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AASHTO -- Prestressed two-beam bridges
- To: seaint(--nospam--at)seaint.org
- Subject: AASHTO -- Prestressed two-beam bridges
- From: Michael Hemstad <mhemstad(--nospam--at)mbjeng.com>
- Date: Mon, 08 Mar 2010 11:09:58 -0600
Bill is correct. Here in Minnesota we
routinely use two lines of prestressed beams to carry ped bridges; but we
don't like to do that with steel, because steel beams are susceptible to fatigue
and so two-beam structures are fracture-critical. The difference is
that the part of a beam that we mostly worry about in fatigue is the tension
flange. One steel flange can have a crack start and then one cold night,
it propogates and the beam breaks in two. The tension portion of a
prestressed beam, by comparison, is 10 or more strands, with relatively constant
stress levels (due to the pre-tensioning). So it's pretty unlikely that
even one strand will fatigue, much less enough of them to let the beam
The exception to this is when they might be susceptible to
a traffic strike. Then, redundancy still matters. If such
a two-girder ped bridge (they're almost always ped bridges, or cart-path
bridges such as on golf courses) is built over traffic, they use increased
clearances so that overheight loads get stopped by the next bridge up the
Bill Polhemus replied to g r:
g r wrote:
> Does anyone know if AASHTO addresses the use of only two
> concrete) beams to support the deck per span? Is it
> If not prohibited are their other requirements to be met
> make it dificult or practically impossible to utilize
only two beams?
> Thanks in advance!
A few years ago there was some controversy regarding the use
of only two
steel girders (or ONE steel trapezoidal "tub" girder) for a
due to concerns of fatigue and redundancy. That is the only
ever heard a limit placed on the number of girders in a
Now, that's not to say there aren't other restrictions I don't
about, but that's the only instance I've ever heard tell of.
If you have
a very narrow section where "only" two prestressed girders
I'm not sure why you couldn't do that so long as the design