Robustness of Bridges Under Multiple Extreme Events

Abstract eng:
Extreme earthquakes like the Nankai Trough scenario will trigger further hazards that may affect already damaged bridges, like landslides, fires or tsunami. Bridges are the backbone of road networks and must provide a minimal functionality after such a sequel of events, which they are typically not designed for. This requires a certain “robustness”, which in this context may be defined as a required “residual functionality” of a damaged bridge. This residual functionality is the type and amount of traffic that the bridge has to be able to accommodate after the event sequence in order to support the necessary emergency and evacuation activities. It depends on the role of the bridge within the road network. This paper presents a road map to such robustness of bridges under multiple events that are triggered by extreme earthquakes. It addresses the importance of hazard sequencing (earthquake followed by tsunami, land slide, fire etc.) and discusses the reduction of functionality due to certain damage types (e.g. pier damage due to shear, un-seating of bearings, permanent soil deformation etc.). In this context, it is recognized that some damage types lead to zero functionality. This leads to a definition of “acceptable” and “un-acceptable” damage states for high-impact bridges. Considering only ground shaking, un-acceptable damage states can be avoided in general by selecting proper structural systems, in particular seismic control concepts like the Hyde System, which not only provides this “robustness” but is also an economic retrofitting solution for deck bridges, the most common type in todays road networks. In the case of landslides and tsunami, robustness may be achieved with known and even simple concepts, but this needs further study. The hazard from un-controlled fire under such circumstances is largely unknown and needs further study before reasonable and economic measures towards fire robustness can be developed. A combination of traffic management tools (especially agent-based simulations) is able to identify bridges that are critical for emergency response and evacuation. They need realistic estimates of residual emergency functionality, which in turn is a function of the remaining load bearing capacity. ROADERS (Road Networks for Earthquake Resilient Societies), an international network of researchers in bridge engineering and traffic management provided a first framework how to approach this. Pre-disaster structural studies must be performed and validated for various levels of damage in order to obtain a realistic picture of a bridge’s remaining functionality. This information can then enter advanced and dynamic disaster management systems (like IDDSS) to allow realistic agent-based simulations for well-informed decision making within minutes. This may be required especially for emergency evacuation when, like for Osaka, only 30 minutes are available before the tsunami arrives.

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