Volcano Watch — The hazards of post and pier foundations

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The column last week summarized the types of damage to structures resulting from shaking during large earthquakes beneath Hawai‘i.

The hazards of post and pier foundations...

The hazards of post and pier foundations

(Public domain.)

The column last week summarized the types of damage to structures resulting from shaking during large earthquakes beneath Hawai‘i. The reports on the earthquakes in Kona in 1951, Kalapana in 1975, and Ka'oiki in 1983 each specifically noted that homes on post and pier foundations were more likely to sustain damage than homes on slab foundations. In an effort to mitigate against damage from future large earthquakes to homes on such post and pier foundations, we asked for help from the American Institute of Architects (AIA), Hawai‘i Island Section.

Unfortunately, the key part of last week's column, the figure showing an inexpensive retrofit for the most common home type on post and pier foundation, was not published due to space limitations. Much of the article was difficult to understand without being able to examine the accompanying figure. To alleviate any confusion this may have caused, we have rerun parts of last weeks column.

The current County code which incorporates design for high wind conditions and which became effective in November 1993 permits only slab on grade or continuous exterior foundations. Alternate systems require engineering design by a licensed professional. Post and pier foundations which have been common to the Islands have proven inadequate for high wind conditions and for earthquakes.

Appropriate building codes that are strictly enforced are the primary method to reduce structural damage, injuries, and deaths caused by strong winds or earthquakes. The remarkably small number of deaths caused by the Northridge earthquake in California demonstrates that appropriate building codes do indeed save lives. Most of the serious injuries and deaths occurred in buildings that were built long ago, before the code was adequate for the earthquake hazard in that part of California. The unified building code (UBC) is constantly changing as we learn new, and usually painful, lessons from each large earthquake.

Light wood frame structures which are not securely tied together and to a solid foundation system are particularly vulnerable to wind damage. However, that very lightness can be a benefit to earthquake resistance because seismic forces on a building increase with the weight of the building. Even a single wall building with sufficient wall areas and interior partitions may survive an earthquake intact; the weak point has historically been the local style post and pier foundation systems which do not provide adequate connection between the building and the ground.

While a continuous foundation is logical for high wind resistance, affording an adequate anchor to resist uplift loads, the generally smaller lateral loads induced by an earthquake can be resisted by a system of short length shear panels strategically located.

A workable system for retrofitting existing rectangular frame houses is illustrated here with the hope that owners of such homes will consider the benefits of adding such elements to their house. There are numerous homes like the one illustrated on the island. The cost of this installation, if constructed by a contractor, would be from $2,500 to $3,000 according to estimates we have informally obtained. Homeowners who have sufficient skills could perform this work themselves.

The County Building Department officials will grant permits for this work without involvement of design professionals for the condition illustrated when the lot is roughly level and the floor is no more than four feet above grade. Similar measures could be effective on sloping lots or for houses with irregular (non-rectangular) floor plans, or for those with attached carports or lanais, although professional engineering would be necessary.

The booklet from the California Seismic Safety Commission, "The Homeowners Guide to Earthquake Safety" is available at public libraries and at cost ($3) at the office of the following AIA Architects: Taylor Cockerham in Hilo, Aza Summers in Waimea, Virginia Macdonald in Volcano, and Jack Parazette in Kona (see yellow pages for addresses and phone numbers). This is the best booklet we have seen on personal earthquake safety.