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RE: NASCAR Pedestrian Bridge Collapse

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Roger,

Even if the bridge was subjected to light vehicles, the applied load would
be less than the 100 psf pedestrian design live load.  Passenger car parking
garages are designed for 50 psf.  Consider that the applied pedestrian live
load that caused the failure of the Hyatt walkways was slightly over 6 psf.
People consider themselves packed in when pedestrian loads exceed about 30
psf.

I don't know how significant the cracks are in the rest of the section that
failed.  Consider that the failure was sudden and the sections fell about 18
feet.  I was actually a bit more surprised that it did not break up more on
impact.

If I were speculating on the cause of the collapse, I would first look to
what generally causes sudden catastrophic failures in double tees in the
bending mode.  I would investigate over-reinforcing resulting in a concrete
crushing mode of failure.  Five years is a pretty short time to corrode the
strands.  

Consider the thousands of precast parking garages that get salt tracked in
for 30 years with no inspection, no maintenance, and no wash down.  When
precast parking garage double tees fail due to gravity load, they sag
excessively due to the large sustained dead load of the precast and reduced
reinforcing steel strand section.  In testing, generally, precast double
tees sag an incredible amount when they are loaded to failure.  

Ruling out over-reinforcing, I would look at hydrogen embrittlement
corrosion.  Hydrogen embrittlement is a problem with hard steel.  It is more
of a problem with galvanized hard steel.  Hydrogen embrittlement failure
mode is sudden with no necking down and little elongation under load like
you get with regular carbon steel.  

I would also look to see if there was a joint at mid span.  ( I know that is
a terrible place for a joint.)  If there was a joint, the connection of the
reinforcing steel is a major issue.

Regards,
Harold Sprague


> -----Original Message-----
> From:	Roger Turk [SMTP:73527.1356(--nospam--at)compuserve.com]
> Sent:	Wednesday, May 24, 2000 8:35 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: NASCAR Pedestrian Bridge Collapse
> 
	 ...... Was this bridge also used for pickup trucks or 
> other light vehicles. ...... 
> 
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
>