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Levels of Modifications
Seismic retrofit is primarily applied to achieve public safety,
with various levels of structure and material survivability determined
by economic considerations:
- Public safety only. The goal is to protect human life, ensuring
that the structure will not collapse upon its occupants or
passers by, and that the structure can be safely exited. Under
severe seismic conditions the structure may be a total economic
write-off, requiring tear-down and replacement.
- Structure survivability. The goal is that the structure,
while remaining safe for exit, may require extensive repair
(but not replacement) before it is generally useful or considered
safe for occupation. This is typically the lowest level of
retrofit applied to bridges.
- Structure usability. The structure is to be undiminished
in its utility, although it may be necessary to perform extensive
repair or replacement of components in preparation for the
next major seismic event. This is typically the lowest level
of retrofit applied to hospitals, fire
fighting stations, public safety (police) command centers,
and the like and is often the most economical level of retrofit
and design for transportation infrastructure such as rail and
highway roadways, bridges, and tunnels. This level of retrofit
is required for water supplies used for fire fighting - reservoirs,
water lines, and hydrants, and is also needed for a few hours
after a seismic event for household water supplies, which may
be used for emergency fire fighting.
- Primary structure undamaged and the structure is undiminished
in utility for its primary application. A high level of retrofit,
this ensures that any required repairs are only "cosmetic" -
for example, minor cracks in plaster, drywall and stucco.
This is the minimum acceptable level of retrofit for hospitals.
- Structure unaffected. This level of retrofit is preferred
for historic structures of high cultural significance.
- The most
common structures requiring extensive retrofit are bridges,
road viaducts, towers, and mid-size buildings.
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