Volcano Watch — Earthquake building tips for a hyper-seismic isle

Over the last several weeks, we have run suggestions of specific things that you can do to reduce your exposure to damage caused by earthquakes. There have been numerous damaging earthquakes affecting most parts of the Big Island during the last few decades. The following excerpts from reports issued assessing the damage caused by the magnitude 6.9 Kona earthquake of 1951, the magnitude 7.2 Kalapana earthquake of 1975, and the magnitude 6.6 Ka'oiki earthquake of 1983 indicate that post-and-pier construction has been a significant factor in much of the damage to homes.

In order to provide some remedies for retro-fitting such construction to survive earthquakes, we have asked for some help from the Hawai‘i Island Section of the American Institute of Architects (AIA). Jack Parazette (AIA) of Kona kindly provides the following retrospective of damage from prior earthquakes and a relatively simple and inexpensive retrofit for the most common style of post-and-pier construction. As the population on Hawai‘i continues to increase, the amount of damage from similar future events will certainly be greater.

The magnitude 6.9 Kona earthquake of August 21, 1951, was centered offshore Kealakekua Bay. The report issued noted that "Damage extended from Holualoa to Honu'apo. It was greatest from Kealakekua to Ho'okena. At Ho'okena the underpinning of two old houses gave way and the houses dropped to the ground. Near Kealia two other houses were similarly damaged. Damage to frame buildings can be traced to the failure of underpinning which was unsound because of age, poor material, or inadequate bracing. The most striking example of all was the Honaunau School where failure of the underpinning allowed the building to collapse partly and slump to the ground, deforming it so badly that it is considered a total loss."

The magnitude 7.2 Kalapana earthquake of November 29, 1975, was centered near Kamoamoa in the Puna District. The U.S. Geological Survey report noted that "a preliminary survey showed damage to 80 homes. Five poorly constructed or old houses were reported completely demolished." In Hilo, a house and garage shifted from their foundation. In Kalapana, a wood frame house shifted 1 meter from its foundation. In Kurtistown, a house and garage moved from their foundation. In Pahoa, three homes moved from their foundations. In Hawaiian Paradise Park, one house shifted off its foundation and cabinets toppled off walls. In Ka‘u, a house moved from a concrete foundation.

The magnitude 6.6 Ka'oiki earthquake of November 16, 1983, was centered near the Volcano Golf Course. The report issued by the U.S. Geological Survey noted the following damage: "houses with post and pier construction sustained greater structural damage, primarily because of lateral displacement of the piers and consequent decoupling of the piers from the houses. The piers often moved with respect to each other as well as to the ground. Most structures with concrete slab foundation, on the other hand, had little damage." In Wood Valley, "several houses of post-and-pier construction moved more than 0.5 m (18 inches) off their foundations." In the Volcano Golf and Country Club Subdivision, "48 houses had been built or were under construction in the subdivision. Of these, 18 houses with post and pier construction were displaced. Of the 30 other houses in the subdivision, 9 moved on their post and pier foundations and suffered minor damage, and the remaining 21 houses, most of which were on concrete slabs, showed no evidence of movement There were reports of extensive breakage inside many houses, regardless of foundation type." As far away as Hilo, "several houses were damaged as they moved downslope off their post and pier foundations in response to the earthquake shaking."

The report specifically noted the relationship between construction styles and damage. "Few houses with post and pier construction had structural ties between the house, the piers, and the ground, and each element was therefore left free to move with respect to the others. In the few cases where structural ties such as rebar and bracing were used between the house and the piers, they appear to have been too few, flimsy, or poorly connected. Neither rebar nor bracing was adequate when used alone."

The American Institute of Architects (AIA), Hawai‘i Island Section offers the following comments. 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 licenced professional. Post and pier foundations which have been common to the Islands have proven inadequate for high wind conditions and earthquakes.

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; 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; they slide.

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

A workable system for retro-fitting existing rectangular frame dwellings is illustrated here with the hope that owners of such structures will consider the benefits of adding these elements to their house. There are numerous dwellings like this on the island, many the product of drafting departments of the building supply firms. 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 would 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.

As we noted in a previous column, the booklet from the California Seismic Safety Commission, "The Homeowner's Guide to Earthquake Safety" will be available at public libraries and at cost ($3) at the office of the following AIA Architects: Taylor Cockerham, Hilo; Aza Summers, Waimea; Virginia Macdonald, Volcano; Jack Parazette, Kona. (See yellow pages for addresses and phone numbers).