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Since hurricanes Katrina and Rita hit the Gulf Coast, some homeowners are choosing to raise their houses to higher elevations before restoring them.
Elevating or rebuilding above base flood level (the level shown on the FEMA flood map) will reduce flood risk; however, elevated foundations must be designed properly or you may face other problems. Because the structure is higher off the ground, the structure and its foundation will be subject to higher wind loads. Also, when a structure is higher off the ground, the length of piles or piers supporting it above ground are longer and require bracing. The length of pile that is under the ground is considered braced.
Choosing and building the right foundation for your home is very important. According to ”Recommended Residential Construction for the Gulf Coast" (FEMA 550, July, 2006), foundations should be designed and constructed to meet three criteria:
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The foundation should properly support the elevated home and resist all loads expected to be imposed on the home and its foundation during a design event (e.g. the 100-year flood, or the 500-year flood).
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The foundation should prevent floatation, collapse and lateral movement of the building in the design event.
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The foundation should be constructed with flood-resistant materials below the BFE (Base Flood Elevation).
The National Flood Insurance Program (NFIP) has established requirements for foundation types in different flood zones.
Open Foundations: These foundations allow water to pass through the foundation, minimizing the lateral load applied on the structure by flood waters. Piers, piles and columns are open foundations if the space between them is not obstructed. Open foundations are required in V Zones (V stands for "velocity"; in V-zones, flooding is accompanied by breaking waves three feet or greater in height). Grade beams with columns or piers and braced pile foundations are used more than any other foundation type in coastal Louisiana.
Closed Foundations: Foundation walls around crawlspaces and stemwall foundations (with slab over backfill) are the two most common types of closed foundations. With backfilled stemwalls, water is not allowed to flow below the elevated structure; thus, to some extent, the backfilled stemwall foundation reduces the area in the community available for storing floodwater.
Foundation wall and stemwall/backfilled slab foundations are not permitted in V-zones because they obstruct the flow of floodwater. When crawlspace foundations are used in A zones, openings in the foundation wall must allow flood waters to get inside the enclosure (i.e., under the building) and equalize the water pressure on either side of the wall.
Grade Beam with Piers or Columns
Grade beams are used in pier or column foundations to increase stability. Basically, grade beams connect all the piers or columns together, which provides a more stable base. They are made of reinforced concrete and should be connected in all directions. They must be designed as self-supporting foundation members and constructed deep enough to resist erosion and scouring.
Braced Piles
Piles are structural members made of wood or steel that were driven into the ground deep enough to reach soil with high bearing capacity or to have adequate friction to carry the weight of the structure that will be placed on top of the foundation. Above the ground, they go to or above the Base Flood Elevation (BFE) to minimize flood risk. These piles are connected to each other at the top with structural members parallel to the ground to set up a platform on which the structure is set or built.
Bracing the piles is necessary in most cases in order to stiffen the piles and increase their resistance to bending. Two kinds of bracing can be used -- diagonal bracing or knee bracing. Both have advantages. They can be either steel or wood, but steel is preferred because of its higher tensile strength, smaller size and durability. Diagonal bracings are slender structural members that work as tension-only members (to resist pulling forces, not compression forces). As a result, they need to be used in pairs on both directions (one pair for north-south direction and one pair for east-west direction).
Many different types of foundations can be designed for elevated structures; however, quality of the design and construction is crucial. Knowing the bearing capacity of the soil, having adequate connections between the foundation and building, and using good quality materials and workmanship are a few important points. It is recommended that you have professional help with foundation design and construction to avoid future problems that could be very costly and might even be deadly.
In V-zones, professional assistance is required, not just recommended. An engineer must certify that the bottom of the lowest horizontal structural member of the lowest floor (excluding the pilings or columns) is elevated to or above the Base Flood Elevation (BFE) and that the pile or column foundation and structure attached thereto is anchored to resist flotation, collapse and lateral movement due to the effects of wind and water loads acting simultaneously on all building components.
Written by Emre Ergen, graduate student, University of New Orleans