FLOOD04878

• County: Statewide
• Basin: Statewide
• Title: Openings in Foundation Walls
• Date: 4/1/1993
• Prepared For: NFIP
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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TECHNICAL BULLETIN 1-93

Openings in Foundation Walls
Required for Buildings
Located in Special Flood Hazard Areas
in accordance with the
National Flood Insurance Program

Introduction

An important objective of the National Flood Insurance Program (NFIP) is to protect buildings
constructed in floodplains from structural damage caused by flood forces, In support of this
objective, the NFIP regulations include building desigu criteria that apply to new construction
and substantial improvements of existing buildings in Special Flood Hazard Areas (SFHAs),
According to these criteria, residential buildings constructed in A zones (Zones A, AE, Al-A3D,
AR, AD, and AH) must have their lowest floors at or above the base blood elevation (BFE).
Non-residential buildings constructed in A zones must either have their lowest floors at or above
the BFE or be dry floodproofed (made watertight) to or above the BFE. Residential and non-
residential buildings whose lowest floors have been constructed at or above the BFE usually are
elevated on piers, columns, piles, extended foundation walls, or fill. While the main portion of
such a building is protected from the lOO-year and lesser-magnitude floods, the foundation and
any enclosures below the BFE used for parking, building access, or limited storage will be
exposed to flood forces.

For buildings constructed on extended foundation walls or that have other enclosures below the
BFE, these flood forces include the hydrostatic pressure of floodwaters against the foundation or
enclosure walls, If the walls are not designed to withstand hydrostatic pressure, they can be
weakened or can fail and the building damaged, Therefore, the NFIP regulations require that
foundation and enclosure walls that are subject to the lOO-year flood contain openings that will
permit the automatic entry and exit of floodwaters, These openings allow floodwaters to reach
equal levels on both sides of the walls and thereby lessen the potential for damage from hydro-
static pressure. The requirement for openings applies to all new and substantially improved
buildings in A zones, This Technical Bulletin explains the requirement for openings and pro-
vides guidance for designing and constructing foundation and enclosure walls that include the
required openings,

Extended foundation and enclosure walls below the BFE may also be threatened by hydrody-
namic forces resulting from velocity flows and debris impact. The requirement for openings is
intended to reduce flood damage associated with hydrostatic not hydrodynamic forces. These
forces are described within this bulletin, and additional design guidance is given for buildings in
areas subject to velocity flood flows, which may include debris.

For buildings in V zones (Zones V, VE, and Vl-V3D), more stringent design and construction
requirements have been established for the portions of the buildings below the BFE, For infor-
mation on V-zone design and construction requirements, refer to the NFIP regulations, the
Technical Bulletin series, and FEMA's "Coastal Construction Manual."

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