From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Wed, 24 May 2000 21:34:42 -0400
Like Stan, I, too, am awaiting the outcome of the failure. It just shouldn't
have happened. With the design live load of 100 psf, which is equivalent to
200 pound people standing 1'-5" apart everywhere on the bridge, there is
also a safety factor, or load factor, of 1.7 on the live load if it was
designed under ACI standards, or 1.3*1.67 if it was designed under AASHTO
The thing that really attracted my attention was the report that there were
cracks, "2-feet long" in other locations. If prestressed concrete cracks,
except when it is starting to exceed its load capacity, then there is
something very, very, wrong. Was this bridge also used for pickup trucks or
other light vehicles.
One of the things that seems to be common now with regard to P/S is that
stress conditions at each and every stage of loading are not investigated, or
if investigated, not recognized for the problems that may result. In one
case that I have seen, the compressive stress under dead load was higher at
the top of the member than it was at the bottom. In that case, the concrete
at the top is going to creep faster than the concrete at the bottom, and the
beam will start sagging. This can result in high edge loading on the support
if very thin (1/2") bearing pads are used.
The speed in which the debris was removed surprised me. There are many
things that can be seen if everything is left where it fell than can be seen
if the failed structure is disturbed.
Let's hope that the investigators are forthright with their reports and don't
try to protect the people that have hired them by obscuring the facts.
A. Roger Turk, P.E.(Structural)