building has what is called a "soft story" condition.
This term is used to describe any building that has a habitable room
or rooms above a garage, carport, or porch area that was not specifically
designed to transmit shear (sideways moving) forces to the story
above. The failure of this kind of building with a soft story condition
was the primary cause of loss of life in the Northridge Earthquake.
In Santa Monica, all commercial soft story buildings have been required
to have a retrofit and most of that work has been done. Los Angeles
is currently drafting an ordinance to require retrofitting of all "soft
In California there is an earthquake hazard disclosure law requiring the seller
to disclose to the buyer at the time of sale the existence of certain known earthquake
hazards, such as lack of bolting, existence of cripple walls with no shear paneling,
hot water heaters that are not properly strapped, etc. One of the conditions
required to be disclosed as a hazard is the existence of a habitable room or
rooms over a garage.
Large open areas such as carport or garage door openings do not transfer shear
(sideways moving forces) unless they have an element specifically designed to
transfer shear built into them, such as a ridged steel frame. Today, whenever
you see a home being built where there will be a room over the garage, you always
see a steel frame built first. Try to visualize two dominos standing on their
small ends, opposite each other. I can load four or five bricks on the dominos
with no difficulty because the dominos are strong enough to carry the load, or
downward force due to the weight. But if I go to the side of my structure and
apply a tiny shear force with my breath, the whole thing comes crashing down.
Or imagine a shoe box bottom turned upside down with a brick on top of it. Even
if I jiggle this a little, it is pretty stable. Now imagine removing one of the
sides of the box. A rectangle can become a parallelogram without breaking any
of the sides.
Current construction for house with rooms above garages typically have steel
frames built into them at the time of construction. Steel frames can adequately
transfer the shear forces to the second story. It is, of course, somewhat more
difficult to install after the house has been built, but this can, and very frequently
is, be done.