If you need the roof as a diaphragm, you can't!
From: Mark E. Deardorff [mailto:MarkD(--nospam--at)DandDEng.com]
Sent: Friday, March 31, 2000 11:22 AM
Subject: RE: Texas Twisters
Being from California I have never had to design for a tornado. In fact, I
was under the impression that the most design anyone ever actually does is
to pray that the tornado misses the building.
If required to endure the extreme loads a tornado can produce, how could a
light-weight flat roof ever be economically designed?
> -----Original Message-----
> From: Paul Ransom [mailto:ad026(--nospam--at)hwcn.org]
> Sent: Thursday, March 30, 2000 5:40 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Texas Twisters
> > From: "Caldwell, Stan" <scaldwell(--nospam--at)halff.com>
> > There are 6 to 8 industrial buildings just west of
> downtown, however, that
> > have fully collapsed and are currently the object of an
> intensive search for
> > survivors. These buildings appear to have been one story
> reinforced masonry
> > and/or concrete tiltwall construction with open web steel
> joist roof framing
> > and steel deck diaphragm roofs. Now they are piles of
> rubble. My guess is
> > that the overhead doors blew open and the roof decks then
> failed in uplift.
> > At that point, the decks could no longer serve as
> diaphragms and the walls
> > lost their lateral stability. In the next few days, we'll
> see if I'm right.
> > Also, as you might imagine, buildings with standing seam
> metal roofs also
> > fared quite poorly, including a large state office building
> in Arlington.
> I'm not completely surprised by your matter-of-fact assessment of
> standing seam roofs. Unfortunately, it's one of those comments that
> paints with a very broad brush.
> From testing that I have done, there are certain types of
> standing seam
> roof that are more prone to wind damage than others. Of course, the
> failure mode that you describe is completely true for many buildings
> except that damage to a ssr should not lead to progressive structural
> collapse as you described for masonry/tilt-up buildings which rely
> completely on the diapragm - and we see the result.
> There are good metal roofing systems with poor implementation (design,
> detailing, erection). There are some roofing systems, which tend to be
> targeted to the architectural market, that need serious help at the
> project design/detailing stage to avoid failure during wind events.
> Both the ssr and tilt-up designers will be able to point to
> the building
> codes and prove compliance to the letter. So much for codes ....
> I wonder if some of the perceptions are due to the fact that
> huge sheets
> of metal can blow for miles whereas other materials may drop
> much closer
> to the point from which they were plucked. It makes good
> press to show a
> helicopter view of an open roof or cladding wrapped around a
> tree a mile
> from its source but a failure in any roof is still a failure.
> I expect that the area will be crawling with metal building
> representatives (among others) looking for examples that came through
> the event unscathed. The ads in the industry magazines will
> appear very