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Architecture links, Didactic

Designing for Hurricanes and Floods

biloxi

A general overview of some of the things we do here in Biloxi to mitigate the risk of building houses in a hurricane-prone area. By no means a comprehensive guide.

Elevation. The house’s finished floor should be above the expected flood elevation. The building codes here currently go by the city’s maps, but FEMA has issued new Base Flood Elevation maps which are typically higher than the city’s. So, if your property is 12 feet above sea level, and FEMA’s maps show a flood elevation on your property of 17 feet, you would need to elevate approximately 5 feet to meet FEMA’s recommendations.

Foundations. Since many of our houses are elevated, we do very little slab-on-grade construction. Options include steel-reinforced concrete block piers, pressure-treated posts set into concrete footings, or poured concrete columns. A simple and effective system that we have recently begun to use extensively is to drive pressure-treated piles into the ground. Depending on the system used and the height above ground, some foundations may require bracing.

Framing. One of the keys to hurricane-resistant construction is good uplift connections. Every vertical connection is important: the columns are tied to the footings by steel reinforcement, the floor joists are bolted to the columns, etcetera. This way the house cannot be simply lifted off its foundation by the floodwaters or wind, as many were in Katrina. In the wall, a 1/2″ shear siding (OSB or similar) extending from the bottom of the rim joist to the top plate creates a good uplift connection. Additionally, some studs will be secured vertically with steel straps.

Shear walls. The shear siding mentioned above also helps prevent shearing or racking caused by a high wind force coming from one direction. Building codes limit the number and location of openings in the exterior walls of a house so they can function effectively as shear walls. In some houses, especially longer ones, an additional interior shear wall may be added.

Windows, etc. One danger of hurricanes is their propensity for throwing debris around at high velocity. Having a window break is very bad because it ruptures the building’s envelope and allows wind and destructive pressure differences to go to work on the inside. Windows should therefore be chosen for good impact resistance. Hurricane shutters are a good idea too, although they are expensive and I rarely see them used.

Roof. Rafter ties at each rafter provide an uplift connection between the wall and roof. The roofing material should obviously be well fastened. Large overhangs and things like covered porches can create problems if the wind is able to get underneath and exert a strong upward force. In this case, it may be possible to use columns (strapped to the roof and floor/foundation) to provide vertical reinforcement. Alternatively, some portions of the roof could be designed to blow away without taking the rest with them, acting as sacrificial lambs for the house.

More information can be found online, including at Wikipedia.

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