Earthquake Retrofitting


Loma Prieta Earthquake - 1989 Cripple Wall Strengthening
1989 Loma Prieta Earthquake – San Francisco                                 Residential Cripple Wall
Protecting Your Largest Investment

Older buildings throughout California are vulnerable to extensive damage from seismic ground shaking.  All jurisdictions allow voluntary structural strengthening (earthquake retrofitting), and some have mandatory programs.  Buildings erected before 1981, when stricter structural design standards were adopted by the state, are more susceptible to earthquake damage.  While some jurisdictions have adopted minimum design standards for earthquake retrofitting, building owners may generally choose any prescriptive (“cook book”) or performance (engineered) structural standard for retrofitting their buildings.  The key to selecting the “right” standard is understanding the expected amount of damage your building will have after an earthquake:

  • Collapse safeguard               –  costly repairs for re-occupancy or demolition
  • Re-Use safeguard                   –  less costly repairs for re-occupancy
  • Re-Occupancy  safeguard  –  cosmetic repairs for re-occupancy
Soft Story Buildings

In general, buildings must be able to adequately resist two forces-of-nature:

  • Vertical Forces (gravity)        –  building’s self-weight and weight of its contents
  • Horizontal Forces (lateral)  –  wind and earthquake.

Many older buildings have a built-in structural weakness called “Soft Story” which makes them particularly vulnerable to earthquake forces.  A Soft Story has less than 80% of the horizontal strength of the story-above.  Soft Stories are typically found in buildings which have large wall openings on the ground floor (e.g., store-front windows, garage doors) or have cripple walls sandwiched between the foundation and the building above (see photographs above).

Earthquake Soft Story explained by Bay Area Soft Story Retrofit Resources. Includes Videos and Photos 

Measuring Earthquakes

There are three widely used methods to compare the amount of energy released by earthquakes:

  • The Mercalli Intensity Scale (circa 1900) measures the observed effects of an earthquake (level I  =  unnoticeable, level XII  =  total destruction).
  • The Richter Magnitude scale (circa 1935), measures the amplitude of an earthquake’s “S” waves (ground-shaking force) on a seismograph.
  • The  Moment Magnitude Scale (circa 1970)  measures the amount of movement along an earthquake’s fault zone using ground seismometers and computers.

The Richter and Moment Magnitude Scales are logarithmic.  A 7.0 earthquake releases about 31 times more energy than a 6.0 earthquake.   The maximum magnitude of a earthquake is estimated to be 9.

Check the Links below for additional information about Earthquake Retrofitting.



  • Forums
    • Earthquake Retrofitting Blog
  • Library

California Office of Emergency Services

California Earthquake Authority

California Seismic Safety Commission

California Building Code, Existing Building Code, and Historical Building Code

Association of Bay Area Governments

Federal Emergency Management Agency

Structural Engineers Association of Northern California

Hardy Frame


Hardy Frame & Zone Four
Hardy Frame        &         Zone Four