Mitigation Assessment Team Report: Hurricane Opal in Florida (FEMA 281)
Hurricane Opal made landfall on Wednesday, October 4, 1995. It was classified as a Category 3 storm on the Saffir-Simpson scale. Fifteen counties in the Florida Panhandle were declared Federal disaster areas.
The most severe damage caused by Opal was concentrated along a 200-mile stretch of Florida's Gulf of Mexico shoreline. This is the area where the MAT conducted its field inspections.
Observations
Most of the structural damage associated with the storm was caused by coastal flood forces - storm surge, wind-generated waves, storm-induced erosion, and floodborne debris.
Slab Foundations. Many slab failures were noted in all types of structures. The major reason for these failures was the loss of support coupled with a lack of reinforcing in the slabs.
Pile and Pier Foundations. Three to seven feet of vertical erosion at the seaward row of piles was common. This erosion, coupled with insufficient penetration of the piles, led to structural damage to or collapse of primarily pre-FIRM structures. Undersized piles (6-inch diameter timber in some instances) were not sufficient to resist storm forces.
Framing Systems. The MAT found many examples of poor framing of timber floor beams and joists in platform-type construction. In particular, poorly fashioned beam-to-beam and joist-to-beam connections were common.
Recommendations
Application of V-Zone Construction Requirements. National Flood Insurance Program (NFIP) V-Zone construction requirements specify that new construction be elevated on piles, posts, columns, or piers and that the bottom of the lowest horizontal structural member (e.g., floor beam, joist) be at or above the Base Flood Elevation (BFE). Depending on the dimensions of those structural members, the resulting lowest floor elevations can be as much as 1.5 feet above the BFE.
Construction Materials. All materials should meet or exceed the minimum requirements for building materials in the Standard Building Code. All materials subject to flooding should resist damage, deterioration, corrosion, and decay due to inundation, precipitation, wind-driven water, salt spray, or other corrosive agents. Special consideration should be given to structural connectors such as hurricane straps and hangers to ensure that structural integrity is not compromised by corrosion.
Slabs and Grade Beams. Slabs-on-grade in areas known to be subject to storm-induced erosion and scour should be designed as freestanding structural elements and reinforced to withstand the loss of underlying soil.
Pile, Post, Column, and Pier Foundations. Foundations should be designed to accommodate all design flood, wind, and other loads acting simultaneously. Documented erosion and conical scour from Hurricane Opal should be considered in the determination of foundation embedment.
You may obtain a copy of this publication by contacting FEMA's Distribution Center at 1-800-480-2520, and requesting publication number FEMA 281.
Last Modified: Tuesday, 18-Apr-2006 14:36:49 EDT