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ASCE-7 and the Building Codes require that breakaway walls be designed to withstand the greater of: the design wind load, the design seismic load, or 10 psf, whichever is greater; but be designed to fail due to flood loads during a Base Flood. Any wall designed to withstand lateral loads greater than 20 psf must be certified by a registered design professional. Just like the building envelope of the elevated structure, breakaway walls must be designed to resist wind loads. However, breakaway walls can and do fail during hurricane flooding, and will offer no protection against wind, wind-driven rain or internal pressurization to the elevated structure.  Thus, the elevated structure and its foundation must be designed to resist wind in two cases: 1) with the breakaway walls intact and transferring wind loads to the main structure, and 2) with the breakaway walls washed away and wind affecting the building envelope on the underside of the elevated building.
 As Mr. Cramer has already pointed out, the updated (August 2008) NFIP Technical Bulletin 5 (http://www.fema.gov/plan/prevent/floodplain/techbul.shtm) states on page 7, "Access stairs are sometimes constructed inside a breakaway enclosure, with an entry door at the bottom of the enclosure, but without an entry door into the elevated building. This practice leads to building damage. The lack of an entry door at the top results in a large opening in the building envelope when the enclosure breaks away. This exposes the building interior to higher internal wind pressures and wind-driven rain, and provides floodwaters an easy path into the building." The practice described is a code violation, but the solution is simple -- place an exterior door, capable of resisting design wind pressures and windborne debris, at the top of any stairs leading into an elevated structure.
By the way, three other NFIP Technical Bulletins were revised and published in August 2008: TB-9 on Design and Construction of Breakaway Walls, TB-2 on Flood Damage-Resistant Materials, and TB-1 on Flood Openings in Enclosure and Foundation Walls. They are available at the link listed above.

John Ingargiola, Civil Engineer
FEMA Mitigation Directorate

John Ingargiola
Blackberry cell 202 557 9455

----- Original Message -----
From: codetalk-bounces at myfloridacode.com <codetalk-bounces at myfloridacode.com>
To: codetalk at myfloridacode.com <codetalk at myfloridacode.com>
Sent: Wed Feb 11 19:27:26 2009
Subject: [Florida Code Talk] (no subject)

It is my understanding that the structure has to meet wind load ratings with the blow out walls gone, as the blow out walls do not meet the wind load rating, so the rest of the structure has to meet the wind load rating.
 
Bret Walley
CBC059520
CAC1816055


      
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