ABSTRACT
In recent years, frequencies of flood events in Australia have increased. It is noted that flood events cause the most damage to infrastructure compared to any other natural hazards in the world (IBISWORLD, 2011). Bridge structures located over waterways are prone to failure under flood events. Failure of a bridge can impact on the community significantly by reducing the evacuation capability and recovery operations during and after a disaster. A recent research project commenced at RMIT University aims to examine failure of road bridges under flood events. The paper has reviewed different bridge design codes used over several years in Australia for designing the bridges. Various failure mechanisms of bridges due to flood events have been investigated and the most common failure mechanisms of the bridges in Queensland have been identified by examining bridge inspections conducted after the 2011 and 2013 flood events. A case study bridge, which failed under flood loading, has been modeled and the effects of different flood scenarios have been investigated. The impacts of different types of debris, urban and regional, have also been studied. Damage indices have been derived for a concrete girder bridge to demonstrate the methodology for vulnerability modeling of bridge structures.
Reinforced or pre-stressed concrete girder bridges are a common design configuration used in Australia. During the Lockyer Valley floods in 2013, vulnerability of girder bridges was observed by significant damage to these structures. Structural performance of Kapernicks Concrete Girder Bridge has been studied in this paper. For the girder not to fail under flood loading, the existing moment capacity of the girder (ØMu ) must be greater than the moment induced by the flood force (M*). In other words ØMu/M* > 1. The maximum allowable flood velocity to satisfy this condition could be read from the above structural vulnerability curves. For Kapernicks Bridge under investigation, the maximum allowable flood velocity is shown in Table 1. Maximum allowable velocity for different types of flood impact.
Type of Flood Impact
Maximum allowable flood velocity (m/s)
Flood only
3.66
Flood+Debris
2.46
Flood+Debris+Container
0.89
It has been observed that when the intensity of flood increases, the bridge structure becomes more vulnerable. The intensity of flood accounts for flood velocity, the accumulation of debris that it carries along in it and any other floating objects such as containers, vehicles and river-crafts. The outcomes will enable identification of the vulnerable girder bridges in the road networks and will assist road authorities to make optimized hardening decisions. On the other hand, emergency management services will be able to avoid vulnerable structures in determining evacuation routes.
The new design for the Kapernicks bridge has used 4 m/s as the design flood velocity.
The research is being continued to improve the vulnerability models considering the fluid structure interaction replacing the code equations and replacing the deterministic approach with reliability-based approach to cover variability of the flood loading and the material degradation of the structures.
