Concrete, as a structural material and as the building exterior skin, has the ability to withstand nature’s normal deteriorating mechanisms as well as natural disasters. Properly designed, reinforced concrete is resistant to earthquakes and provides blast protection for occupants. Concrete safe rooms help provide protection from earthquakes, tornadoes, hurricanes, fires, and other disasters.
Masonry Safe Room
Concrete offers noncombustible construction that helps contain a fire within boundaries:
- As a separation wall, concrete helps to prevent a fire from spreading throughout a building.
- As an exterior wall or roof, concrete helps to prevent a fire from jumping from building to building.
- During wild fires, concrete walls and roofs help provide protection to human life and the occupants' possession within a building.
- Concrete helps contain an fire even if no water supply is available, whereas sprinklers rely on a water source.
- Concrete that endures a fire can often be reused when the building is rebuilt.
Image after a fire, concrete and masonry may be all that remains (PCA). ASTM International
E119, "Standard Test Methods for Fire Tests of Building Construction and Materials," describes test procedures for determining fire endurance of building materials. A two-hour fire endurance for a concrete wall will most likely mean the wall gets hot (experiences an average temperature rise of 250 degrees Fahrenheit at any one point.) The fire endurance of concrete can be determined by its thickness and type of aggregate using American Concrete International
(ACI) procedures. In fire endurance tests, concrete generally fails by heat transmission long before structural failure, whereas other construction materials fail by heat transmission when collapse in imminent.
Stucco is fire-resistant, which is one of the main reasons this home was the only house left standing on this California hillside after the wild fire. (PCA No. 13560)
Concrete also performed well during the urban-wildland interface fires that have destroyed billions of dollars of property in parts of the western United States. Hilly terrain, hot and dry winds, combustible vegetation, and closely spaced dwellings create favorable conditions for these types of fire. This trend is expected to continue as populations continue to expand into wildland areas. Data collected after these fires shows a correlation between fire damage and the exterior surfaces of buildings, including:
Concrete or clay tile roofs performed much better than wood shale or shingle roofs.
- Buildings having non-combustible exterior walls surfaces, such as masonry or stucco, achieved a higher level of survival.
- Double-pane windows are need to minimize heat transfer to the building interior.
- Minimal roof projections or the use of non-combustible materials to protect combustible eaves and projections plus the elimination of soffit vents will also increase a structure's chances of surviving a wildland fire.
Tornado, Hurricane, and Wind Resistance
Concrete is resistant to tornadoes, hurricanes, and wind. Following Hurricane Katrina, a concrete house was the sole house left standing in a Pass Christian, Mississippi, neighborhood.
The Sundbergs' home, in the Pass Christian, Mississippi, area affected by Hurricane Katrina, is shown in the yellow circle and is a prime example of the durability of concrete homes. (PCA photo from FEMA)
Investigators have learned from previous hurricanes that:
- Asphalt shingles often failed due to holes created by staple guns. Nails held better than staples if they were properly placed.
- Clay roof tiles resisted wind forces better than asphalt shingles but were apt to shatter if hit by flying debris.
- Concrete roof tiles suffered similar damage as clay roof tiles from debris, but were more resistant to shattering than clay tiles.
- Asphalt gravel roofs, if not well maintained, were flaked off in layers by the wind, exposing sub-layers.
- Plywood sheathing failures were due to inadequate nailing.
- Particle board does not provide a good base for the attachment of surface roofing materials.
- Gables were more prone to failure than hip roofs. Gables constructed of concrete masonry fared much better than frame construction. Inadequate attachment to walls and inadequate lateral support caused many failures of gables, particularly wood frame gables.
- Concrete block walls performed well. Concrete masonry construction was more forgiving of poor craftsmanship than wood frame construction. Compliance with the Standard for Hurricane Resistant Residential Construction (SSTD 10-93), or the provisions of ACI 530/ASCE 5/TMS402-95 would have probably reduced the amount of damage observed in these structures.
- Masonry veneer also performed well when properly constructed and connected to the structure. Damaged veneers were invariably a result of corroded, inadequate, or improperly embedded ties. Masonry veneer structures subjected to storm surges were able in many cases to withstand the storm surge better than wood frame houses without veneer.
- Wood frame walls performed poorly unless well designed and constructed.
- Loads on building components and connections are significantly increased when the envelope is breached by high wind or flying debris. Masonry systems appeared to resist breaching as well, if not better, than other wall systems.
- Windows and doors need to be carefully installed. Windows must be protected with hurricane shutters.
This wood 2-by-4 impaled a wood frame home due to a tornado spawned by Hurricane Katrina (http://www.noaa.gov/)
Debris driven by high winds presents the greatest hazard to homeowners and their homes during hurricanes and tornados. Tests show that concrete wall systems suffer no structural damage when impacted by debris carried by hurricane and tornado-force winds.
As another example, in 1967, a series of deadly tornadoes hit northern Illinois, killing 57 people and destroying 484 homes. Damages at the time were estimated at $50 million. Two prestressed concrete structures, a grocery store and a high school, were in the direct path of two tornadoes that struck almost simultaneously. Repairs to the structural system of the grocery store were less than $200. In the high school, structural damage was also limited.
Concrete is not damaged by water; concrete continues to gain strength in the presence of moisture. Concrete submerged in water absorbs very small amounts of water over long periods of time, and the concrete is not damaged. In flood-damaged areas, concrete buildings are often salvageable. Concrete dams and levees are used for long-lasting flood control.
In the rebuilding of New Orleans after Hurricane Katrina, architects and engineers are looking at structures that will keep water out and not shift or float away when submersed in floodwaters. One solution is reinforced concrete walls to the roof with a 12-inch thick concrete slab. In one example, the slab will be kept in place with 8-inch helical anchors drilled 10 to 13 feet into the ground (Architect Hank Browne and engineers DMK Group, April 2006 Building Design and Construction).
Concrete will only contribute to moisture problems in buildings if it is enclosed in a system that traps moisture between the concrete and other building materials. For instance, a vinyl wall covering in hot and humid climates will act as a vapor retarder and moisture can get trapped between the concrete and the wall covering. For this reason, impermeable wall coverings (such as vinyl wallpaper) should not be used on concrete walls without a pathway for vapor drying.
High Humidity and Wind-Driven Rain
Concrete is not affected by wind-driven rain and moist outdoor air in hot and humid climates because it is impermeable to air infiltration and wind-driven rain. Moisture that enters a building must come through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Good practice for all types of wall construction is to have permeable materials that breathe (are allowed to dry) on at least one surface and to not encapsulate concrete between two impermeable surfaces. Concrete will dry out if not covered by impermeable treatments.
Concrete is resistant to earthquakes. Earthquakes in Guam, the United States (Richter scale 8.1); Manila, the Philippines (Richter scale 7.2); and Kobe, Japan (Richter scale 6.9) have subjected concrete buildings to some of nature’s deadliest forces. Concrete framing systems have a proven capacity to withstand these major earthquakes. Another pertinent example is the 1994 Northridge, California, earthquake (Richter scale 6.8). It was one of the costliest natural disasters in U.S. history, with total damages estimated at $20 billion. Most engineered structures within the affected region performed well, including structures with concrete components. It should be noted that parking structures with large open plan areas—regardless of structural system—did not perform as well as other types of buildings.
Built according to good practices, concrete homes can be among the safest and most durable types of structures during an earthquake. Homes built with reinforced concrete walls have a record of surviving earthquakes intact, structurally sound and largely unblemished. In reinforced concrete construction, the combination of concrete and steel provides the three most important properties for earthquake resistance: stiffness, strength, and ductility.
The Los Angeles Metro Blue Line Bridge was designed to sustain no functional damage from the worst earthquake expected in the next 75 years, and only minimal damage from an earthquake 10 times stronger than the 1995 Northridge quake. (PCA No. 10037)
Studies of earthquake damage consistently show well-anchored shear walls are the key to earthquake resistance in low-rise buildings. Optimal design conditions include shear walls that extend the entire height and are located on all four sides of a building. Long walls are stronger than short walls, and solid walls are better than ones with a lot of openings for windows and doors. These elements are designed to survive severe sideways (in-plane) forces, called racking and shear, without being damaged or bent far out of position. Shear walls also must be well anchored to the foundation structure to work effectively. Properly installed steel reinforcing bars extend across the joint between the walls and the foundation to provide secure anchorage to the foundation.
Properly anchored walls are key to earthquake resistance in low rise buildings (http://www.cement.org/)
Low-rise buildings most vulnerable to earthquakes do not have the necessary stiffness, strength, and ductility to resist the forces of an earthquake or have walls that are not well anchored to a solid foundation, or both. Three types of buildings sustain the most significant damage:
Multi-story buildings with a ground floor consisting only of columns;
Wood-frame houses with weak connections between the walls and foundation;
Unreinforced masonry or concrete buildings
Reinforced concrete walls work well because of the composite system: Concrete resists compression forces, and reinforcing steel resists tensile forces produced by an earthquake. Even a lightly reinforced concrete shear wall has over six times the racking load resistance as framed wall construction.
Fortified…for Safer Living
The Fortified…for Safer Living® program, an initiative of the Institute for Business & Home Safety, provides design, construction, and landscaping guidelines to increase a new home's resistance to natural disaster. “Fortified” techniques and construction materials raise a home’s overall disaster-resistance above the minimum requirement of local building codes. Extra attention is given to areas especially vulnerable to harsh elements, including doors and windows, roof construction and the foundation.
Extreme weather events.
Homes can be exposed to one or more extreme weather events, such as high wind, wildfire, flood, hail and earthquake. The Insurance Institute for Business and Home Safety website indicates major threats depending on the region of the country. A “fortified” home under construction in Illinois will have added protection against tornadoes, hail and severe winter weather – three of the state’s most destructive natural elements. “Fortified” construction features in this home will include:
Impact-resistant roof materials that better withstand high winds and are fire resistant.
Windows and doors with higher wind and water design pressure ratings and a garage door capable of withstanding impact from large objects.
Construction materials and sitework that minimizes the threat of flood or wildfire.
Disaster impact conditions are not limited to mother nature. Concrete has demonstrated blast resistance through tests. The Insulating Concrete Form Association (ICFA) and the Northern Virginia Concrete Advisory Council successfully demonstrated the blast-resistant properties of ICF building systems during the Force Protection Equipment Demonstration (FPED V) April 26–28, 2005, at Quantico Marine Corps Base in Northern Virginia. During the blast demonstrations, 11 separate ICF reaction boxes, weighing 13 tons apiece and with walls measuring 8 feet tall and 6 inches thick were subjected to explosion from 50 pounds of TNT at differing distances (3.5 to 10 feet) and to pressures from 300 pounds per square inch (psi) to over 7,000 psi. Known for decades for its impact resistant properties, expanded polystyrene (EPS), the primary forming material in ICFs, has shown great potential in combination with concrete as a blast-resistant product. In each instance during six different blast demonstrations, expanded polystyrene compressed against the face of the concrete wall and reduced the pressure of the blast.
Insulated Concrete Forma (ICF) reaction boxes prior to blast test
An ICF wall after a 50 pound TNT detonation from 10 feet away.
In addition, high performance concrete can be designed to have improved blast resistant properties. These concretes often have a compressive strength exceeding 14,500 pounds per square inch (psi) and contain steel fibers. These blast-resistant structures are often used in bank vaults and military applications.
Ubiquitous precast concrete planters provide protection to federal buildings, museums, and national landmarks. These barriers are attractive while still being a deterrent to wayward vehicles. Attractive concrete barriers that also provide seating are becoming commonplace
Concrete planters in Washington, D. C. (National Precast Concrete Association)
September 11, 2001 World Trade Center
Comparing the present with the past in the world around us can be an important learning experience. Such was the case for the Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers (ASCE), in the difficult task of conducting an evaluation of the World Trade Center (WTC) and surrounding buildings.
On September 11, 2001, airplanes struck two 110-story office towers in New York and the Pentagon in Washington, D.C. The towers World Trade Center 1 and World Trade Center 2 collapsed in less than two hours, and another building in the complex World Trade Center 7 collapsed later in the afternoon. These buildings had few or no masonry components. All of the surrounding buildings suffered damage from falling debris, wreckage, and fire from the towers.
Buildings surrounding World Trade Center collapse Sept. 29, 2001 (FEMA Photo 5695, http://www.fema.gov/)
While the impact of portions of the collapsing buildings did the majority of harm, there was also damage from flying debris to the masonry cladding used in their construction. Examples demonstrate how masonry helped prevent greater destruction during the World Trade Center disaster. Some of the lessons learned:
- Older framed buildings with masonry cladding performed generally better than newer buildings with lightweight curtain wall construction.
- Masonry (walls, beams, partitions, infill) served as fireproofing and provided significant structural redundancy.
- Masonry infill absorbed impact energy to minimize damage locally.
- Masonry veneers and panelized systems are readily repaired.
Masonry proved in this event that it does more than simply enclose space; it provides fire protection, structural capacity, and even structural redundancy. It can provide safer enclosures for stairways or other exit routes, affording egress in high-rise buildings during emergencies.