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Re: "Blown Away"

[Subject Prev][Subject Next][Thread Prev][Thread Next] Looking at this from a slightly different perspective, there are situations where portions of structures are purposely designed to be 'blown away' such as in damage limiting construction per NFPA 68 "Guide for Venting Deflagrations". This practice is common in process plants where wall and/or roof panels are designed to blow out to relieve internal pressure due to accidental explosions or deflagrations thereby preventing failure of the superstructure. However, the important point here is that the panels are designed to remain intact and tethered to the main structural system.

Walter Sawruk
EQE International, Inc.
Shillington, PA
Email: ws(--nospam--at)

At 09:06 AM 05-03-01 -0600, you wrote:
Recently, an architect client of mine and I were discussing a feature of a
commercial building we were working on. He does lots of convenience store/fast
food combination facilities. One client has a "doghouse" at the front entrance
of the store, a "penthouse" type structure that allows a sign-logo above the
entrance that extends about eight feet or so above the parapet line.

The doghouse itself is CFS supporting metal cladding, which sits on the beams
and joists of the main structure.

The design drawings given me showed a CFS structure that was WAY under-designed
to resist code-specified wind forces, and I proceeded to design it correctly,
which made for a much "beefier" structure, particularly where the roof members
and overall connnections were concerned. When he saw this, he had a negative
reaction. "Too much", he said.

I pointed out to him that we were building in an area where the design wind
speed is 120 mph (three-second gust). His response was "oh, that doesn't matter,
if they had a hurricane, the doghouse would just be blown off, and they could
build it back again".

I had NEVER heard of this approach before! In essence, he's saying "just design
it to handle gravity loads and don't worry about wind, this is not an occupied
portion of the structure. If it gets scoured off, no big deal".

Have any of you designed "components and cladding" type portions of a structure under this approach? The architect insists it is perfectly valid: "we do it all
the time". Of course, this guy is the "Wal-Mart" of architects in our market,
and is used by developers here because he works cheaply, and doesn't get in
their way.

I'm starting to rethink my relationship with him on this basis, but I'd like to
know if this is perhaps more commonplace than I realize, and if I'm
"overreacting". I just can't imagine a building department anywhere allowing a
portion of a structure the possibility of being "blown away".