Earthquake Damage
Your home and business are your biggest financial assets while their contents and personal possessions
are your lifetime investments. Your assets and investments may be at risk when a moderate-to-large
earthquake strikes your area. Basic knowledge of the potential earthquake damage and the structural
issues of your building would help you decide on seismic retrofit the structural components as well as risk
mitigation of the nonstructural components and contents, thus reducing the risk to lives and insuring your
business and home safety. Northridge Earthquake (1994) and Kobe Earthquake (1995) that occurred in
urban areas demonstrated that earthquake damage is influenced by construction materials, anchor bolts
not effectively connected to the concrete foundations, unbraced cripple walls of the crawl space, soft story
in the first floor due to large openings without effective bracing, year built, and number of stories.
Your home and business are your biggest financial assets while their contents and personal possessions
are your lifetime investments. Your assets and investments may be at risk when a moderate-to-large
earthquake strikes your area. Basic knowledge of the potential earthquake damage and the structural
issues of your building would help you decide on seismic retrofit the structural components as well as risk
mitigation of the nonstructural components and contents, thus reducing the risk to lives and insuring your
business and home safety. Northridge Earthquake (1994) and Kobe Earthquake (1995) that occurred in
urban areas demonstrated that earthquake damage is influenced by construction materials, anchor bolts
not effectively connected to the concrete foundations, unbraced cripple walls of the crawl space, soft story
in the first floor due to large openings without effective bracing, year built, and number of stories.
| Earthquake Awareness and Preparedness |
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| Figure 1 - Steel Braced Frame (created using SDS/2 software) |
| Figure 2 - Welded Moment Connection |
| Figure 3 - Bolted Moment Connection |
| Photos 4 - Soft Story Building that leaned during 1995 Kobe Earthquake |
| Photo 2 - Behavior of Reinforced Concrete Structures would Save Lives and reduce Property Damage |
| Photos 1a and 1b - Collapse of the Sixth Floor of Kobe City Hall due to the Non-Ductile Behavior of Reinforced Concrete Columns having Inadequate Transverse Reinforcement |
| Photo 3 - Collapse of the First Story of a Wooden House |
Construction Materials
Earthquake damage to you home/business is influenced by construction materials. Older
construction materials like unreinforced masonry and non-ductile reinforced concrete pose the
greatest risk to life safety and are no longer allowed to be built in California because they have a
documented poor performance during earthquakes. However, many of these buildings still exist
and are not retrofitted. Severe damage or structural collapse is expected within 30 miles from
active major fault lines and moderate damage for buildings further away. On the other hand,
modern reinforced masonry construction, wood construction, ductile reinforced concrete
construction, and steel construction generally perform better and are less likely to have severe
earthquake damage. Detailed information on construction materials follows:
Masonry Construction
Unreinforced masonry is an old construction method where bricks, hollow clay tiles, stone,
concrete blocks, or adobe with mortar form the bearing walls without using steel reinforcing bars.
However, the mortar holding the masonry together is not strong enough to resist earthquake
lateral forces even during moderate ground shaking, where the masonry walls may buckle or pull
away from floor or roof anchorage leading to severe damage or partial collapse. Earthquake
damage may be reduced by anchoring and bracing the exterior and interior masonry walls to the
structural members of the roof and floor framing, or by installing an inside steel frame and bolting
it to the masonry wall. In case the masonry walls are damaged during an earthquake, it would be
very expensive to shore up the building, remove the damaged walls, and put in new walls.
Concrete Construction
Non-ductile reinforced concrete construction is generally the case for buildings 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-to-column or beam-to-wall connections is inadequate. During ground shaking, the
concrete walls may buckle or pull away from floor or roof anchorage and the building, or a portion
of it, may collapse. These buildings can be strengthened by adding new concrete walls, steel
bracing, or wrapping the columns with confining material. Earthquake pictures from Kobe
Earthquake illustrate the difference in the structural performance of non-ductile versus ductile
reinforced concrete structures. Photos 1a and 1b show a complete collapse of the sixth floor of the
eight-story reinforced concrete Kobe City Hall that was built in the 1960's. This type of non-ductile
behavior that leads to shear failure of the columns was due to the very light transverse
reinforcement that was widely spaced. Photo 2 shows a "relatively" ductile behavior of a reinforced
concrete frame garage structure. Although the structure is seriously damaged, however, no
injuries were reported or even damage to the vehicles beneath it.
Wood Construction
This is by far the most prevalent type of construction used for single-family homes and apartment
buildings in the United States, Japan and elsewhere. They generally perform better during ground
shaking than other construction materials and are less likely to be damaged especially wooden
houses with structural plywood shear walls. However, earthquake damage is most likely in older
timber homes built before the 1990’s relying on materials like gypsum wallboard (drywall) and
stucco that would crack and lose both strength and stiffness during a moderate-to-large
earthquake. Numerous traditional Japanese timber buildings collapsed during Kobe Earthquake
due to the unbraced soft story of the first floor that had large garage and/or window openings as
shown in the earthquake picture (photo 3). Such non-engineered wooden houses are typically
unbraced or lightly braced one- and two-story post and beam construction with heavy tile roofs.
Earthquake damage to you home/business is influenced by construction materials. Older
construction materials like unreinforced masonry and non-ductile reinforced concrete pose the
greatest risk to life safety and are no longer allowed to be built in California because they have a
documented poor performance during earthquakes. However, many of these buildings still exist
and are not retrofitted. Severe damage or structural collapse is expected within 30 miles from
active major fault lines and moderate damage for buildings further away. On the other hand,
modern reinforced masonry construction, wood construction, ductile reinforced concrete
construction, and steel construction generally perform better and are less likely to have severe
earthquake damage. Detailed information on construction materials follows:
Masonry Construction
Unreinforced masonry is an old construction method where bricks, hollow clay tiles, stone,
concrete blocks, or adobe with mortar form the bearing walls without using steel reinforcing bars.
However, the mortar holding the masonry together is not strong enough to resist earthquake
lateral forces even during moderate ground shaking, where the masonry walls may buckle or pull
away from floor or roof anchorage leading to severe damage or partial collapse. Earthquake
damage may be reduced by anchoring and bracing the exterior and interior masonry walls to the
structural members of the roof and floor framing, or by installing an inside steel frame and bolting
it to the masonry wall. In case the masonry walls are damaged during an earthquake, it would be
very expensive to shore up the building, remove the damaged walls, and put in new walls.
Concrete Construction
Non-ductile reinforced concrete construction is generally the case for buildings 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-to-column or beam-to-wall connections is inadequate. During ground shaking, the
concrete walls may buckle or pull away from floor or roof anchorage and the building, or a portion
of it, may collapse. These buildings can be strengthened by adding new concrete walls, steel
bracing, or wrapping the columns with confining material. Earthquake pictures from Kobe
Earthquake illustrate the difference in the structural performance of non-ductile versus ductile
reinforced concrete structures. Photos 1a and 1b show a complete collapse of the sixth floor of the
eight-story reinforced concrete Kobe City Hall that was built in the 1960's. This type of non-ductile
behavior that leads to shear failure of the columns was due to the very light transverse
reinforcement that was widely spaced. Photo 2 shows a "relatively" ductile behavior of a reinforced
concrete frame garage structure. Although the structure is seriously damaged, however, no
injuries were reported or even damage to the vehicles beneath it.
Wood Construction
This is by far the most prevalent type of construction used for single-family homes and apartment
buildings in the United States, Japan and elsewhere. They generally perform better during ground
shaking than other construction materials and are less likely to be damaged especially wooden
houses with structural plywood shear walls. However, earthquake damage is most likely in older
timber homes built before the 1990’s relying on materials like gypsum wallboard (drywall) and
stucco that would crack and lose both strength and stiffness during a moderate-to-large
earthquake. Numerous traditional Japanese timber buildings collapsed during Kobe Earthquake
due to the unbraced soft story of the first floor that had large garage and/or window openings as
shown in the earthquake picture (photo 3). Such non-engineered wooden houses are typically
unbraced or lightly braced one- and two-story post and beam construction with heavy tile roofs.
Steel Construction
Steel construction became one of the most common types of construction used for buildings although it is
a young concept. Numerous factors have contributed to the growth of this market, especially in California
and Japan, where engineers had confidence in structural steel as a reliable construction material for
earthquake resistant design. They also believed that steel moment frames would behave in a ductile
manner, bend under earthquake loading, but not break. However, numerous steel frame structures were
subjected to some of the strongest motions ever recorded during Northridge and Kobe earthquakes
leading to unexpected earthquake damage that required launching extensive research projects.
Steel frame construction is composed of steel beams which are horizontal framing members, steel
columns which are vertical framing members, and steel braces which are diagonal members in the
vertical planes. Braced steel frame (shown in Figure 1) has simple beam-to-column connections in which
the braces resist the lateral earthquake forces. Earthquake damage includes stretching or buckling of
braces with cracked welds or failed bolted connections. Unbraced steel frame (also called moment frame)
has rigid beam-to-column connections that are welded (shown in Figure 2) or bolted (shown in Figure 3) .
Earthquake damage includes major cracks in welded connections, broken bolts, and enlarged bolt holes.
Moment frame is more flexible than braced frame resulting in much larger displacements during
earthquakes that will increase the risk of earthquake damage to nonstructural components and home
contents (refer to last title of this web page for more information).
Steel construction became one of the most common types of construction used for buildings although it is
a young concept. Numerous factors have contributed to the growth of this market, especially in California
and Japan, where engineers had confidence in structural steel as a reliable construction material for
earthquake resistant design. They also believed that steel moment frames would behave in a ductile
manner, bend under earthquake loading, but not break. However, numerous steel frame structures were
subjected to some of the strongest motions ever recorded during Northridge and Kobe earthquakes
leading to unexpected earthquake damage that required launching extensive research projects.
Steel frame construction is composed of steel beams which are horizontal framing members, steel
columns which are vertical framing members, and steel braces which are diagonal members in the
vertical planes. Braced steel frame (shown in Figure 1) has simple beam-to-column connections in which
the braces resist the lateral earthquake forces. Earthquake damage includes stretching or buckling of
braces with cracked welds or failed bolted connections. Unbraced steel frame (also called moment frame)
has rigid beam-to-column connections that are welded (shown in Figure 2) or bolted (shown in Figure 3) .
Earthquake damage includes major cracks in welded connections, broken bolts, and enlarged bolt holes.
Moment frame is more flexible than braced frame resulting in much larger displacements during
earthquakes that will increase the risk of earthquake damage to nonstructural components and home
contents (refer to last title of this web page for more information).
Home Safety
Earthquake damage to your home or business is dependent of its proximity to active fault lines; soil
properties, depth and strength below the concrete foundations; earthquake-related natural hazards like soil
amplification, landslides and liquefaction susceptibility; and several other structural-related factors such as
construction materials discussed above as well as:
Concrete Foundations Conditions
Your home or work place may be old and does not meet the seismic requirements of the current building
codes. The structure should have enough anchor bolts connecting the frame to the concrete foundations to
prevent overturning and collapse during strong ground shaking. Upgrading the anchor bolts to 3/4 inch
diameter at a maximum of 4 feet spacing and using large and thick square washers to secure the
anchorage of the sill plate to the concrete foundations is the first and most important step toward home
safety.
Crawl Space is Unbraced
The crawl space or basement may not be adequately braced or completely unbraced. The cripple walls
can be effectively braced using 3/8 inch structural grade plywood panels on the inside surface of the crawl
space or basement extending from the sill plate to the base of the floor joist and adequately nailing them to
the studs. This fix can help your home to survive strong ground shaking.
Soft Story is Unbraced
Buildings with soft story in the first floor (shown in Earthquake Pictures 4) without strengthening or effective
bracing are structurally weak due to the presence of large openings such as garage doors or windows.
They may lean or collapse during an earthquake, regardless construction materials, which was the case of
numerous buildings during Northridge and Kobe earthquakes. This can be fixed by bracing the walls of the
soft story using steel members or strengthening the walls using specially-detailed plywood panels.
Year Built
Buildings 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
Buildings with two-story or above are more likely to suffer earthquake damage than single-story homes.
Earthquake damage to your home or business is dependent of its proximity to active fault lines; soil
properties, depth and strength below the concrete foundations; earthquake-related natural hazards like soil
amplification, landslides and liquefaction susceptibility; and several other structural-related factors such as
construction materials discussed above as well as:
Concrete Foundations Conditions
Your home or work place may be old and does not meet the seismic requirements of the current building
codes. The structure should have enough anchor bolts connecting the frame to the concrete foundations to
prevent overturning and collapse during strong ground shaking. Upgrading the anchor bolts to 3/4 inch
diameter at a maximum of 4 feet spacing and using large and thick square washers to secure the
anchorage of the sill plate to the concrete foundations is the first and most important step toward home
safety.
Crawl Space is Unbraced
The crawl space or basement may not be adequately braced or completely unbraced. The cripple walls
can be effectively braced using 3/8 inch structural grade plywood panels on the inside surface of the crawl
space or basement extending from the sill plate to the base of the floor joist and adequately nailing them to
the studs. This fix can help your home to survive strong ground shaking.
Soft Story is Unbraced
Buildings with soft story in the first floor (shown in Earthquake Pictures 4) without strengthening or effective
bracing are structurally weak due to the presence of large openings such as garage doors or windows.
They may lean or collapse during an earthquake, regardless construction materials, which was the case of
numerous buildings during Northridge and Kobe earthquakes. This can be fixed by bracing the walls of the
soft story using steel members or strengthening the walls using specially-detailed plywood panels.
Year Built
Buildings 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
Buildings with two-story or above are more likely to suffer earthquake damage than single-story homes.
Earthquake Damage to Nonstructural Components and Home Contents
Nonstructural components include exterior wall panels, cladding, interior partitions, and suspended
ceilings. Home contents such as electrical and mechanical equipment, ducts, water and gas pipes, water
heaters, hanging objects, furniture, home electronics, and dishes. Falling exterior walls and cladding, and
falling interior ceilings, light fixtures, pipes, equipment, and other nonstructural components or home
contents also cause deaths and injuries. As building codes improve and buildings remains standing
during earthquakes due to seismic retrofit, the relative importance of earthquake damage to nonstructural
components and home contents increases. Recent earthquakes demonstrated that the economic losses
from nonstructural components and home contents have typically been comparable to the structural
losses. Risk mitigation of nonstructural components and home contents has not been regulated in any
building code until very recently, which is still a simple approach that is not tested yet in actual
earthquakes. In fact, many of these losses are simple to prevent by securing nonstructural components
and home contents to the studs of the interior walls or floors of the building. This is one of the most
important ways that individuals can reduce their own economic losses as such risk mitigation techniques
require no special expertise, materials or tools to implement.
Nonstructural components include exterior wall panels, cladding, interior partitions, and suspended
ceilings. Home contents such as electrical and mechanical equipment, ducts, water and gas pipes, water
heaters, hanging objects, furniture, home electronics, and dishes. Falling exterior walls and cladding, and
falling interior ceilings, light fixtures, pipes, equipment, and other nonstructural components or home
contents also cause deaths and injuries. As building codes improve and buildings remains standing
during earthquakes due to seismic retrofit, the relative importance of earthquake damage to nonstructural
components and home contents increases. Recent earthquakes demonstrated that the economic losses
from nonstructural components and home contents have typically been comparable to the structural
losses. Risk mitigation of nonstructural components and home contents has not been regulated in any
building code until very recently, which is still a simple approach that is not tested yet in actual
earthquakes. In fact, many of these losses are simple to prevent by securing nonstructural components
and home contents to the studs of the interior walls or floors of the building. This is one of the most
important ways that individuals can reduce their own economic losses as such risk mitigation techniques
require no special expertise, materials or tools to implement.