Coastal Subsidence Study 2005 - Laserfiche WebLink

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are built for wave or flood risks by elevating on piers or columns. For example, the higher a house is elevated on a pier or column, the more stress the pier or column will be subject to during earthquake shaking. The building of a house on a pier or column can create a top heavy structure and a “soft” story (the area between the ground surface and the base of the elevated structure). The stress on the columns and piers would be a function of many factors including the amount of elevation and the weight on top. These are especially important factors for Kapoho because of the possibility that any new structures may need to be elevated even higher due to subsidence (i.e., building in freeboard as discussed in Section 2.4). In Figure 2-3, earthquake risk for the Hawaiian Islands is expressed as a percentage of gravity for events that have a 10% chance of exceedance every 50 years. Earthquake risk is the greatest for the southeast portion of Hawaii County, including the Kapoho area. This is attributed to the active volcano in the vicinity. Because Hawaii has the greatest earthquake risk, it is in seismic zone 4 under the Uniform Building Code. Kauai is in seismic zone 1, Oahu seismic zone 2a and Maui zone 2b. While it was outside the scope of this study to analyze the adequacy of the building codes for Hawaii County, the building department should make sure that any structures elevated on piers and columns to mitigate the damage from flooding or wave action under the National Flood Insurance Program should also be able to handle anticipated earthquake risk. As expressed previously, this is important because there will be a tendency to build higher on piers and columns given the subsidence recorded in the area. Some measures to increase column strength can be found in the Federal Emergency Management Agency’s Coastal Construction Manual and includes the use of cross bracing or knee bracing (see Chapter 4). 14