"An acceptable concrete release strength could not be found."

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Visitor (not verified)
"An acceptable concrete release strength could not be found."

I am new to PGSuper and am using it to initially analyze a single span, spread box beam bridge. Each time I "design girder", I get the error I posted above. Is this because the beam shape I have chosen is too slender to handle the tensile strength or have I input my limits incorrectly? Any assistance would be greatly appreciated.

Thanks,

Lee

Rick Brice
Welcome to PGSuper. Thanks

Welcome to PGSuper. Thanks for giving it a try.

The design algorithm can't always arrive at a successful design. In your case, it can't find a concrete strength that will satisify the allowable stress limits. The tension stress limits tend to be the culprit in this case. The tension stress limit is in the general form x*sqrt(f'c) <= max, where x is a coefficient like 0.0948 and max is a maximum allowable tension like 0.2KSI. The designer will compute the tension caused by external loads and reduce it by the compression caused by the prestress. The remaining tension must be less than the tension stress limit. Using some simple math, we can compute the f'c required to make the tension limit check work. However, if the tension stress exceeds the maximum allowable tensile stress, there isn't a concrete strength that will work.

Other causes of this can occur in the girder stress and stability checks for lifting and hauling.

The best way to figure out what needs to be altered in your design is to accept the outcome of the PGSuper designer (even though it doesn't arrive at a good design), and run a Spec Check Report. You will be able to see where there is excessive tension and why a value of f'c cannot be computed.

Feel free to post your file if you can't figure it out.

Rick

LTalley (not verified)
Thanks Rick. I have attached

Thanks Rick. I have attached the file.

It could be too small of a beam section..

Lee

Rick Brice
I think the root cause of the

I think the root cause of the issue is that the PGSuper designer is not able to debond any strands. The excessive tensile stresses are occurring in the top of the girder near the ends at release and when the deck is placed. Debonding would help to reduce these stresses. In your girder library entry ( PennDot 48” x 21” Box Beam (Spread) ) none of the straight strand positions are designated as “debondable”. Without any strands that can be debonded, the designer is struck with increasing concrete strength until either the tension stress limits are satisfied or it has been increased to its maximum value (is this case it is the value that hits your maximum allowable tension limit of 0.3 KSI).

If debonding of strands is permitted, go back to your girder library entry and mark the appropriate strand positions as “debondable”. To do this, go to the Permanent Strands tab and double click on the strand grid entries. This will open the strand position editing window. Check the box that says “Allow straight strand(s) to be debonded” to permit strands at that position to be debonded. The designer will also constrain debonding to the limits specified on the Debond Criteria tab.

LTalley (not verified)
Thank you Rick. After

Thank you Rick. After posting my file I came to the same conclusion. Debonding strands worked. I also have another question. For shear design, I am getting a failure in my spec check at the end of the beam, about 2.8 ft from the end, even though my end block is 4' long. I thought it had to do with the support at the abutment, but since it is an integral abutment, the critical section for shear should be at the interface between the end block and start of the beam voids. The splitting force is also rather high. Any off the cuff ideas?

Thanks again,

Lee

Rick Brice
I don't have the current

I don't have the current state of your design, but based on the file you posted the issues are

1) Splitting resistance is too low (or conversely the splitting force is too high). This is probably because there are too many strands in the model I'm looking at. You can see the details of the splitting resistance calculations in the Details Report.

2) Confinement zone length is too short. This can be adjusted by adding confinement bars in additional stirrup zones. Just be sure to limit the bars to #3 at 6" per LRFD 5.10.10.2

3) There isn't enough longitudinal reinforcement for shear (LRFD 5.8.3.5). This occurs only at the face of support. The C/D ratio is 0.92 so it's pretty close. I would guess that adding a couple of bars would do the trick.

Even though you are modeling the left abutment as integral, PGSuper is treating it as a simple support because you've got the analysis mode set to Simple Span. The Simple Span analysis mode treats all spans as simple spans regardless of the specified boundary conditions (this is a common design policy). Change the analysis mode if you want PGSuper to account for the continunity at the abutment.

One last note, to understand better why spec checks aren't passing, look at the details report. This report has all the details of the calculations so you can see exactly which parameters can be adjusted to make the design work.

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