The materials used to construct a structure affect its ability to weather an earthquake. The
type of material that is best depends on the size of the building. Small structures, like
houses, do better if they are constructed of materials that bend and sway such as wood and
steel rather than brick, stone, and adobe, which are brittle and will break. Brittle materials
are less likely to break if they are reinforced by steel or wood. Larger buildings must sway,
but not so much that they touch nearby buildings. Counterweights and diagonal steel beams
are used to hold down sway. A completely different approach for large buildings is to place
them on rollers so that they move with the ground but do not collapse. Buildings may also
be placed on layers of steel and rubber, which absorb the shock of the passing seismic waves.
Structures that fail usually do so because they are weak at the connections, such as where the
walls meet the foundation. Earthquake safe buildings are well connected. In a multi-story
building, the first story must be supported or the structure may collapse (Figure7.47).
Figure 7.47: The first floor of this San Francisco building is collapsing after the 1989 Loma
Prieta earthquake. The building is being held up by the two walls that are not in the
photograph and the strength of the upper floors. The two walls that are in view are not
strong because they had doors in them. ( 42 )
Older structures can be retrofitted to be more earthquake safe. Retrofitting includes adding
steel or wood to reinforce a buildings structure and its connections (Figure7.48). Elevated
freeways and bridges can also be retrofitted so that they do not collapse. The goal of
retrofitting is different depending on the type of structure being altered. Most structures are
retrofit only to a strength that protects human life. More important structures, like bridges,
are made to survive intact, but may need extensive repair after the earthquake. Structures
that need to be used in an emergency, like hospitals, are retrofit to higher standards so that
they will need only superficial repairs after an earthquake. The highest level of protection is
a retrofit that will allow a building to survive unaffected. This is very expensive and is only
done for buildings that are of great historical or cultural significance.