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

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Jim,

Be careful with the BOCA. It was known in the mid 1990's that the model building codes were going to merge to what became the IBC. The effort to update the wind provisions in the 1996 BOCA were only half hearted. BOCA already allowed the use of ASCE 7, so they really did not feel the need to go through the effort.

BOCA also maintained the old "fastest mile" wind velocity maps. The fastest mile maps had not been updated in decades, and were not all that accurate when they were first developed. The ASCE 7 switched over to the "3 sec" wind velocity maps in 1995 as a result of the 1992 Hurricane Andrew. BOCA did not have the resources or the desire to go through a major update for just one last code cycle. Another aspect was that the majority of BOCA members were from the northeastern US, and did not have the urgent concern to upgrade wind provisions as did the members that comprised the Standard Building Code.

There has been a lot of consideration, studies, and testing done in preparation for the ASCE 7. That same level of effort was not present for the 1996 BOCA.

Generally a comparison of BOCA 1996 to ASCE 7-02 indicates that the BOCA will result in significantly lower pressures on elements and on the building structure.

I am not familiar with the Mass Code and can not comment.

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: RE: Building corner wind effect - when is a corner a corner?
Date: Thu, 16 Sep 2004 10:41:48 -0700 (PDT)

No, I have never done a wind tunnel test. But if that's what the testing results have shown, then I guess that's a good lesson learned. Now I just have to compare ASCE and BOCA against the Mass Code to see if I am overly conservative or not.

Thanks for the info!

Jim


Harold Sprague <spraguehope(--nospam--at)hotmail.com> wrote:
Jim,
Now I understand, and your observation is correct. The loading on the
leeward side of a building is the same from top to bottom, and is dictated
by the roof height. The leeward load at the top is the same as the leeward
load at the bottom. The only exception is at the parapet (if you have one),
and that is where you apply the section on parapets.

I don't know if you have had the occasion to do a wind tunnel test on a
building, but the gross pressures indicated in the ASCE 7 are pretty close
to what you will get in a wind tunnel test.

Regards,
Harold Sprague


>From: Jim Wilson
>Reply-To:
>To: seaint(--nospam--at)seaint.org
>Subject: RE: Building corner wind effect - when is a corner a corner?
>Date: Thu, 16 Sep 2004 04:39:26 -0700 (PDT)
>
>ASCE 7-02 Page 44 shows that walls have vertical corner zones only. The
>corner zone does not continue along the top edge of the wall. Figure 6-17
>on page 67 shows the same thing.
>
>If I wasn't clear when I originally mentioned roof edge, I was trying to
>describe the leeward edge at the top of the wall. That is where BOCA
>1609.8.2 does indicate there are corner/end zones for C&C, but ASCE does
>not.
>
>Jim
>
>Harold Sprague wrote:
>Jim,
>
>I am not sure which issue of ASCE you have, but the ASCE 7-02 page 44 shows
>the corner zone for components and cladding.
>
>Regards,
>Harold Sprague
>
>
> >From: Jim Wilson
> >Reply-To:
> >To: seaint(--nospam--at)seaint.org
> >Subject: RE: Building corner wind effect - when is a corner a corner?
> >Date: Tue, 14 Sep 2004 19:30:12 -0700 (PDT)
> >
> >ASCE 7 seems remiss in not visually depicting the roof edge as a corner
> >zone. The figures are quite clear that only the vertical corners receive
> >corner effects. I agree with my limited knowledge of wind testing that
>the
> >roof edge IS a corner, but it could more easily be argued the other way,
> >given what is in the figures.
> >
> >And thanks for the update on ASCE 7-05. I'll start surcharging my clients
> >now.
> >
> >Thanks,
> >Jim Wilson
> >
> >
> >Harold Sprague wrote: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
> > >Reply-To:
> > >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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