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Abstract: Numerous terrorist events during the last decade, including the 2001 attack on the World Trade Center, have heightened concern about the safety of bridges during intentional/unintentional blast load effects. Analysis of highway bridges under blast loads requires accurate generation and application of blast loads and good understanding of the behavior of components of a bridge during high strain rate loading encountered during blast loads. In this paper, a new approach for the application of blast loads on bridge components has been presented. This approach can apply realistic loads and can simulate both reflection and diffraction of blast loads. Using this approach, verification of simulation of blast loads in LS-DYNA has been carried out by using available blast tests on two types of beams. A high fidelity model of a typical three-span highway bridge has been developed for investigation of blast load effects on a three-span reinforced concrete bridge. It is observed that the range of demands imposed on bridge components during blast loads may be significantly higher than those during other extreme hazards, e.g., seismic, for which bridge components may have been designed. Detailed results on various failure modes of this bridge and seismic blast correlations using this model of the bridge are presented in the companion paper.21609
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Author keywords: Multihazard; Bridge; Blast; Failure mode; Seismic.
Introduction
The number and intensity of domestic and international terrorist activities, including the September 11, 2001, attack on the World Trade Center towers in New York, have heightened the concerns have been considered attractive targets because of their accessibility and potential impacts on human lives and economic activity. The Federal Highway Administration (FHWA) and
AASHTO jointly sponsored a Blue Ribbon Panel (BRP) consisting of bridge and tunnel experts from professional practice, academia, federal and state agencies, and toll authorities in 2003 to examine bridge and tunnel security and develop strategies for their protection from terrorist activities. They identified that substantial casualties, economic disruption, and other societal ramifications may result from isolated attacks on 1,000 out of 600,000 bridges in the country. The recent collapse of the I-35W Bridge over the Mississippi in Minneapolis on August 1, 2007, has clearly demonstrated the vulnerabilities in highway bridge infrastructures and their societal and
economic impacts nationwide. Hence, there is a need to develop guidelines for the design of bridges that are redundant and resilient with respect to unforeseen blast load events.
Bridges are very complex systems. Decision making on blast threats (charge type, size, and location), identification of bridge components affected by the direct blasts, and redundancies of existing bridges can be daunting, even for the simplest of bridges. The Blue Ribbon Panel placed first priority on deterrence, denial, and detection of blasts, second priority on defense with standoff, and third priority on structural modifications through design and detailing (FHWA 2003). Highway bridges are readily accessible to vehicles that can carry explosives. Continuous monitoring of even just critical bridges and inspection of vehicles approaching these bridges will require significant financial and other resources and may cause persistent traffic congestions. Barrier standoffs may be effective in reducing the destructive effects of blast loads on bridge piers. The BRP has recommended minimum barrier standoffs for different vehicular threat types in terms of explosive weight (kilogram of TNT). However, it may not be possible to provide adequate standoff to protect existing bridge piers on busy highways due to traffic requirements. In such cases, strengthening of bridge components becomes the only viable protective option.
Loads imposed on highway bridge components during a blast loading event can exceed the design capacity of those members. In some cases, the loads can be in the direction opposite to those of