Wind Damage >> How To Survive A Hurricane

Foundations are at relatively low risk from wind forces, but are at higher risks from water forces in wave erosion zones and flood zones. In flooding and wave erosion zones, How To Survive A Hurricane homes are typically elevated above base flood elevation (BFE), defined as the elevation having a 1% chance of being reached in any given year. 

The primary methods that can be used to elevate homes in hurricane hazard areas are piers, posts and columns, and pilings. FEMA (1986a, 1995) makes several recommendations for their use. Piers, How To Survive A Hurricane which are the most common elevation support structures, are shallow supports installed in pre-dug holes. 

Piers are vertical support members supported entirely by reinforced concrete footings and designed primarily for vertical loads. Piers generally have shallow embedment, How To Survive A Hurricane with footings extending only slightly below the frost line. Piers can be used in areas with little velocity flow and shallow depth flooding. 

Because of shallow embedment, piers should not be used in areas where scour is a factor. The minimum standard for masonry block piers is 12 inches by 12 inches, with footings that are no smaller than 24 inches by 24 inches, and no less than 8 inches deep. Whether block or poured-in-place concrete, How To Survive A Hurricane piers should have steel reinforcing in both the pier and the footing. 

Piers offer the least lateral strength of the three elevation methods. Pier height should not exceed the shortest horizontal dimension of the pier by more than 10 times (Pilkey et al., 1981). Posts and How To Survive A Hurricane columns can be used in areas of moderate velocity flow and depth. Posts and columns differ only in size and application. 

Posts are smaller and can be made of wood, steel, or precast reinforced concrete. They are installed in pre-dug holes that are then backfilled with earth, crushed stone, or How To Survive A Hurricane gravel. Posts are usually square, making attachment to the house superstructure easier. While piers act as individual support members, posts must normally be braced. 

There are several techniques that can be used: wood knee and cross bracing, steel rods, and guy wires. Piles provide the most protection against flooding. Unlike posts, columns, and How To Survive A Hurricane piers, piles are driven, or water-jetted, deeper into the ground, making piles less susceptible to high-velocity flood waters, scouring, and debris impact. 

Piles are more slender than posts and usually made of wood, but steel and concrete piles are used in some areas. Wood is the most common material for residential piles, How To Survive A Hurricane however. The most common piling sizes acceptable for coastal areas are 8-inch and 10-inch diameters. 

Eight-inch is the minimum size generally approved for high-wind areas. In areas where the design wind speed is greater than 100 mph, How To Survive A Hurricane 10-inch piles should be used (FEMA, 1986). Driven piles offer superior pullout resistance to piles that are water-jetted in. When piles are driven, the driving process forces soil outward. 

This compresses the soil and increases friction between the soil and the pile. Jetting, on the other hand, loosens the surrounding soil, and the soil by the tip of the piling. This reduces the load capacity of the pile, and How To Survive A Hurricane produces a void around the pile that must be filled (FEMA, 1986). Because piles must resist the greatest storm forces and resulting loads, proper installation is essential. 

Piles must resist downward loads from the weight of the building, upward loads due to wind or flooding uplift, and How To Survive A Hurricane lateral forces from both wind and water. Piles must either rest on bedrock or be driven deeply enough so that friction transfers the vertical load to the surrounding soil. The following factors must be considered when deciding proper embedment depth. 

1. pile depth necessary to resist vertical, uplift, and horizontal loads; 2. anticipated scour depth or How To Survive A Hurricane elevation at the site; 3. existing ground elevation; 4. base flood elevation. Proper embedment is the most critical factor in pile installation. Standard construction practice for determining pile embedment is inadequate in many areas. 

Rules of thumb such as "piles should be embedded as much below ground as above ground" generally underestimate the required embedment depth and How To Survive A Hurricane have not taken scour into effect (FEMA, 1986). Soil load bearing is a key factor in determining proper embedment depth. In poor soil, longer piles must be used. 

Pilkey et al. (1981) roughly categorize soils into three types: 1. Below average soil - soft clay, poorly compacted sand, and How To Survive A Hurricane clays containing a large amount of silt. Vertical bearing capacity is 1,500 psf, and lateral bearing capacity is 100 psf per linear foot of embedment. 2. Average soil - loose gravel, medium clay, or any more compact composition. 

Vertical bearing capacity is 3,000 psf, and lateral bearing capacity is 200 psf per linear foot of embedment. 3. Good soil - compact, How To Survive A Hurricane well-graded sand and gravel, hard clay, and graded fine and coarse sand. Vertical bearing capacity is 6,000 psf, and lateral bearing capacity is 400 psf per linear foot of embedment. 

As a guideline, FEMA recommends that piles in wave and erosion zones penetrate sand to a depth of 5 feet below mean sea level, How To Survive A Hurricane if the base flood elevation is less than 10 feet above mean sea level. Mean sea level is the expected height of water plus waves. 

If the home is elevated more than 10 feet above mean sea level, then piles should extend to at least 10 feet below mean sea level. FEMA's Coastal Construction Manual contains more detailed design tables for determining pile size selection and embedment depths. To withstand horizontal forces, piles must be cross-braced with knee braces, trusses, or How To Survive A Hurricane grade beams. 

Bracing members are as important to the structural resistance of the foundation as the piles themselves. For homes elevated about 8 to 10 feet, FEMA recommends diagonal or knee braces, How To Survive A Hurricane sized according to construction tables in the Coastal Construction Manual. 

Truss bracing is recommended when a home is elevated more than 10 feet above grade, or How To Survive A Hurricane when the design wind speed is greater than 100 mph. Grade beams are a third method of pile reinforcement. Grade beams are horizontal supports such as 8x8's connecting the piles around the perimeter of the home at ground level. 

FEMA notes some disagreement regarding the use of grade beams. One view is that the use of grade beams increases wave forces on the piles, and How To Survive A Hurricane scour around the foundation. FEMA recommends their use, however, noting that the stiffer foundation system more than offsets the risk of additional scouring. It is important to note that all bracing must be installed with bolts and not nails (FEMA, 1986).

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