# Recent Advancement in Assessment and Control of Structures under Multi-Hazard
## 2 Risk: Hazard, Exposure, and Vulnerability
### Vulnerability and Risk  
Vulnerability lies within the characteristics or properties of the elements (structures) at risk, making them susceptible to impacts of hazards. The UNGA [24] defines vulnerability as \"the conditions determined by physical, social, economic and environmental factors or processes which increase the susceptibility of an individual, a community, assets or systems to the impacts of hazards\". The concept of vulnerability is used in a broad sense and with different meanings in different fields. Vulnerability is the risk determinant that is the most feasible one to manage/control/reduce through human action or interference. Vulnerability reduction is, therefore, one of the most effective forms of risk reduction. However, the quantification of vulnerability of civil engineering structures even to individual hazards, such as earthquakes, is a challenging task with many uncertainties (see, for example, [38; 39; 40]). In a multi-hazard scenario, the overall vulnerability can be different from the vulnerability to a single hazard, which makes the definition of vulnerability especially challenging. Itscomplexity is due to the variations in structural material types, geometries, environments, exposure to hazards, usage, age, maintenance, and many other factors. Quantification of structural vulnerability to different types of hazards is a popular and growing research field. Structural vulnerability is mostly expressed in terms of fragility or vulnerability curves, which quantify, in a probabilistic sense, the chance of exceeding undesired states of damage conditioned to a given intensity of hazard. On a larger geographical scale, vulnerability classification of structures relies on general information about the structures, their usage, and exposure to hazards. Such classifications are commonly used for buildings. This method of vulnerability assessment was used by [PERSON] et al. [41] to rank school infrastructure in the Philippines, considering flood, wind, and earthquake hazards. A vulnerability assessment methodology for building, subjected to both single and multi-hazards, was presented by [PERSON] et al. [42]. By following the principles of the European Macroseismic Scale 1998 (EMS-98, [43]), they developed vulnerability tables for different hazards (wind, flood, and earthquake). A framework to create multi-dimensional vulnerability models from vulnerability tables was also presented and applied in a few test cities in Germany. [PERSON] and [PERSON] [44] present multi-hazard damage to structures in central Nepal caused by the 2015 Gorkha Earthquake and the 2017 Chhatiune Khola flash flood. A conceptual model for multi-hazard assessment of the vulnerability of historic buildings is presented by [PERSON] et al. [45] (2006) with an example application considering English parish churches. A comprehensive review of single and multi-hazard vulnerability and risk in historic urban areas is presented by [PERSON] and [PERSON] [46]. They also present interesting examples of the use of multi-hazard risk analysis in historic urban areas.  
An interesting methodology for the risk evaluation of offshore structures subjected to ocean waves, wind, and ground motion is proposed by [PERSON] and [PERSON] [47]. They consider failure probabilities under short-term loads, as well as overall risks, due to loads of different intensities. Their results show that limit states (of failure), structural characteristics, as well as features of different types of loads interact in a complex way, controlling the relative importance of different hazards. Aggravating effects in a multi-hazard scenario are clearly demonstrated in their case-study example of ambient (everpresent wind over the sea) and seismic loads. An outline for identification of different hazards and subsequent risk assessment has been introduced by the United States Federal Emergency Management Agency, 1997, [48]. [PERSON] et al. [49] present a comprehensive report of recent developments in multi-hazard processes and risks related to research, policy, and industry.