Forensic diagnosis on flood-induced bridge failure. II: Framework of quantitative assessment

The failure of the Shuang-Yuan Bridge in Taiwan caused by floods associated with Typhoon Morakot in August 2009 prompted this forensic investigation into its causes. The investigation found multiple factors for the bridge failure: (1) flood flows, (2) river-bend-induced skewed flow, and (3) the effects induced by extended foundation caps and rafted woody debris. A qualitative assessment framework, which includes hydraulic computation, a fluid-solid-coupled model, and foundation stability analysis, is introduced to develop scenario simulations and forensic diagnosis to identify the main causes of the bridge failure. The watershed-wide and local hydraulic conditions are reconstructed using hydraulic computation. The flow field, dynamic water-pressure distributions, scour formation, flood resistance, and flood thrust are efficiently simulated using a fluid-solid-coupled model and foundation stability analysis around the bridge foundations under different hydraulic and environmental conditions. The forensic diagnosis results show that river flooding, extended foundations, skewed flow, and rafted wood aggravated the scour depths and undermined the flood-resistant capacity of the bridge at different severities. In particular, rafted wood contributed to individual fracturing of the bridge piles.