LLDF for exterior girders

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LLDF for exterior girders
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Hello,

I have a 3-span Tx70 (Texas 70 I-Girder) girder bridge with 13 girders on spans 1 &2, and 16 girders on span 3. I have the option set to "Compute Live Load Distribution Factors in accordance with current Project Criteria" in the Live Load Distribution Factors menu.

For each span, the program reports the LLDF for one of the exterior girders as the same as the adjacent interior girder, but not the other one. For example, Girders 12 & 13 both have LLDF of 0.774, but Girders 1 & 2 have LLDF of 0.724 & 0.746, respectively.

According to the exception of AASHTO LRFD Bridge Design spec article 4.6.2.2.2, that "For exterior beam design with a slab cantilever length equal to or less than one-half of the adjacent interior beam spacing, use the live load distribution factor for the interior beam. The slab cantilever length is defined as the distance from the center line of the exterior beam to the edge of the slab."

I'm running TxDOT config and 2010 TxDOT specification. Attached is my model

Please advise.

Regards,

Ink

Pickings
TxDOT's reply

TxDOT has responded to this through their support channels. For completeness, I post their response below:
 
Response to 1:  The Bridge Design Manual 3.5, states "For exterior beam design with a slab cantilever length equal to or less than one-half of the adjacent interior beam spacing, use the live load distribution factor for the interior beam."  This is a little off.  The manual will be updated to indicate that for exterior beam design with a slab cantilever length equal to or less than one-half of the adjacent interior beam spacing, the exterior beam should be treated as if it is an interior beam to determine the live distribution factor.  What this means is the equations for the Interior LLDF should be used using the exterior beam values.  This is exactly what PGSuper is doing and what we intended for these cases. 
 
For your structure, Girder 1 is a different length than Girder 2, so has a different distribution factor. Girder 1 is longer so has a higher non-distributed moment, which results in a lower distribution factor.  Girder 1 actually ends up having a slightly higher live load moment than Girder 2.  The dead load however is less for Girder 1, so you will likely end up with the design algorithm calling for fewer strands.
 
Response to 2:  Thank you for letting us know about this.  We have already addressed the fix for this in the next edition of PGSuper.  I am currently testing this version, but we should have it out in the next couple of months.
 

Richard Pickings, P.E.
BridgeSight Inc.

inktomei (not verified)
Thank you very much for your

Thank you very much for your reply!

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