Flood Resistant Design and Construction
American Society of Civil Engineers (ASCE) Guide helps apply IBC in Floodprone Areas
 Is your state one of the more than 40 states that have adopted a building code based
on the model International Building Code®? If so, then you need to order a copy
of ASCE 24, Flood Resistant Design and Construction [http://www.asce.org]. ASCE
24 is referenced in the section of the code that outlines requirements for buildings
and structures in flood hazard areas. The most recent edition was published in 2005
(the first edition was dated 1998).
In addition, the International Residential Code® requires dwellings in floodways
to be designed in accordance with ASCE 24. And the 2009 edition of the IRC will
include an alternative that allows communities to require homes in V Zones to be
designed in accordance with ASCE 24.
Since the 2003 editions, FEMA has noted that the International Code Series has provisions
that are consistent with the NFIP’s regulations for buildings. The IBC’s consistency
is achieved partly by reference to ASCE 24. It is important to note, however, that
ASCE 24 is not simply a restatement of all of the NFIP regulations. It includes
some additional specificity, some additional requirements, and some limitations.
Here are a few notable highlights.
- Freeboard is required as a function of the nature of occupancy and the flood zone.
Dwellings and most other buildings have 1 foot of freeboard; certain essential facilities
have 2–3 feet; only agricultural facilities, temporary facilities, and minor storage
facilities are allowed to have their lowest floors at the base flood elevation (BFE).
- Flood loads and other loads are as specified in ASCE 7.
- Performance of foundations exposed to flood loads and load combinations is specified;
soil characteristics and underlying strata, including soil consolidation, expansion
or movement, erosion and scour, liquefaction, and subsidence all must be considered.
- Fill is required to be stable under conditions of flooding, including rapid rise
and rapid drawdown, prolonged inundation, and erosion and scour; structural fill
compaction is specified or an engineering report is required; side slopes are required
to be no steeper than 1:1.5.
- Specifications for slabs-on-grade are listed, depending on the purpose and location
of the slabs.
- Two alternatives are specified for flood openings to allow for the automatic entry
and exit of floodwaters in below- BFE enclosures: nonengineered openings that do
not require certification (1 square inch per square foot of enclosed area), and
engineered openings that must be certified by a registered design professional.
- In V Zones and Coastal A Zones,
- Structures shall be supported on piles, columns, or shear walls (with shear walls limited in width and oriented parallel to the flow and wave direction).
- Mat or raft foundations, with columns extending upward, are permitted in limited
circumstances.
- Foundation depth shall take into account erosion and scour.
- Walls designed to break away shall not produce debris that is capable of damaging
structures (breakaway walls in Coastal A Zones require openings, while breakaway
walls in V Zones do not).
- Mechanical, heating, ventilation, and air conditioning elements shall be located
on the landward side of structures.
- Erosion control structures (bulkheads, seawalls, revetments) shall not be attached
to buildings nor shall they focus or increase flood forces or erosion impacts on
structures.
- Decks, concrete pads, and patios shall be structurally independent of buildings
and constructed to break away without producing damaging debris.
- Pools shall be elevated, designed to break away without producing damaging debris,
or sited to remain in the ground without obstructing flow that causes damage.
Order ASCE 24 from http://www.asce.org (click on “publications” to reach the bookstore.
Download FEMA’s summary from
http://www.floods.org/PDF/ASCE24_Highlights_1008.pdf.
Source: ASFPM, News & Views (December 2008)
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