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Thanks for the replies so far!
 
This is for HL-93 loads.  They'll be one-lane ramp or direct connector bridges.
 
They'd be non-typical, large prestressed concrete beams with very wide top flanges.  7' or 8' deep??? 5' or 6' wide top flanges???
 
One concern I do have would be the distribution factor.  Not sure if having two beams would throw a DF calculation "out of whack".
 
I'm not sure if there are other concerns out there.  It doesn't sound like it.  At least, not that I've seen yet.
Please let me know if y'all have any more comments, things to look into, etc.
 

 
---------- Forwarded message ----------
From: admin <admin(--nospam--at)seausa.org>
Date: Tue, Mar 9, 2010 at 2:00 AM
Subject: seaint Digest for 8 Mar 2010
To: gr7070(--nospam--at)gmail.com


6: AASHTO -- Prestressed two-beam bridges
            by Michael Hemstad <mhemstad(--nospam--at)mbjeng.com>


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From: Michael Hemstad <mhemstad(--nospam--at)mbjeng.com>
To: seaint(--nospam--at)seaint.org
Subject: 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.

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<HTML>
<HEAD>

<META content=3D"MSHTML 6.00.6000.16850" name=3DGENERATOR></HEAD>
<BODY>
<DIV><FONT face=3DArial size=3D3>Bill is correct.&nbsp; Here in Minnesota=
 we =

routinely use two lines of&nbsp;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&nbsp;fracture-critical.&nbsp; The difference=
 is =

that the part of a beam that we mostly worry about in fatigue is the=
 tension =

flange.&nbsp; One steel flange can have a crack start and then one cold=
 night, =

it propogates and the beam breaks in two.&nbsp; The tension portion of a =

prestressed beam, by comparison, is 10 or more strands, with relatively=
 constant =

stress levels (due to the pre-tensioning).&nbsp; So it's pretty unlikely=
 that =

even one strand will fatigue, much less enough of them to let the beam =

fall.</FONT></DIV>
<DIV><FONT face=3DArial></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial>The exception to this is when they might be=
 susceptible to =

a traffic strike.&nbsp; Then, redundancy&nbsp;still matters. &nbsp;If such =

a&nbsp;two-girder ped bridge (they're almost always ped bridges, or=
 cart-path =

bridges such as on golf courses)&nbsp;is built over traffic, they use=
 increased =

clearances so that overheight loads get stopped by the next bridge up the =

road.</FONT></DIV>
<DIV><FONT face=3DArial></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial>Mike Hemstad</FONT></DIV>
<DIV><FONT face=3DArial>MBJ</FONT></DIV>
<DIV><FONT face=3DArial>Minneapolis, Minnesota</FONT></DIV>
<DIV><FONT face=3DArial size=3D3></FONT>&nbsp;</DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D3>Bill Polhemus replied to g=
 r:</FONT></DIV><FONT =

size=3D1>
<DIV><FONT size=3D3>g r wrote:</FONT></DIV>
<DIV><FONT size=3D3>&gt; Does anyone know if AASHTO addresses the use of=
 only two =

(prestressed</FONT></DIV>
<DIV><FONT size=3D3>&gt; concrete) beams to support the deck per span? Is=
 it =

specifically</FONT></DIV>
<DIV><FONT size=3D3>&gt; prohibited?</FONT></DIV>
<DIV><FONT size=3D3>&gt;</FONT></DIV>
<DIV><FONT size=3D3>&gt; If not prohibited are their other requirements to=
 be met =

that would</FONT></DIV>
<DIV><FONT size=3D3>&gt; make it dificult or practically impossible to=
 utilize =

only two beams?</FONT></DIV>
<DIV><FONT size=3D3>&gt;</FONT></DIV>
<DIV><FONT size=3D3>&gt; Thanks in advance!</FONT></DIV>
<DIV><FONT size=3D3>&gt;</FONT></DIV>
<DIV><FONT size=3D3>&gt; GR</FONT></DIV>
<DIV><FONT size=3D3>A few years ago there was some controversy regarding=
 the use =

of only two</FONT></DIV>
<DIV><FONT size=3D3>steel girders (or ONE steel trapezoidal "tub" girder)=
 for a =

bridge unit,</FONT></DIV>
<DIV><FONT size=3D3>due to concerns of fatigue and redundancy. That is the=
 _only_ =

time I've</FONT></DIV>
<DIV><FONT size=3D3>ever heard a limit placed on the number of girders in a=
 =

unit.</FONT></DIV>
<DIV><FONT size=3D3>Now, that's not to say there aren't other restrictions=
 I don't =

know</FONT></DIV>
<DIV><FONT size=3D3>about, but that's the only instance I've ever heard=
 tell of. =

If you have</FONT></DIV>
<DIV><FONT size=3D3>a very narrow section where "only" two prestressed=
 girders =

would work,</FONT></DIV>
<DIV><FONT size=3D3>I'm not sure why you couldn't do that so long as the=
 design =

provisions</FONT></DIV>
<DIV><FONT size=3D3>are met</FONT>.</DIV></FONT></BODY></HTML>
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