Nonlinear Dynamic Analysis for the Structural Robustness Assessment of a Complex Structural System

The review of the results provided from the performed investigations permit to identify some characteristics in the behavior of the bridge after a damage in the hanger suspension system, that are intrinsic to the structural system (see Fig.7), that are summarized in the following 4 aspects: 1) Minimum number of removed hangers and most sensitive location for the triggering of the progressive collapse: the bridge results to be more sensible to the damage at mid-span, where the removal of just 5 hanger for the symmetrical rupture and 7 hangers for the asymmetrical rupture is needed in order to trigger the collapse propagation. Shifting the initial damage location aside (about at 1/3 of the span) the asymmetrical rupture of 9 hangers is required for the collapse propagation, while moving the initial damage near the tower even the asymmetrical removal of 12 hangers has no global effects on the structure and very 7 hangers must be symmetrically removed on both sides in order to trigger the propagation of the ruptures on the adjoining hangers. 2) Preferential direction for the collapse propagation: to the higher damage sensibility of he bridge central zone counterpoises a lower acceleration of the collapse progression triggered by central ruptures, with respect to that one triggered by lateral ruptures. This effect is due to the particular configuration of the structural system that requires a growing hanger length from the centre to the sides of the bridge: when a chain rupture trigger, the ultimate elongation required to the hangers adjoining the failed ones increases as the collapse propagates (because the unsupported deck length also increases). If the initial damage occurs at mid-span, it involves the shortest hangers and the collapse propagation is partially slowed down from the growing element ductility of sideward hangers. On the contrary, a more intense initial damage is required sideways to trigger chain ruptures, but then the hanger breakdowns speeds up when moving toward the centre, where the hanger length decreases. 3) Qualitative measure that could possibly lead the collapse to an halt: in the case of a central rupture a closer increment in the section of the hangers (that remain instead the same for about 5/6 of the span length) could possibly provide for a collapse standstill. In the case of a chain rupture triggered in a lateral zone the preferential direction showed by the progressive collapse would probably make less effective such a measure. 4) Sensibility to modality of damage (asymmetrical or symmetrical failure): another consideration about the possible collapse standstill concerns the higher susceptibility of the bridge to an unsymmetrical hanger failure than to a symmetrical one: in the last case the symmetrical hinge formations determines a symmetrical moment increment on the deck box-girders, thus possibly allowing for an early deck segment detachment that would arrest the collapse. © 2008 ASCE