Collapse of partially or fully submerged river bridges as a result of river floods is investigated. To this end, post-failure analysis of a bridge in Japan’s Iwate Prefecture that collapsed in 2013 is conducted. Failure of this particular bridge is atypical because piers have also toppled over, which can be attributed to the spread footing foundation not anchored to the bedrock.
Two hypotheses are formed regarding the failure mode of this bridge. One assumes that initially the deck was dislodged and the piers subsequently toppled over, while the other follows the scenario of simultaneous failure of deck and pier. In order to predict which failure mode is governing, estimates of hydrodynamic forces based on design standards and previous research are formulated in conjunction with an analysis of the capacity of a potential bearing type.
Experiments in a flume are performed at a 1:37 scale to simulate steady-state hydrodynamic forces on the bridge. The flow depth and Froude number of the flow are varied across a wide range. Simulated debris is also lodged against the bridge model during some experiments. Observations on under which conditions a free-standing bridge model will or will not fail are carried out. These conditions are then replicated while the bridge model is connected to a load cell to determine the forces associated with failure.
Results show that the resistance against horizontal movement of a bridge deck actually increases as Froude number increases, as negative lift forces dominate over drag forces. Standalone piers always have sufficient resistance against overturning as long as no debris is present. Addition of simulated debris against the pier breaks down its streamlined shape and can cause toppling over of a pier.
Failure of the bridge as a whole (intact deck-pier connection) is not observed under any conditions during the experiments and this hypothesis is discarded. Subsequent failure of deck and pier only happens when simulated debris accumulations are introduced into the experiment. Deck displacement occurs at inundation ratios greater than 2.0 at moderate Froude numbers (0.14 ≤ Fr ≤ 0.32). Pier failure occurs at Fr ≥ 0.34 when the flow depth exceeds the height of the pier.
Possible countermeasures and improvements that may be incorporated into the design of bridges so that they may be more resilient to collapse during a flood in the future are suggested.