Positive moment capacity

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mdmetzger21@gma...
Positive moment capacity

why might i be passing all the spec checks except the Strength I positive moment capacity, and only at the extreme ends (FoS, 1.0Ls) supported by piers? I am modeling a 3-span, prestressed box girder, and it is passing all other checks.

Rick Brice
PGSuper reduces the

PGSuper reduces the effectiveness of the prestressing strands due to the lack of full development in the development and transfer zone. This may be causing the issue.
If you post your PGSuper project file, I'd be happy to take a look at it.
Best regards,
Rick
Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office

mdmetzger21@gma...
project file

Thanks Rick, I have attached the file. I also do not understand why I am failing the longitudinal reinforcement for shear check only at the abutments.

Matt

Rick Brice
Matt,   The reason you are

Matt,
 
The reason you are getting zero capacity at the ends of your precast girders for flexure and longitudinal reinforcement for shear is because the strands do not have force in them at that section.
 
At the face of a precast girder, the stress in the strands is zero. Consider the length of strand that extends outside of the concrete after fabricating the girder. After the strand is cut and the prestressing released into the girder, the force in this length of strand is zero as it is unstressed. For equilibrium to be satisfied, the force just inside the face of the girder must also be zero. The force in the strand increases linearly over the prestress transfer length to the effective prestress force (See LRFD 5.11.4.2).
 
For your PGSuper model, you have used a zero length connection. For this connection geometry, the distance from the end of the girder to the face of support is zero. Prestress force cannot be transferred over this length.
 
You can work around this issue by modeling the end distance and bearing offset as they really are in your bridge. Alternatively, you can use a connection that is virtually zero length by modeling a bearing offset and girder end distance of 1” and then changing transfer length model to be “zero” length. Edit your project criteria to change the transfer length model. At the bottom of the Losses tab, you’ll find a setting for transfer length per LRFD or “zero” length.
 
If I can shed any more light on this subject, please let me know.
 
Rick
 
Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office

Pickings
Transfer length

Matt,

It looks to me that both problems are due to the fact that the bearing locations are at the very ends of the beams. This leaves zero distance for the strands and rebar for transfer/development. Normally you will have some overhang over the bearing, and also the bearing in your example has a zero support width. PGSuper computes the longitudinal reforcement for shear requirement at 1/2 of the  support width in from the bearing.

Both of these issues can be addressed by editing the connection library entry in the library editor to match the actual beam end overhangs and bearing width dimensions.

Regards,

Richard Pickings, P.E.
BridgeSight Inc.

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