Establishing Common Nomenclature, Characterizing the Problem, and Identifying Future Opportunities in Multihazard Design
Strong interest in extending the service life of critical infrastructure, compounded by the severity of damages during major disasters, such as Hurricane Katrina in 2005 (FEMA 2006) and the Tohoku earthquake in 2011 (NILIM and BRI 2011) has triggered a growing interest in design concepts that account for cascading effects and the interaction of multiple hazards. Traditionally, design is focused on the effects of various kinds of individual single hazards. In today's structural design practice, the impacts of various single hazards are translated into equivalent forces. Modern design codes account for concurrence and combinations of multiple hazards by suggesting load combinations and load factors intended to include uncertainties and significance of different hazards. A structural system that is designed to resist maximum load effects is expected to survive the damaging effects of multiple hazards. In recent years, other concepts such as displacement based design and performance based design were developed for hazards such as earthquakes. However, current design philosophies fail to consider the complex and intertwined effects of multiple hazards at system wide and societal levels. Some of the shortcomings of the current design philosophies in reflecting the complex nature of multiple hazards can be identified as follows: The effects of many hazards cannot be meaningfully translated into "equivalent forces" for instance, the damage caused by a fire is better represented bymaterial decay than by thermal forces; Successions of hazards impacting a structure are not explicitly included; for example, earthquakes can have a different impact on structures that suffer from corrosion damage compared with pristine structures (Burke and Bruneau 2016; Shiraki et al. 2007) similarly, scour has a great impact on the seismic fragility of reinforced concrete bridges (Wang et al. 2014) Magnifying effects of hazards acting together are typically ignored; in the case of the catastrophic collapse of the World Trade Center towers, the impact due to the airplane crash shattered the fire protection coatings, which exposed the load carrying elements to extreme heat effects (FEMA 2002) Analyses are commonly performed on models of intact structures; during the 2011 Tohoku earthquake, several structures that had been damaged by the mainshock suffered further damage in aftershocks (Li et al. 2014) and Systemand society level consequences of multiple hazards are not explicitly included; current design philosophies tend to focus on individual components of a system in isolation (e.g. bridges in a transportation system) to minimize the adverse social and economic impacts of multiple hazards, a holistic approach is necessary to account for different scenarios that may impair system function; for example, an earthquake that causes a landslide that blocks access to a hospital could have a catastrophic system level impact once supplies dwindle that would be similar to that of the earthquake causing structural damage to the hospital directly. In designing for the effects of multiple hazards, several other deficiencies may be present in current design practice. The concept of superposition of different hazards cannot always accurately predict the risk of damage. The effects of multiple hazards, acting concurrently or over time, can significantly increase the damaging impact of individual hazards. Therefore, an explicit multihazard design is necessary to achieve robustness and resiliency (as further defined later) at a large scale. Multihazard design requires an in depth understanding of the nature of various hazards and their interactions. It must also include the effects that the hazards have on one another and on the behavior of structures or physical components of a system. Design for multihazard mitigation is a multifaceted and complex challenge that may prohibit the development of a unified approach.