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RE: Texas Twisters

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Bill,

In most smaller commercial type of buildings.  The failure mode due to
tornadic wind loads is initiated by compromising the building envelope.
With the building envelope compromised, the uplift load on the roof is
greatly increased.  (Compare an enclosed building with an open building)  If
the roof lifts off, you loose a critical element required for the stability
of the wall.  The roof is often a diaphram which stabilizes the walls.

When you do a wind tunnel test on a structure, and you remove a small area
(window) on the windward exposure, you greatly increase the internal
pressures.

Maintaining the building envelope has been a major effort in the hurricane
areas like Dade County.  The Standard Building Code developed a very
rigorous glass window testing qualification protocol which is optional.  The
development came out of the efforts of Texas Tech. shooting 2x4's and BB's
at glazing assemblies.

Also bear in mind that tornados are generally small localized areas of high
winds.  The wind velocities which place them into the various Fujita
catagories are designated in 1/4 mile wind velocities.  It is markedly
different when you place the building structure in a straight line wind.

Regards,
Harold Sprague


> -----Original Message-----
> From:	Chanel1096(--nospam--at)aol.com [SMTP:Chanel1096(--nospam--at)aol.com]
> Sent:	Thursday, March 30, 2000 7:10 AM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: Texas Twisters
> 
> In a message dated 3/29/00 5:48:45 PM Eastern Standard Time, 
> scaldwell(--nospam--at)halff.com writes:
> 
> > It will take some time to evaluate the structural integrity of the
> dozens of
> >  affected buildings.  My guess is that the mid-rise and high-rise steel
> and
> >  concrete frame office buildings in downtown Fort Worth will ultimately
> be
> >  found to be structurally sound, albeit with some localized damage.  The
> >  glass curtainwalls that are common on these buildings failed quickly,
> like
> >  "structural fuses", allowing the wind to blow right through the open
> >  building frames.  Once the sail is gone, the forces become manageable.
> By
> >  the way, Fort Worth uses UBC, with a 70 mph basic wind speed.  
> >  
> That goes to one of the items they were mentioning on the news this
> morning 
> which did not make much sense to me.  They were saying that Fort Worth was
> 
> beginning to look into changing the building code to require tougher glass
> 
> that would not be destroyed by tornadoes.  Wouldn't this actually increase
> 
> the building's chance of being destroyed by tornados?  
> 
> -Bill King, EIT
>