Flood Zone Foundations

Design Considerations

To successfully elevate a home on an open foundation, site specific conditions must also be identified. Once the site specific conditions are identified and evaluated, a proper design can then be implemented. McEvoy Homes can help you navigating the complex FEMA requirements and counsel you on the most cost effective method for you to move forward as well as understand the entire FEMA Compliant House Raising process.

General Compliant Designs for Homes Located in Zone A

All new homes and homes determined to have sustained Substantial Damage or that will be Substantially Improved must adhere to the following criteria:

  • All buildings must be properly anchored to resist, flotation, collapse, and lateral movement.
  • The top of the lowest floor must elevated to, or above, the required elevation.
  • Homes can be elevated on perimeter foundation walls, or on piles, piers, or columns. If permitted by the community, elevation can also be achieved by placing fill under the structure. If the community permits fill to be placed below the BFE, the fill must be compacted and protected against scour and erosion. It is easiest to place fill before a home is constructed or, for existing homes, when the home is temporarily relocated.
  • Basements are not permitted. To be compliant, existing below-grade areas must be backfilled.
  • Walls of enclosed areas below elevated homes must have flood openings that allow floodwaters to automatically equalize during an event. For more information, consult FEMA NFIP Technical Bulletin 1, Openings in Foundation Walls and Walls of Enclosures (2008).
  • Enclosed areas below elevated buildings are permitted to be used only for parking, building access, and storage.
  • Utilities, including electrical, heating, ventilation, plumbing, air-conditioning equipment (including ductwork) must be elevated above the BFE, or specifically designed to prevent water from entering or accumulating within the components during flooding.
  • Flood damage-resistant construction materials must be used below the BFE. For more information, consult FEMA NFIP Technical Bulletin 2, Flood Damage Resistant Materials Requirements (2008).
  • When the lowest floor is set, and again prior to final inspection, builders must obtain elevation certificates to document compliance. Owners need these certificates to obtain NFIP flood insurance.
  • Construction of the home and other development must not result in any increase in flood levels within the community during the occurrence of the Base Flood discharge.

General Compliant Designs for Homes Located in Zone V

All new homes and homes determined to have sustained Substantial Damage or that will be Substantially Improved must adhere to the following criteria:

  • All residential buildings must be properly anchored. The NFIP requires open pile or column foundations and structures attached thereto to be anchored to resist flotation, collapse, and lateral movement due to the effects of wind and water loads acting simultaneously on all building components.
  • Open foundations (piers, pilings, or columns) are required. Closed foundations, such as solid masonry or concrete walls, and use of fill, are not permitted.
  • Basements are not permitted. To be compliant, existing below-grade areas must be backfilled to or above the adjacent ground surface.
  • The bottom of the lowest horizontal structural member of the lowest floor must elevated to the required elevation.
  • Enclosed areas below elevated buildings are permitted to be used only for parking, building access, and storage.
  • Areas below elevated buildings must be free of obstructions. The space below the lowest floor can be enclosed by non-supporting breakaway walls, open-wood lattice work, and insect screening intended to collapse under wind and water loads. For more information, consult FEMA NFIP Technical Bulletin 5, Free of Obstruction Requirements for Buildings Located in Coastal High Hazard Areas (2008).
  • Walls must be designed to break away under base flood conditions when used to enclose areas below elevated buildings. For more information, consult FEMA NFIP Technical Bulletin 9, Design and Construction Guidance for Breakaway Walls (2008).
  • Flood damage-resistant construction materials must be used below the BFE. For more information, consult FEMA NFIP Technical Bulletin 2, Flood Damage Resistant Materials Requirements (2008).
  • Utilities, including electrical, heating, ventilation, plumbing, and air-conditioning equipment (including ductwork) must be elevated on platforms, which may be attached to the building above the BFE.
  • When the lowest floor is set, and again prior to final inspection, builders must obtain elevation certificates to document compliance. Owners need these certificates to obtain NFIP flood insurance.
  • The design and methods of construction must be certified by a registered design professional to be in accordance with an accepted standard of practice for meeting Zone V design requirements.
  • The current accepted standard of practice is ASCE 24, Flood Resistant Design and Construction. The IRC allows ASCE 24 to be used as an alternative to the Zone V requirements. FEMA recommends that designers use it when designing homes in Zone V. ASCE 24 has more detailed criteria

Solutions Specifics for Elevated Construction on Open Foundations

Homes on small lots or with limited access require special methods to elevate-in-place. Small lots may not have adequate space on which to relocate a home while new foundation piles are driven. Horizontal and vertical clearances needed to elevate-in-place and drive traditional piles may also be inadequate. Alternative (non-timber pile) open foundations may be more feasible when lot size is a constraining factor.

One solution for elevating a home on a small lot is to place a system of concrete columns and construct grade beams to support the elevated home. For more information on grade beams, consult Section 10.5.6 of FEMA P-55, Coastal Construction Manual (2011).

Another option for elevating a home on a small lot, if permitted by local building codes, is to incorporate deep foundation elements, such as micropiles, into the footings to increase building support and resist the lateral and uplift loads caused by high winds, flooding, scour, or erosion. Foundation elements such as micropiles or helical piles canbe installed even if there are minimal horizontal or low vertical clearances, and when there are limitations on disturbance to neighboring homes.

Achieving proper open foundation designs for small lots may require owners to consult with a geotechnical engineer and a structural engineer, who may also need to work with an elevation contractor and a specialty foundation contractor. The need to protect neighboring homes and utilities also affects construction risks and costs, and will need to be considered when selecting the appropriate foundation alternative.

  1. Pier Foundations
  2. Pile Foundations
  3. Micropile Foundations

A) Pier Foundations

Pier (a type of column) foundations are typically constructed of either reinforced concrete or reinforced masonry columns. Piers are generally placed on footings to support the elevated home.

Without footings, piers function as short piles and rarely have sufficient capacity to resist uplift, lateral, and gravity loads. Additionally, when exposed to lateral loads, discrete footings can rotate and therefore, piers supported by discrete footings are not recommended in coastal environments.

Piers supported with continuous concrete footings provide much greater resistance to lateral loads because the footings can act as an integrated unit to resist rotation. The integrated footing system must be steel reinforced to resist moment forces that develop at the base of the piers as a result of lateral loads on the foundation and elevated home.

Pier foundations are typically shallow due to excavation constraints and are appropriate only where there is limited potential for erosion and scour. To prevent the continuous footing system from being undermined, the foundations must extend below the maximum estimated depth for long- and short-term erosion and localized scour. In some case, existing pier foundations may be retrofitted with grade beams to provide enhanced lateral support.

See the section titled “Pile Foundations” for more information on grade beams.

B) Pile Foundations

Pile foundations are required in Zone V coastal environments so that waves can pass more easily under elevated homes. Traditional piles are typically constructed of treated timber, steel, or precast concrete and are driven into the ground to a depth required to resist vertical and lateral loads from gravity, wind, and flood forces. Pile foundations use the soil’s resistance to support the elevated home.

Critical aspects of a pile foundation include the pile size and spacing, installation method, embedment depth, bracing, and the connection to the elevated home. Piles that are properly sized, spaced, installed, and braced, and have adequate embedment into the soil (with consideration for erosion and scour effects) will perform properly and allow the home to remain standing and intact following a design flood event. For more information about scour and erosion, refer to section 3.1.1.2 of FEMA’s Hurricane Ike in Texas and Louisiana: Mitigation Assessment Team Report (2009).

Piles can be used with or without grade beams. When used without grade beams, piles extend to the lowest floor of the elevated home. Improved performance is achieved when the piles extend beyond the lowest floor to an upper floor level, although owners should check with an insurance agent to understand how the extended piles will be rated for flood insurance. Using grade beams provides resistance to rotation (also called “fixity”) in the top of the embedded piles and improves stiffness of the pile foundation system against lateral loading. When used together, piles and grade beams work together to support the elevated home and transfer vertical and lateral loads imposed on the elevated home and foundation to the ground below.

Design and installation of grade beams should include the following concepts:

  • Grade beam design criteria should include resisting lateral flood loads from both hydrodynamic forces and flood-borne debris impacts.
  • Grade beams are to provide horizontal bracing of piers or piles and should not directly support any vertical load-carrying elements such as floor slabs. They should be designed to be self-supporting between vertical foundation members, such as piles, to account for cases when erosion and scour extend below the grade beam. According to NFIP requirements, grade beams that are structurally connected to slabs are considered to be the lowest horizontal structural member supporting the slab, which is a nonconforming use below BFE and severely increases flood insurance premiums if present.

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