Definition of %g
What is "% g"?
When acceleration acts on a physical body, the body experiences the acceleration as a force. The force we are most experienced with is the force of gravity, which caused us to have weight. The units of acceleration of the map are measured in terms of g, the acceleration due to gravity. An acceleration of 11 feet per second per second is 11*12*2.54 = 335 cm/sec/sec. The acceleration due to gravity is 980 cm/sec/sec, so an acceleration of 11 feet/sec/sec is about 335/980 = 0.34 g. Expressed as a percent, 0.34 g is 34 % g.
What is the relation to building damage?
Pre-1940 dwellings are likely to perform poorly in earthquake shaking
Pre-1975 dwellings are likely to have some vulnerabilities to earthquake shaking
Some post-1985 dwellings, built to California earthquake standards, have experienced
severe shaking (60 % g) with only chimney damage and damage to contents.
Simple retrofit measures can greatly reduce the vulnerability of dwellings:
- Bolt house to foundation
- Provide resistant paneling or bracing to cripple walls
- Reinforce brick chimneys or replace with patent metal chimneys
NOTE: The 10 percent g value was chosen because on the average it corresponds to
Modified Mercalli Intensities VI to VII, levels of threshold damage, in California, for
ground motions within 25 km of the earthquake epicenter. This value should not be
used in the case of particular buildings, because
- the relation between intensity and peak acceleration is quite variable,
- for more distant sites, longer duration ground motions may cause damage
at lower acceleration values, and- buildings differ greatly in their vulnerability.
Furthermore, the hazard maps combine near and distant ground motions indiscriminately.
