Earthquake Risks in Southern California

By John van Blerck, P.E.
JVB Construction Management, Inc.

July 9, 2010

 

Introduction

As earthquakes in Southern California appear to be occurring more frequently, its time to look at the buildings we occupy and consider what work may or should be done to some of the structures to reduce the risk of injury, loss of life or property damage.

During an earthquake, the structural system of a building is put to the test; ground forces try to tear the building apart, sometimes resulting in severe damage.

A building endures both lateral and upward forces. Lateral forces shake the building, weaken the frame and can result in the structure sliding off the foundation. Upward and downward forces may result in overturning and the building being lifted off the foundation.

Structures and buildings that may be more susceptible to sustaining damage and causing life threatening injuries or deaths include the following:

• Buildings constructed without anchor bolts securing walls to foundations
• Un-reinforced or partially reinforced masonry structures
• Buildings with un-braced parapet walls
• Buildings on raised foundations
• Buildings with living space above garages, with inadequate, missing or undersized shear walls.
• Buildings with incomplete “load paths” or structural connections from foundation to walls, to subsequent floors and roof.

Solutions

Due to advances in Structural Engineering, as well as lessons learnt from previous earthquakes, newer buildings built to current codes have a far greater ability to withstand earthquake forces.

Design basics focus around developing a continuous load path through the building from the foundation through each subsequent wall and floor up to the roof. The building frame and structural system needs to be strong enough to endure and transfer the earthquake forces through the building. A building with a complete load path will have increased ability to withstand earthquake forces with less damage if all components are connected together, and the building is secured to the foundation.

There are a variety of repairs that can be done to existing, deficient building structures of various types to improve the life safety of the buildings, and significantly reduce the extent of damage that may occur in the event of an earthquake, including the following:

• Attachment of interior columns to the beams supported,
• Installation of plywood or OSB to exterior cripple and other walls to increase shear strength.
• Installation of anchor bolts, or retrofit plates to secure wood framing to the foundation,
• Installation of plywood shear panels, wood blocking and framing angles,
• Installation of straps and connectors between floors.

Benefits of a structural retrofit include better chances of withstanding an earthquake, possibly lower earthquake insurance costs, and increases in resale value of the building.

John is a registered Civil Engineer, and President of JVB Construction Management, Inc., a San Diego based Construction Management Company. Visit JVB at www.jvbcm.com.