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RE: Texas Twisters

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When you bust the numbers on a tornado strike for an individual structure
even in the heart of tornado alley, the probabilities are pretty long.  That
said, many structures are designed for tornadic load resistance in various
fashions.  Some structures are designed with an enhanced tornado protection
area (ETPA).  A concept developed by Mehta and McDonald of Texas Tech and
used for some schools.  Some structures like electric power control
buildings must resist a direct hit from an F2 or greater tornado
(probability or the design tornado defined by the owner).  Missile impact is
a major consideration.  Nuclear power plants must resist a much higher load
and missile impact.  

What got the high rise buildings was missile impact on the glazing (rarely a
design consideration).  What gets the residential and light commercial world
is no design consideration for wind uplift.  What gets the light metal roofs
are local compromises in the envelope due to uplift.  Once that happens, it
is self exacerbating.

If an owner chooses to design for tornadic load resistance, it can be done.
The trick then is to characterize the nature of wind forces and missile
impact.  Once you consider missile impact gage metal roofs are out.

The "economical" aspect comes into play when you do the loss and business
impact studies.  Granted those structures and roofs will be more expensive
than the standard Code minimum structures, but it can be done.

Regards,
Harold Sprague


> -----Original Message-----
> From:	Mark E. Deardorff [SMTP:MarkD(--nospam--at)DandDEng.com]
> Sent:	Friday, March 31, 2000 10:22 AM
> To:	seaint(--nospam--at)seaint.org
> 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?
> 
> Mark
> 
> > -----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
> > quickly.
> > 
> > -- 
>