Simple
Facts about Earthquakes
An
earthquake is a sudden, rapid shaking of the Earth caused by the
breaking and shifting of rock beneath the Earth's surface.
Buildings with foundations resting on unconsolidated landfill or
unstable soil are most vulnerable
Earthquakes
send out explosive shock waves that travel in all directions up
the surface of the earth. These waves impart horizontal and
vertical forces on buildings. The vertical forces ordinarily
cause the building to move up and down safely with the ground.
However, horizontal force waves cause buildings to move laterally
and vibrate back and forth during an earthquake. Unless the
building is properly constructed, these horizontal forces will cause
damage.
Building's
Earthquake Resisting Systems
Buildings
are usually designed to resist earthquake forces with two structural
systems, Gravity Force Resisting System (fig.
1) and Horizontal Force Resisting System (fig.
2).
The
Gravity Force Resisting System supports the vertical gravity loads
on the building. It consists of beams (joists or roof rafters)
and vertical framing (wood stud walls or posts). They transfer
the gravity loads to the foundation.
The
Horizontal Force Resisting System transfers the horizontal forces
a building faced safely to the ground. It consists of diaphragms
(roof and floor structures) and shear walls. To be effective,
each part in the system must be adequate and properly connected
to other parts in the system. They must form a continuous
load path.
Seismic
Retrofit Basics
Seismic
retrofitting typically deals with four areas in the house structure
(cripple walls, shear walls, connections and
foundation). It tries to reinforce all these elements
to strengthen the Gravity and Horizontal Force Resisting System
of a building.
Cripple
Walls
A
cripple wall is a wall that is less than full story height.
It usually occurs between the first floor and the foundation and
is generally the weakest part of older buildings.
Because seismic forces in the building accumulate all the way down
to the ground, they are greatest at the base of the building.
That is why retrofitting usually strengthens
the cripple walls.
Shear
Walls
Shear
walls are typically wood frame stud walls covered with a structural
sheathing material like plywood. They provide resistance both
to the horizontal forces and the uplift forces.
After
the 1994 Northridge Earthquake, the City of Los Angeles and Structural
Engineers of Southern California formed a joint task force to re-evaluate
seismic provisions of the building code. The task force recommended
significant changes to the design and construction of wood framed
shear walls. These changes included lowering the allowable
height-to-width ratios and shear strength values. (Check the Photos
& Drawings web page for additional retrofit diagrams)
Connections
Connections
consist mainly of hold-downs (uplift connections) and shear connections.
Each of these use bolts, anchors, plates and other connectors to
secure elements of the Gravity and Horizontal Force Resisting Systems
to their proper places and form continuous load paths for both resisting
systems.
Foundation
Since
all the forces in a building during an earthquake are transferred
to the foundation through the Force Resisting Systems, it is important
to make sure the condition of the foundation is sound.
There are two concerns for the foundation condition: deterioration
and cracking. For deterioration, if parging or repointing
cannot repair the foundation wall, a full foundation retrofit is
required. As of cracking, depending on the size of the crack,
concrete cracking can be repaired with epoxy injections or cementitious
mortars.
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