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RE: Building corner wind effect - when is a corner a corner?

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The edge of the roof is a corner. BOCA never developed a commentary. The ASCE develops commentary. The other point that should be made is that if you design per the 1993 BOCA, you will have pressures that are too low. The ASCE 7 underwent very marked changes in the mid and late 1990's due to Hurricane Andrew. We were wrong, and the ASCE needed help. We switched over to 3 second gust and changed the way local pressures were calculated especially on fasteners.

I would suggest that you require a Factory Mutual I-90 roof. That should keep you out of the code problems and get you the performance that you need.

By the way, I am in San Francisco Friday and Saturday working on the next evolution of the seismic section of the ASCE 7-05. I think that you will like the improvements.

Regards,
Harold Sprague

From: Jim Wilson <wilsonengineers(--nospam--at)yahoo.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Building corner wind effect - when is a corner a corner?
Date: Fri, 10 Sep 2004 11:04:33 -0700 (PDT)

ASCE 7-02 Figures 6-3 and 6-17 illustrate that the end zone of a wall receives a higher negative wind loading for components and cladding. But there is no end zone treatment along the leeward edge of the roof.

BOCA Figure 1609.8.2, on the other hand, does treat the roof edge as a building corner for components and cladding. My intuition tells me that BOCA is right on this, but I would otherwise prefer to follow the more current recommendations in ASCE.

To complicate my interpretation more, I am designing to the Mass Building Code which is modeled on BOCA 1993. They provide tabulated wind pressures by elevation, zone (location within the state) and exposure. Corner type areas are defined as "The salient corner shall be defined as the vertical surface located within a distance of 1/10 the least width of the structure, but not more than ten feet, from a prominent (salient) corner." A "prominent (salient) corner "sounds like it should include the roof edge.

Thus my question - When is the roof edge considered a "corner" for increased wind loads on components and cladding? I have already concluded and designed my system for corners at the roof. But now that the contractor did not install the roof edge as if it were a "corner".....

Jim Wilson, PE
wilsonengineers(--nospam--at)yahoo.com
Stroudsburg, PA



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