Performance of Cable Supported Bridge Decks Subjected to Blast Loads

Study of performance of major bridges subjected to blast loads is an important first step in developing technologies that can increase resistance of these important transportation links and prevent their catastrophic failure. In this study, the behavior of two types of typical bridge deck structures, steel orthotropic box girder and composite steel-concrete plate girder, is studied. The bridge types considered in these studies are long span cable-supported bridges, in particular, anchored suspension bridges, self-anchored suspension bridges and cable-stayed bridges. The studies were performed by subjecting the non-linear finite element models of the typical deck structures to simulated blast effects. The main variables in the study were: the type of the deck structure, the size of explosive device, the amount of axial forces in the deck, and mechanical properties of the steel and concrete material used in the deck structures. The high strain rate effects of the blast on the mechanical properties of the material were also considered. The blast load levels used in the study were representative of various levels of car bombs located on the decks. From the results of these analyses, the failure modes of the two common types of bridge deck structures were identified and strengthening measures were developed and recommended to enhance blast-resistance and prevent progressive collapse of these systems and bridge as a whole. The study indicated that the most important parameters for blast resistance and progressive collapse prevention of steel orthotropic box girder and composite steel-concrete plate girder are ductility of material and redundancy in the system rather than strength of the material. For orthotropic decks, steel with medium range strength and high ultimate strain performed much better than the deck with a very high strength steel but relatively smaller ultimate strain. For composite decks, the deck with normal strength concrete slab performed much better and had much less damage than the deck with high strength concrete. The axial force in the deck adversely affected the behavior of bridge decks. Due to presence of large axial force in the deck, the vertical deflection of the deck generated by blast created large P-Delta effects causing the collapse of the steel orthotropic deck while the loss of the concrete slab created the lateral buckling in the composite plate girder deck system causing the collapse of the composite plate girder deck. The decks in the traditional anchored suspension bridges performed well due to the axial load in these bridges being almost zero. The performance of self-anchored bridge studied here and subjected to blast was poor due to presence of large axial load in the deck f self-anchored bridge throughout the length of the bridge. Performance of cable stayed bridges subjected to blast depended on the location of the blast. If the location was such that there was negligible axial load, the performance was good. But, if the axial load in the deck was high, the performance tended to be similar to the not-so-good performance of self-anchored suspension bridges which by their characteristic nature have very high axial load in the deck.