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Re: AASHTO Plate Girders

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I've done some additional digging.
 
The 17th ed. 10.5.4 states to use contraflexure pt. distance.  However I cannot find similar language in the LRFD code, which governs in this case.  I found what might be contradictory language in article 6.7.1 which states:
      6.7.1 Effective Length of Span
      Span lengths shall be taken as the distance between centers of bearings or other points of support.
 
Except this article doesn't specifically state to use this defined length when looking at span:depth ratios while 17th does.
 
In addition, to the NSBA example problem I mentioned (http://www.steelbridges.org/pdfs/DesignExample1.pdf Page 8, section 4.1) using c-c length there is this publication http://www.dot.state.tx.us/publications/bridge/steel_bridge.pdf which states:
    The recommendations in AASHTO LRFD Bridge Design Specifications, Article 2.5.2.6.3, provide a good estimate of a minimum web depth for straight girders. Consider this depth a starting point. If vertical clearance is not a problem, adding depth can result in lighter girders. For curved girder web depth, use either the AASHTO recommended minimum depth for straight girders, increased by 10 to 20 percent, or use LRFD Equation 2.5.2.6.3-1 as a starting point. Aesthetics also has a role in girder depth. A rule of thumb for a well-proportioned superstructure is to have total section depth (slab plus girder) in the range of 0.033L to 0.04L (L = c–c brg length).
This bridge will be in TX.  The bolded statment matches closely the .032L item in the AASHTO table and I suspect that .033-.040 is to produce a more optimum section like the NSBA example shows.
 
So while I have a number of credible sources some are potentially contradictory and it appears per the LRFD code the formulas for suggested min. depths use L as a c-c bearing length and no longer use contrflexure distances.
 
Anything that could help clear this up would be very appreciated.

 
On Feb 5, 2008 1:21 PM, g r <gr7070(--nospam--at)gmail.com> wrote:
Thanks a lot for your prompt response Tom!
 
http://www.steelbridges.org/pdfs/DesignExample1.pdf
 
The above is a design example from the NSBA.  Page 8, section 4.1 uses the table I referenced.  They use L = max span length, as did I.  I don't see anything that defines L in the AASHTO table.  Can you help me out by pointing to a part of the code that I may use to document your proposed L?
 
Also,  if I'm reading the table correctly, the formula I used specifically states "depth of I-beam portion of composite I-beam".  I take this to mean the outside dimension of flange to flange.
 
Again,  thanks for the response Tom.  If you or others can help with the additional issues above I'd appreciate it!

 
On Feb 5, 2008 11:48 AM, Cummings, Thomas J. <tcummings(--nospam--at)gfnet.com> wrote:

GR,

 

The depths indicated in the table are the total structure depth, including the deck slab and haunches. Also, the span length "L" is taken as the distance between dead load points of contraflexure for continuous spans. These two criteria should reduce your girder depths to near the 6ft. deep section.

 

Good luck with the design.  Hope this helps.

 

Regards,

Tom

 

Thomas J. Cummings, PE

Manager - Bridge Design Practice

Gannett Fleming Engineers and Architects, P.C.

One Penn Plaza, Suite 2222

250 West 34th Street

New York, NY 10119

P:  (212) 967-9833 Ext. 5244

F:  (212) 268-6684

E:  tcummings(--nospam--at)gfnet.com

 


From: g r [mailto:gr7070(--nospam--at)gmail.com]
Sent: Tuesday, February 05, 2008 12:34 PM
To: seaint(--nospam--at)seaint.org
Subject: AASHTO Plate Girders

 

I'm taking over a past dormant project that has a bridge where 3 of the 26 spans are continuous steel plate girders.  Only layouts have been completed (and approved), but they show a plate girder section of 6' deep - that's the girder depth.

 

The spans are 200'-280'-200'.

 

I'm not very experienced with steel plate girder design.

 

Article 2.5.2.6.3 Optional Criteria for Span-to-Depth Ratios contains a Table 2.5.2.6.3-1.  This table provides a minimum recommended steel girder depth of 0.027L.

 

0.027 * 280 = 7.56'

 

That's much greater than the current 6'.

 

So how worthwhile is the value in this table.  That's a stark difference.  Did the original producers of the layout just have no idea what they were doing?

 

Thanks in advance for any comments or helpful criticisms!