# Resilience (engineering and construction)

A home in Gilchrist, Texas, designed to resist flood waters survived Hurricane Ike in 2008. (Photo courtesy of FEMA/Joselyne Augustino)

In the fields of engineering and construction, resilience is an objective of design, maintenance and restoration for buildings and infrastructure, as well as communities. It is the ability to absorb or avoid damage without suffering complete failure.[1][2][3] A more comprehensive definition is that it is the ability to respond, absorb, and adapt to, as well as recover in a disruptive event. A resilient structure/system/community is expected to be able to resist to an extreme event with minimal damages and functionality disruptions during the event; after the event, it should be able to rapidly recovery its functionality similar to or even better than the pre-event level.

The concept of resilience originated from ecology and then gradually applied to other fields. It is related to that of vulnerability. Both terms are specific to the event perturbation, meaning that a system/infrastructure/community may be more vulnerable or less resilient to one event than another one. However, they are not the same. One obvious difference is that vulnerability focuses on the evaluation of system susceptibility in the pre-event phase; resilience emphasizes the dynamic features in the pre-event, during-event, and post-event phases[4].

Resilience is a multi-facet property, covering four dimensions: technical, organization, social and economic [5]. Therefore, using one metric may not be representative to describe and quantify resilience. In engineering, resilience is characterized by four Rs: robustness, redundancy, resourcefulness, and rapidity. Current research studies have developed various ways to quantify resilience from multiple aspects, such as functionality- and socioeconomic- related aspects[4].

## Functionality-related resilience metrics

The first influential quantitative resilience metric based on the functionality recovery curve was proposed by Bruneau et al. [5], where resilience is quantified as the resilience loss as follows.

${\displaystyle R_{L}=\int _{t_{0}}^{t_{f}}[100\%-Q(t)]dt}$

where ${\displaystyle Q(t)}$ is the functionality at time ${\displaystyle t}$; ${\displaystyle t_{0}}$ is the time when the event strikes; ${\displaystyle t_{f}}$ is the time when the functionality full recovers.

The resilience loss is a metric of a only positive value. It has the advantage of being easily generalized to different structures, infrastructures, and communities. This definition assumes that the functionality is 100% pre-event and will eventually be recovered to a full functionality of 100%. This may not be true in practice. A system may be partially functional when a hurricane strikes and may not be fully recovered due to uneconomic cost-benefit ratio.

Resilience index is a normalized metric between 0 and 1, computed from the functionality recovery curve [6].

${\displaystyle R={\frac {\int _{t_{0}}^{t_{h}}Q(t)dt}{t_{h}-t_{0}}}}$

where ${\displaystyle Q(t)}$ is the functionality at time ${\displaystyle t}$; ${\displaystyle t_{0}}$ is the time when the event strikes; ${\displaystyle t_{h}}$ is the time horizon of interest.

## Socioeconomic-related resilience metrics

Socioeconomic resilience metrics fall into two categories: system-based and capital-based. System-based socioeconomic resilience metrics focus on quantifying the post-event business continuity and operability, whereas capital-based resilience metric measure resilience from the capital of individuals and communities [4].