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AASHTO -- Prestressed two-beam bridges

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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 fall.
 
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 road.
 
Mike Hemstad
MBJ
Minneapolis, Minnesota
 
 
Bill Polhemus replied to g r:
g r wrote:
> Does anyone know if AASHTO addresses the use of only two (prestressed
> concrete) beams to support the deck per span? Is it specifically
> prohibited?
>
> If not prohibited are their other requirements to be met that would
> make it dificult or practically impossible to utilize only two beams?
>
> Thanks in advance!
>
> GR
A few years ago there was some controversy regarding the use of only two
steel girders (or ONE steel trapezoidal "tub" girder) for a bridge unit,
due to concerns of fatigue and redundancy. That is the only time I've
ever heard a limit placed on the number of girders in a unit.
Now, that's not to say there aren't other restrictions I don't know
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 would work,
I'm not sure why you couldn't do that so long as the design provisions
are met.