponsored a series of experiments to study the blast response of a particular type of roof system. The Defense Threat Reduction Agency (DTRA) was the technical manager for the TSWG portion of the study. A broad range of roof system designs are common in conventional construction. Any of these systems could have served as the object of this study. The system selected is frequently used for on-base military construction and consists of standing seam metal decking placed on purlins that span between open web steel joists. Experiments were conducted to evaluate the blast resistance of the original configuration of the roof system. Pretest analyses assisted in designing the experiments, while posttest analyses supported validation of the analytical methodology. A methodology for designing retrofits to improve the resistance of the roof system to blast loads was developed and used to provide a retrofit for this roof system. That retrofitted roof system was tested at a higher pressure level than was used in the previous experiments. Experimental Program