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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