by Kerwin Lee, AIA and Steven Winkel, FAIA, CASp
The earthquake on August 24th was just a reminder that we live in earthquake country. It has been about 25 years since Loma Prieta shook the Bay Area on October 17, 1989, and over 100 years since the great San Francisco earthquake on April 18, 1906. Although earthquakes can be disastrous event, in my opinion they may be a little overrated. Compared to hurricanes, tornadoes and other natural disasters, there has only been one loss of life from an earthquake in 25 years.
What earthquakes do tell us is that our current building regulations are working. The majority of the damage I saw on the news involved the same six buildings or so, all of which are associated with unreinforced masonry building (UMB) or masonry construction. For the most part, their construction predated the current building regulations and standards. The current standards have been evolving for over 40 years. Each time the standard is changed, the bar for earthquake standards has been raised.
In the beginning, the mindset was to have buildings survive an event and to avoid total collapse. The key to this philosophy was to protect the occupants and allow them to safely evacuate the building. However, the building was not always salvageable or repairable. This philosophy has changed. There are many types of facilities that need to not only survive a major event, but continue to be usable afterward. Hospitals and essential service facilities (police, fire, and emergency responders) all fall into this category. The University of California (UC) system has also adopted this philosophy because they cannot afford to lose the use of a facility after an event. The UC wants to continue to operate with little or no disruption. In part, this is an economical decision.
So, what about the most vulnerable buildings, the UMB’s? Under current building codes (IBC and CBC), existing buildings that are not coming under any improvements need not make changes. However, if the building undergoes any improvements, these changes must comply with current code, but only for those elements that are changed. One of the triggers for seismic improvement is a change in use. When, say, a warehouse is converted into a business office (B-Office) or school (E-Educational),an evaluation of the building is required to assure that it is in compliance with current seismic requirements. [See Change in Use under Chapter 3408.] Things get tricky if it is determined that seismic improvements are required. What standard are the improvements required to meet? For the most part, an existing building cannot comply with current standard without a complete structural upgrade, and even then a building is unlikely to meet 100% of present standards. The final decision on how to approach a structural conversion is left up to the Building Department and structural engineer, who must determine what is practical and economically viable for the project.
The problem still remains, however: existing UMBs that are not undergoing any changes. This now becomes an economic issue, because it takes money to correct the hundreds of buildings that are at risk. Some jurisdictions, such as San Francisco and Los Angeles, have enacted local ordinances to address this issue. San Francisco has a UMB Ordinance requiring masonry buildings make improvements. This ordinance includes public money to help complete this process. There were close to 2,000 buildings on the UMB list in 1992. At current date, more than 150 buildings still remain unimproved.
I concu rwith Kerwin’s comments about UMBs. I would add that there are a couple of other broad categories of damage that touch every person who owns a residence, including many of our own members. The way I finally came to understand earthquake forces was to visualize the effect of one “G” of lateral acceleration, which could well happen during a major quake on the Hayward fault. This force is capable of turning a house on its side. The force of gravity, normally acting downward, pulls sideways on anything not well anchored with the same force that causes a glass to fall to the floor when you drop it.
Many houses have been damaged by chimney and cripple wall failures. I urge everyone who lives in the East Bay chapter area to take a close look at their foundations and at masonry chimneys. Chimneys should be braced above the roofline, and tall masonry chimneys reduced or replaced with modern flues. In addition, many older houses are not bolted to their foundations and often also have unbraced cripple walls. If there is a sound concrete foundation the sill can simply be bolted down with expansion anchors on code mandated spacing. Cripple walls should be braced with plywood to provide a shear connection between the house and the foundation. For a minimal investment older houses can be made more quake resistant. Many local building departments have good handouts describing simple earthquake resistant construction. I would urge each of our members to examine their own houses and tells their friends and clients to do the same. Note also that even if you have earthquake insurance– which likely only applies to a small percentage of you–the deductible is very high. The damage prevented by mitigating cripple wall and chimney failures will more than pay for itself in non-insured damage prevention. Also, while undertaking these measures be sure to strap your water heater tank to the structure to prevent it from overturning in an earthquake. That is a cheap and easy mitigation measure for loss prevention.
Kerwin Lee, AIA, CASp is an ICC-Certified Accessibility Inspector and Building Plan Examiner. Contact him at firstname.lastname@example.org.
Steven R Winkel, FAIA, PE, CASp can be contacted via The PREVIEW Group, Inc. Architects Providing Regulatory Solutions.