In an earthquake the ground rocks, twists, heaves and subsides, changing direction and speed all the while. Such violent and chaotic ground movement sets buildings in motion. Houses tend to shift off their foundations and some structural elements may overturn (Figure A). Houses literally come apart at the seams, section by section and piece by piece. But woodframe houses, if properly attached to the foundation and tied together structurally, can resist seismic loads and reduce the likelihood of earthquake damage.


The Effect of Lateral Force on Structural Elements

The light weight of wood frame buildings results in less force from inertia. Less force means less damage (Figure B). Wood's natural flexibility also is an advantage when seismic forces are brought to bear and the nailed joints in wood frame buildings dissipate energy and motion.

But wood's inherent earthquake resistance must be accompanied by design and construction techniques that take advantage of those characteristics. Structural wood panels nailed to wall framing add rigid bracing, help resist lateral loads and help tie framing members together Bolted connections at the sill plate/foundation joint help keep the house in one spot. Securely connected wall, floor and roof framing also help tie a house together and make it a single, solid structural unit. Proper connections will do more to hold a house together during an earthquake than any other single seismic design element.

Modern building codes require seismic design elements in new construction. Those elements typically include the measures mentioned above. Consult your local building codes for the requirements in your area. Older houses frequently need retrofitting if they are to withstand earthquakes. While this brochure deals primarily with retrofit applications, the same principles apply to new construction.

Figure B - When the Ground Moves
This series of illustrations shows, in an exaggerated way, what a house goes through during and after a simple north-south lurch.

In frame #1, the building and ground are at rest.

In frame #2, the Southern lurch begins and the building follows.
In frame #3, the ground is still pulling the building to the south, but the deformation of the building is reduced. The building is travelling at/near the speed of the ground.
In frame #4, the Northern lurch begins. The building will have to reverse its direction of travel; doubling the forces imposed by the first ground movement.
In frame #5, the building has changed direction. The top floor is the last to feel this change.
In frame #6, the ground is finally a rest, but the building is still in motion.
In frames #7 through #10, the building continues to move until the energy of the earthquake is dissipated.
Energy absorbing construction elements, like plywood shear walls, flex and deform; generating heat in the process.
Less flexible materials, like gypsum wallboard, absorb energy to a point, then they rupture. Diagonal cracks from the corners of doorways and windows are the result.
(Illustration courtesy of Fine Homebuilding magazine.)

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