By Mostafa EL-Engebawy, Ph.D.
What is the Construction Material of your Home?
Building materials include unreinforced masonry, non-ductile reinforced concrete, wood frame, ductile reinforced concrete, steel including prefabricated buildings, or a combination of these construction materials. Unreinforced masonry buildings and non-ductile reinforced concrete buildings pose the greatest risk to life safety. These two types of buildings are no longer allowed to be built in California, however many of these buildings still exist and are not retrofitted. If not retrofitted, these building types will be heavily damaged or destroyed near the fault line, but may suffer less damage if located 20 miles or more away from the fault line. The most common traditional building types in active seismic regions in the United States are:
Unreinforced Masonry (URM) Homes
Bricks, hollow clay tiles, stone, concrete blocks, or adobe with mortar form the bearing walls of these homes without using steel reinforcement. The mortar holding the masonry together is not strong enough to resist earthquake lateral forces. Earthquake hazard may be reduced by anchoring and bracing the exterior and interior URM walls to the roof and floor framing.
Non-Ductile Reinforced Concrete Homes
This type of construction is generally the case for homes built before the 1980's where the confining transverse steel reinforcement in columns or walls is too widely spaced, short overlap lengths of longitudinal bars in columns and walls, or the detailing of steel at the beam-column connections is inadequate. These homes can be strengthened by adding new walls, bracing, or wrapping the columns with confining material.
In addition, anchorage of masonry or concrete walls to the floor and the roof may be inadequate for unreinforced masonry and non-ductile reinforced concrete homes. As a result, the walls may buckle or pull away from floor/roof anchorage and the structure, or a portion of it, may collapse. Tying the walls to the floor and roof and/or installing an inside steel frame and bolting the wall to it are common retrofitting techniques. However, if the walls are damaged during an earthquake, it would be very expensive to shore up your building, remove damaged walls, and put in new walls.
Wood Frame Homes
This is by far the most prevalent type of construction used for single-family homes and apartment buildings in the United States. They generally perform better than the previous types during earthquake shaking and are less likely to be damaged, especially wood frames using plywood shear walls. However, damage is most likely in older homes built before the 1990's relying on materials like gypsum wallboard (drywall) and stucco that crack and lose both strength and stiffness during a moderate-to-large earthquake. Damage may also occur if the building is not bolted to the foundation, the cripple walls are not reinforced, or in buildings with large openings such as garage doors or windows on the first floor without compensating reinforcement.
Steel Braced Frame Homes
These are steel structures with simple beam-to-column connections, and diagonals members in the vertical planes to resist the lateral seismic forces. Typical earthquake damage includes stretching or buckling of braces with cracked welds or failed bolted connections.
Steel Moment Frame Homes
These are steel structures with rigid beam-to-column connections that are welded or bolted. Typical earthquake damage includes major cracks in welded connections, broken bolts, and enlarged bolt holes. Extensive damage or failure of connections leading to partial collapse of the structure may also develop during a large earthquake. Steel moment frames are more flexible than steel braced frames resulting in large displacements that would lead to greater non-structural and contents damage. However, the understanding of the seismic behavior of welded steel moment frame buildings was significantly improved after the 1994 Northridge Earthquake where comprehensive studies on their performance were conducted and currently implemented in various design codes.
What are the Potential Structural Issues of your Home?
In addition to the type of construction material listed above, the location of your home with respect to active faults, soil properties and strength below the foundations, landsliding and liquefaction susceptibility; there are several other factors influencing the structural damage to your home during moderate-to-large earthquakes. These factors are:
Year Built
Homes constructed or remodeled before the 1980's are more likely to suffer earthquake damage because they do not have adequate reinforcement in the concrete walls or masonry walls, or were not constructed according to modern building codes.
Number of Stories
Homes with more than one story are more likely to suffer earthquake damage.
Soft Story at the First Level
Buildings with soft first floor are structurally weak and may lean or collapse during an earthquake. Numerous buildings, regardless type of construction material, were severely damaged or collapsed in 1994 Northridge earthquake and 1995 Kobe earthquake.
Basement / Crawl Space Bracing
If there is a basement or crawl space under your home, proper bracing of the walls between the concrete foundations and the floor joists can help the structure survive strong shaking.
Foundation Conditions
Your home should have enough bolts connecting the frame to the foundation to prevent overturning and collapse during strong shaking.
Concluding Remark
The 2010 Haiti earthquake is a wake up call for anyone who lives in an active seismic region to know how to reduce the risk to lives and investments by establishing an earthquake preparedness plan that include -among other items- understanding the potential structural issues of your home that would help you decide on retrofitting its structural components. Structural damage is influenced by construction material, year built, number of stories, foundation conditions, unbraced soft story and unbraced crawl space. In the United States, these seismic regions include -but not limited to- Alaska and the West Coast especially California; the Midwestern States especially Illinois, Kentucky, Missouri, and Tennessee around the New Madrid and the Wabash Valley Seismic Zones; and the Charleston area in South Carolina.
Additional information on how to secure the contents of your home including illustrative figures and photos can be found at http://hubpages.com/hub/earthquake-preparedness-understanding-the-structural-issues-of-your-home.
Building materials include unreinforced masonry, non-ductile reinforced concrete, wood frame, ductile reinforced concrete, steel including prefabricated buildings, or a combination of these construction materials. Unreinforced masonry buildings and non-ductile reinforced concrete buildings pose the greatest risk to life safety. These two types of buildings are no longer allowed to be built in California, however many of these buildings still exist and are not retrofitted. If not retrofitted, these building types will be heavily damaged or destroyed near the fault line, but may suffer less damage if located 20 miles or more away from the fault line. The most common traditional building types in active seismic regions in the United States are:
Unreinforced Masonry (URM) Homes
Bricks, hollow clay tiles, stone, concrete blocks, or adobe with mortar form the bearing walls of these homes without using steel reinforcement. The mortar holding the masonry together is not strong enough to resist earthquake lateral forces. Earthquake hazard may be reduced by anchoring and bracing the exterior and interior URM walls to the roof and floor framing.
Non-Ductile Reinforced Concrete Homes
This type of construction is generally the case for homes built before the 1980's where the confining transverse steel reinforcement in columns or walls is too widely spaced, short overlap lengths of longitudinal bars in columns and walls, or the detailing of steel at the beam-column connections is inadequate. These homes can be strengthened by adding new walls, bracing, or wrapping the columns with confining material.
In addition, anchorage of masonry or concrete walls to the floor and the roof may be inadequate for unreinforced masonry and non-ductile reinforced concrete homes. As a result, the walls may buckle or pull away from floor/roof anchorage and the structure, or a portion of it, may collapse. Tying the walls to the floor and roof and/or installing an inside steel frame and bolting the wall to it are common retrofitting techniques. However, if the walls are damaged during an earthquake, it would be very expensive to shore up your building, remove damaged walls, and put in new walls.
Wood Frame Homes
This is by far the most prevalent type of construction used for single-family homes and apartment buildings in the United States. They generally perform better than the previous types during earthquake shaking and are less likely to be damaged, especially wood frames using plywood shear walls. However, damage is most likely in older homes built before the 1990's relying on materials like gypsum wallboard (drywall) and stucco that crack and lose both strength and stiffness during a moderate-to-large earthquake. Damage may also occur if the building is not bolted to the foundation, the cripple walls are not reinforced, or in buildings with large openings such as garage doors or windows on the first floor without compensating reinforcement.
Steel Braced Frame Homes
These are steel structures with simple beam-to-column connections, and diagonals members in the vertical planes to resist the lateral seismic forces. Typical earthquake damage includes stretching or buckling of braces with cracked welds or failed bolted connections.
Steel Moment Frame Homes
These are steel structures with rigid beam-to-column connections that are welded or bolted. Typical earthquake damage includes major cracks in welded connections, broken bolts, and enlarged bolt holes. Extensive damage or failure of connections leading to partial collapse of the structure may also develop during a large earthquake. Steel moment frames are more flexible than steel braced frames resulting in large displacements that would lead to greater non-structural and contents damage. However, the understanding of the seismic behavior of welded steel moment frame buildings was significantly improved after the 1994 Northridge Earthquake where comprehensive studies on their performance were conducted and currently implemented in various design codes.
What are the Potential Structural Issues of your Home?
In addition to the type of construction material listed above, the location of your home with respect to active faults, soil properties and strength below the foundations, landsliding and liquefaction susceptibility; there are several other factors influencing the structural damage to your home during moderate-to-large earthquakes. These factors are:
Year Built
Homes constructed or remodeled before the 1980's are more likely to suffer earthquake damage because they do not have adequate reinforcement in the concrete walls or masonry walls, or were not constructed according to modern building codes.
Number of Stories
Homes with more than one story are more likely to suffer earthquake damage.
Soft Story at the First Level
Buildings with soft first floor are structurally weak and may lean or collapse during an earthquake. Numerous buildings, regardless type of construction material, were severely damaged or collapsed in 1994 Northridge earthquake and 1995 Kobe earthquake.
Basement / Crawl Space Bracing
If there is a basement or crawl space under your home, proper bracing of the walls between the concrete foundations and the floor joists can help the structure survive strong shaking.
Foundation Conditions
Your home should have enough bolts connecting the frame to the foundation to prevent overturning and collapse during strong shaking.
Concluding Remark
The 2010 Haiti earthquake is a wake up call for anyone who lives in an active seismic region to know how to reduce the risk to lives and investments by establishing an earthquake preparedness plan that include -among other items- understanding the potential structural issues of your home that would help you decide on retrofitting its structural components. Structural damage is influenced by construction material, year built, number of stories, foundation conditions, unbraced soft story and unbraced crawl space. In the United States, these seismic regions include -but not limited to- Alaska and the West Coast especially California; the Midwestern States especially Illinois, Kentucky, Missouri, and Tennessee around the New Madrid and the Wabash Valley Seismic Zones; and the Charleston area in South Carolina.
Additional information on how to secure the contents of your home including illustrative figures and photos can be found at http://hubpages.com/hub/earthquake-preparedness-understanding-the-structural-issues-of-your-home.
My family and I survived the January 17th 1995 Kobe Earthquake although our building partially collapsed. Check my website for additional information on earthquake awareness and preparedness in the United States including earthquake damage photos of Kobe earthquake at http://earthquake-preparedness.net
