# Recent Advancement in Assessment and Control of Structures under Multi-Hazard
## 2 Risk: Hazard, Exposure, and Vulnerability
### Hazard  
The definition of \"hazard\" in a broader sense is \"any external or internal process or event that might degrade the performance of the system on hand\" [12]. The United Nations General Assembly [24] defines hazard as \"a process, phenomenon or human activity that may cause loss of life, injury or other health impacts, property damage, social and economic disruption or environmental degradation\". Natural events, such as storms, earthquakes, or floods, are well-known hazards with widespread potential to turn into disasters. While these events are mostly sudden and occur in a relatively short time window, slower processes, such as fatigue, corrosion, ageing, etc., can also impact structural performance over their life span. Among the three elements that constitute disaster risk, hazard is the one that is mostly beyond human control. Nevertheless, a proper understanding of the occurrence frequency, spatio-temporal distribution, and intensity of the hazard is important for disaster risk reduction. Recent advances in sensing technology: data collection, processing, storage, sharing capabilities; and modelling/computational tools have improved our understanding how different hazards affect civil engineering structures. Hazards can be of different types. For example, they can be natural events, such as earthquakes, or man-made ones, such as explosions. Different classifications of hazard have been proposed for multi-hazard studies. For example, [PERSON] and [PERSON] [12] discuss the classification of hazards based on Temporal, Frequency, and Newtonian characteristics. Temporal characterization distinguishes between simultaneous occurrence, segregation in time, and cascading effects. Frequency characterization distinguishes continuous processes, such as corrosion, from intermittent processes such as earthquakes. Intermittent processes can be further classified as frequent, intermediate, or rare. Newtonian characterization is another useful approach for hazard classification that is generally used in design codes. In design codes, hazards are generally quantified in terms of loads, such as wind load, earthquake load, etc. The impact of these loads can be quantified by different metrics, such as stress, deformation, etc., and are evaluated based on Newtonian mechanics. Such hazards have been termed as Newtonian [12]. Other processes, such as corrosion, wear and tear, fatigue, etc., are termed as non-Newtonian [12]. Natural hazards can also be classified based on their origin and the geo-atmospheric processes associated with them, such as  
1. Biophysical (wildfire). 2. Atmospheric (wind or thunderstorms, lightning, hail, snow, and climate change). 3. Hydrological (drought and flood). 4. Shallow Earth Processes (erosion, subsidence and uplift, and mass movement). 5. Geophysical (volcanic eruption, tsunami, landslide, earthquake, and snow avalanche). The idea that a structure needs to resist different types of hazards during its service life is well-established in civil engineering. For example, design codes and standards have provisions for different types of actions such as dead load, live load, wind load, seismic load, etc. Simultaneous occurrence of multiple actions is addressed in design codes through load combinations. Such recognition of multiple actions and load combinations does not encompass the real extent of multi-hazard effects and interactions. Multi-hazard generally refers to the concept where two or more hazards interact through structural performance. A multi-hazard interaction, for example, can impact risk due to a hazard when a decision regarding structural exposure and vulnerability against frequency, location, and amplitude of another hazard is made. For example, a change in design wind load and/or structural capacity can impact structural vulnerability to earthquake forces. Multi-hazard interactions may result in common or conflicting design solutions. For example, provision of structural ductility is beneficial for both blast and seismic loads.