Fire risks in suspension bridges

Long‐span suspension bridges provide vital support to society and its development. With ever increasing road traffic, such bridges are increasingly exposed to greater risks from fires. Experience shows that a significant fire in a truck without dangerous goods (like the 2013 New Little Belt Bridge fire in Denmark) can grow rapidly to reach temperatures above 1000°C, thereby making it a fire comparable to the hydrocarbon fire test standard. Compared to fuel tankers, the relatively high frequencies of trucks carrying normal (but flammable) goods make such significant truck fires more likely. Significant fires on a suspension bridge can lead to strength reduction or cable failure that could result in a reduction in bridge load rating and a need for long‐lasting repairs. Such outcomes mean high societal costs due to long‐term disruption of the connectivity provided by the bridge. Hence, there is a need to systematically assess fire risks in suspension bridges, focusing on a) fire frequencies and proportion of significant fires that can affect key bridge elements, b) fire consequences to key bridge elements and c) implementing mitigation measures when needed. Main cables in a suspension bridge are vital for structural integrity and at the same time difficult and expensive to replace during the bridge lifetime. This means that, in addition to ensuring no cable failure and no bridge collapse during fire, non‐exceedance of critical temperatures that could result in extensive damages and/or irreversible strength loss after fire is also important. Evaluating cable performance with respect to such criteria requires a detailed modelling of heat transfer into the cable, taking into account the nature of fire, multiple heat transfer modes, packing arrangement within the cable and geometrical exposure conditions.