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Resist Natural Hazards

by the WBDG Safe Committee

Last updated: 05-01-2008

Overview

Buildings in any geographic location are subject to a wide variety of natural phenomena such as windstorms, floods, earthquakes, and other hazards. While the occurrence of these events cannot be precisely predicted, their impacts are well understood and can be managed effectively through a comprehensive program of hazard mitigation planning.

Mitigation refers to measures that can reduce or eliminate the vulnerability of the built environment to hazards, whether natural or man-made. The fundamental goal of mitigation is to minimize loss of life, property, and function due to disasters. Designing to resist any hazard(s) should always begin with a comprehensive risk assessment. This process includes identification of the hazards present in the location and an assessment of their potential impacts and effects on the built environment based on existing or anticipated vulnerabilities and potential losses.

It is common for different organizations to use varying nomenclature to refer to the components of risk assessment. For example, terrorism and foreign military power are referred to as "threats" by the intelligence community, while hurricanes and floods are referred to as "hazards" by emergency managers; however, both are simply forces that have the potential to cause damage, death, and injury, and loss of function in the built environment. Regardless of who is conducting the risk assessment, the fundamental process of identifying what can happen at a given location, how it can affect the built environment, and what the potential losses could be, remains essentially the same from application to application.

Only after the overall risk is fully understood should mitigation measures be identified, prioritized, and implemented. Basic principles underlying this process include:

RECOMMENDATIONS

Design professionals agree that the most successful way to mitigate losses of life, property, and function is to design buildings that are disaster resistant. This approach should be incorporated into the project planning, design, and development at the earliest possible stage so that design and material decisions can be based on an integrated "whole building approach."

A variety of techniques are available to mitigate the effects of natural hazards on the built environment. Depending on the hazards identified, the location and construction type of a proposed building or facility, and the specific performance requirements for the building, the structure can be designed to resist hazard effects such as induced loads. Later in the building's life cycle, additional opportunities to further reduce the risk from natural hazards may exist when renovation projects and repairs of the existing structure is undertaken. When incorporating disaster reduction measures into building design, some or all of the issues outlined below should be considered in order to protect lives, properties, and operations from damages caused by natural hazards.

Earthquakes

Building design will be influenced by the level of seismic resistance desired. This can range from prevention of nonstructural damage in frequent minor ground shaking to prevention of structural damage and minimization of nonstructural damage in occasional moderate ground shaking, and even avoidance of collapse or serious damage in rare major ground shaking. These performance objectives can be accomplished through a variety of measures such as structural components like shear walls, braced frames, moment resisting frames, and diaphragms, base isolation, energy dissipating devices such as visco-elastic dampers, elastomeric dampers, and hysteretic-loop dampers, and bracing of nonstructural components.

Hurricanes, Typhoons, and Tornadoes

The key strategy to protecting a building from high winds caused by tornados, hurricanes, and gust fronts is to maintain the integrity of the building envelope, including roofs and windows, and to design the structure to withstand the expected lateral and uplift forces. For example, roof trusses and gables should be braced; hurricane straps should be used to strengthen the connection between the roof and walls; and doors and windows should be protected by covering and/or bracing. When planning renovation projects, designers should consider opportunities to upgrade the roof structure and covering and enhance the protection of fenestration. The Additional Resources section of this page includes several FEMA publications for designing community shelters, constructed to protect a large number of people from a natural hazard event, and "residential safe rooms" for occupant refuge during windstorms.

Flooding

Flood mitigation is best achieved by hazard avoidance—that is, proper site selection away from floodplains. Should buildings be sited in flood-prone locations, they should be elevated above expected flood levels to reduce the chances of flooding and to limit the potential damage to the building and its contents when it is flooded. Flood mitigation techniques include elevating the building so that the lowest floor is above the flood level; dry flood-proofing, or making the building watertight to prevent water entry; wet flood-proofing, or making uninhabited or non-critical parts of the building resistant to water damage; relocation of the building; and the incorporation of levees and floodwalls into site design to keep water away from the building.

Rainfall and Wind-Driven Rain

One of the primary performance requirements for any building is that it should keep the interior space dry. All roofs and walls must therefore shed rainwater, and design requirements are the same everywhere in this respect. For example, roof drainage design should minimize the possibility of ponding water, and existing buildings with flat roofs should be inspected to determine compliance with this requirement. Recommendations for addressing rainfall and wind-driven rain can be found in the International Building Code (IBC) series.

Differential Settlement (Subsidence)

Ground subsidence can result from mining, sinkholes, underground fluid withdrawal, hydrocompaction, and organic soil drainage and oxidation. Subsidence mitigation can best be achieved through careful site selection, including geotechnical study of the site. In subsidence-prone areas, foundations should be appropriately constructed, basements and other below-ground projections should be minimized, and utility lines and connections should be stress-resistant. When retrofitting structures to be more subsidence-resistant, shear walls, geo-fabrics, and earth reinforcement techniques such as dynamic compaction can be used to increase resistance to subsidence damage and to stabilize collapsible soils.

Landslides and Mudslides

Gravity-driven movement of earth material can result from water saturation, slope modifications, and earthquakes. Techniques for reducing landslide and mudslide risks to structures include selecting non-hillside or stable slope sites; constructing channels, drainage systems, retention structures, and deflection walls; planting groundcover; and soil reinforcement using geo-synthetic materials, and avoiding cut and fill building sites.

Forest Fires

As residential developments expand into wild land areas, people and property are increasingly at risk from wildfire. Fire is a natural process in any wild land area and serves an important purpose; however, if ground cover is burned away, erosion, landslide, mudflow, and flood hazards can be exacerbated. A cleared safety zone of at least 30 feet (100 feet in pine forests) should be maintained between structures and combustible vegetation, and fire-resistant ground cover, shrubs, and trees should be used for landscaping (for example, hardwood trees are less flammable than pines, evergreens, eucalyptus or firs). Only fire-resistant or non-combustible materials should be used on roofs and exterior surfaces. Roofs and gutters should be regularly cleaned and chimneys should be equipped with spark arrestors. Vents, louvers, and other openings should be covered with wire mesh to prevent embers and flaming debris from entering. Overhangs, eaves, porches, and balconies can trap heat and burning embers and should also be avoided or minimized and protected with wire mesh. Windows allow radiated heat to pass through and ignite combustible materials inside, but dual- or triple-pane thermal glass, fire-resistant shutters or drapes, and noncombustible awnings can help reduce this risk.

Tsunami

A tsunami is a series of ocean waves generated by sudden displacements in the sea floor, landslides, or volcanic activity. In the deep ocean, the tsunami wave may only be a few inches high. The tsunami wave may come gently ashore or may increase in height to become a fast moving wall of turbulent water several meters high. Although a tsunami cannot be prevented, the impact of a tsunami can be mitigated through urban/land planning, community preparedness, timely warnings, and effective response.

Emerging Issues

Hazard Mitigation and Sustainability

Unsustainable development is one of the major factors in the rising costs of natural disasters. Given that hazard mitigation is at the core of disaster resistance, then, many design strategies and technologies serve double duty, by not only preventing losses but serving the higher goal of long-term community sustainability. For example, erosion control measures designed to mitigate flood, mudslide, rainstorm, and other damage to a building's foundation may also improve the quality of runoff water entering streams and lakes.

Cost of Mitigation Measures in Retrofits

The cost of incorporating multi-hazard mitigation measures in existing buildings is an issue of increased importance (for designers, insurers and the public) because a critical mass of facilities were built before multi-hazard construction measures were incorporated in modern building codes. Initially, building codes were developed solely to prevent or reduce the loss of life and property due to fire in buildings.

Relevant Codes and Standards

Regulations, codes, standards, and best practices will guide the design of buildings to resist natural hazards. For new buildings, code requirements serve to define the minimum mitigation requirements, but compliance with regulations in building design is not sufficient to guarantee that a facility will perform adequately when impacted by the forces for which it was designed. Indeed, individual evaluation of the costs and benefits of specific hazard mitigation alternatives can lead to effective strategies that will exceed the minimum requirements. Additionally, special mitigation requirements may be imposed on projects in response to locale-specific hazards. When a change in use or occupancy occurs, the designer must determine whether this change triggers other mitigation requirements and must understand how to evaluate alternatives for meeting those requirements.

Finally, designers should augment the codes and standards to consider the importance of nonstructural elements, assets, and mission of the building, i.e., windows, hoods, parapets and balcony railings, and electrical and mechanical systems, because they may account for more than 70% of the value of a building.

Many states and municipalities have also adopted supplemental codes to meet local requirements for multi-hazard protection. Examples of such codes include:

General Multi-Hazard

Earthquake

Hurricane, Typhoon, and Tornado

Flood

Rainfall and Wind-Driven Rain

Additional Resources

Organizations and Associations

General Multi-Hazard

Earthquake

Hurricane, Typhoon, and Tornado

Flood

Forest Fires

Publications

General Multi-Hazard

Earthquake

Hurricane, Typhoon, and Tornado

Flood

Landslide, Mudslide

Progressive Collapse

NOTE: To order FEMA publications that are not available online, request by title or document number from the FEMA Publications Warehouse at (800) 480-2520

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