I want to model two strands in top of I-beam with debonding in the middle span. Strands will be cut in the field in the middle of prestressed beam. After cutting these top strands I still need prestress at top of beam at transfer length at each end (to meet tension requirements). I do have mild reinforcement in top, so casting yard tension stress is ok.
PGSuper has option to use temporary strands with 10' bonded length at each end. Is this the option I use? At transfer length at each end of the girder once you ramove the strands - at deck placement stage service 1 (non composite loads) I am having hard time meeting tension limit of 0.2 ksi. I think program does not take into account 2 top bars in 10' bonded length at each end.
May 05, 2012
#1
Strands and debonding at top of I-beam
You will want to use the temporary strands. PGSuper assumes them to be bonded for the end 10 ft of the beam and unbonded elsewhere. Immediately prior to the diaphragms being cast, the temporary strands are removed. The effect of the bonded strand over the end 10 ft is ignored once the strands are removed.
The tension stress in the top of the girder can be well controlled by harped strands. WSDOT's practice is to use the temporary top strands only to control stresses during lifting and handling. We rarely have issues with excessive tension in the top of the girder when the temporary strands are removed.
If you could send me your PGSuper project file, I'd like to see what options there are for resolving the issue.
Rick
Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office
I am trying to design girder by debonding bottom strands at each end of girder instead of harped strands. This seems to be general practice here.
Please look at span 2 girder C. I have added two temporary strands at top to reduce tension stresses at transfer length at casting stage. My model also requires additional reinforcement to transfer horizontal shear.
PGSuper is wonderful and user friendly program. Detailed report is very helpful!
Tahnks for your quick reply.
Thanks for posting your file... I think the problem is a matter of interpretation. The allowable tension that you are using is for temporary stresses before losses, though you are checking stresses in an intermediate stage (some losses have occured, but not all of them).
The project criteria that you are using is limiting the allowable tension to 0.0948sqrt(f'c) <= 0.2 KSI. WSDOT policy is to limit the stress in this intermediate state to 0.19sqrt(f'c).
If you change the allowable to 0.19sqrt(f'c) and f'c = 8.6 ksi, you can make the girder work.
This all comes down to the allowable tension the bridge owner will permit for temporary conditions during construction.
Thanks! Allowable tension of 0.19sqrt(f'c) will work for temporary conditions. I do have mild steel in top flange as well.
I have few more questions:)
1. In project criteria, negative moment capacity that includes positive moment on non composite section in Mu in strength limit state - Does that mean negative Mu based on composite loads on continuous span will be reduced by positive Mu based on non-composite loads on simple span (less conservative)?
2. In project criteria, moment capacity - Do you generally include mild steel in capacity calculations?
3. Envelopes for composite dead loads and live loads on simple span are calculated by PGSuper. Where do you use that in design?
4. Horizontal interface shear/length for strength limit AASHTO 5.8.4.3 - Is it standard prctice to intentionally roughen surface to increase shear capacity for AASHTO I-beam?
5. Shear distribution factor at exterior girder, equation g=e(gint.) - program does not show calculations for equation e in detailed output (in my case that equation does not control).
6. Is there a way to export girder section and elevation as a cad or dxf file?
7. Library AASHTO I-beam type III and IV - I changed top flange width in my project library it was not correct.
8. My file crashes when I try to copy girder 1-C to all girders on span 1 and 3, and 2-C to rest of the girders on span 2. I have attached the file.
1. In project criteria, negative moment capacity that includes positive moment on non composite section in Mu in strength limit state - Does that mean negative Mu based on composite loads on continuous span will be reduced by positive Mu based on non-composite loads on simple span (less conservative)?
Yes... you have your option of including or excluding the simple span moments due to girder, diaphragms, slab when computing Mu in the Strength limit state for purposes of evaluating the strength of the deck.
2. In project criteria, moment capacity - Do you generally include mild steel in capacity calculations?
Generally not... the strand alone is usually more than enough.
3. Envelopes for composite dead loads and live loads on simple span are calculated by PGSuper. Where do you use that in design?
It is WSDOT's design policy to design for the envelope of simple span and continuous span behavior.
4. Horizontal interface shear/length for strength limit AASHTO 5.8.4.3 - Is it standard prctice to intentionally roughen surface to increase shear capacity for AASHTO I-beam?
It is standard practice in Washington to intentionally roughen the top flange surface. The plans for your girder will have to match what you put into PGSuper for analysis.
5. Shear distribution factor at exterior girder, equation g=e(gint.) - program does not show calculations for equation e in detailed output (in my case that equation does not control).
Thanks... I'll get that fixed.
6. Is there a way to export girder section and elevation as a cad or dxf file?
No, but that would be a great feature to add.
7. Library AASHTO I-beam type III and IV - I changed top flange width in my project library it was not correct.
Thanks, I'll double check the AASHTO library
8. My file crashes when I try to copy girder 1-C to all girders on span 1 and 3, and 2-C to rest of the girders on span 2. I have attached the file.
I get the same issue. I'll get this feature fixed.
Thank you so much for your time. Great program and support!
Does PGSuper design elastomeric bearing pads? I see it gives girder reactions and rotations at abutments and bents for bearing design. I think at bent program gives total reaction from girder on each side of bent.
PGSuper does not design elastomeric bearing pads. The Bearing Design Parameters Report gives you all the reactions and rotations you need to do the design.
Rick
Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office
Thanks!
I am trying to come up with depth of compression block and moments same as the program. Does PGSuper use different designations then AASHTO 2012, section 5.7.3? How do you come up with fpe and fps.avg in the table? I was able to verify fpe at midspan only (prestress after final losses). I have 6 out of 22 strands debonded at girder ends. I am not sure if PGSuper is using section 5.7.3.1.3b.
PGSuper does not use the simplified analysis per 6.7.3.1.3b. It uses a more rigourous strain compatibility approach. Refer to the chapter "Ultimate Moment Capacity Analysis" in the Theoretical Manual section of PGSuper's built-in help for a detailed description of the approach.
Press the F1 button anytime in the program to bring up help.
Richard Pickings, P.E.
BridgeSight Inc.
Thanks!
I an not sure if PGSuper calculates minimum spacing required for crack control in slab as per equation 5.7.3.4-1.
Also in my output for negetive moment at pier- Mcr in minimum reinforcement table is different than calculated Mcr. I am not sure why?
PGSuper does not check crack control requirements. More robust input for longitudinal steel is needed to make this practical.
Please attach your input file for the Mcr question.
Richard Pickings, P.E.
BridgeSight Inc.
Please see attached file. Look at the detailed report for girder 2-C and calculations for negative moment.
Thanks
It appears that there is a bug in the computation of Mcr for the minimum reinforcement table. The bug is only for the 2012 AASHTO LRFD. I have to defer this one to Rick Brice who is on vacation until July 23.
Richard Pickings, P.E.
BridgeSight Inc.
Thanks!
I don't know if you still have my file or not. The question I have is in Longitudinal Strain- Strength I calculation table (for girder 2-C) why As in negative moment region at bent and Aps in positive moment region changes? I have same As in negative region. I have debonded bars but Aps should change only at these sections and transfer lengths. I don't know why it changes at almoast every point.
Since this is an ultimate condition Aps and As must be reduced unless they are fully developed. Note that the development length is much longer than the transfer length. The transfer and development length details chapter in the details report gives clarification, but the computation by hand gets difficult when you have multiple debond locations as in your model (assuming that "Sledge Road Bridge.pgs" is the correct file.
Richard Pickings, P.E.
BridgeSight Inc.
Detailed report table shear- Live load values per girder for fatigue truck:
If you still have the model I sent you- Shear distribution factor for fatigue for girder 2-C should be 0.6. PGSuper is using different factor (may be DF for moment).
You are correct. Thanks for bringing this issue to our attention. Fortunately, there are not any shear related fatigue checks performed by PGSuper so this issue has no bearing on the outcome of the analysis.
Rick
Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office
You are right we don't use it for checks. I found one more thing!
Detailed report for longitudinal reinforcement for shear and shear check for strength-
Program does not reduce Aps in debonded regions (girder 2-C). This changes Apsxfps value in equation 5.8.3.5-1.
If you look at the table labeled "Longitudinal Reinforcement Shear Check Details - Table 1 of 3" you'll see that fy and fps have been adjusted for development length. In this case we don't want to again reduce As and Aps.
fy and fps come from the moment capacity analysis which does the adjustments.
Thanks a lot!
How does the program handle number of extended strands required? Does it calculate strands or moment required for effects of creep, shrinkage and positive moments? I know I can get positive moments from the tables. I have looked at extreme event 1 requirements in section 5.1.3 of the design manual.
Starting with Version 2.6, you can model extended straight strands with PGSuper. PGSuper does not determine the number of extended strands, you have to input that. In the strength limit states, extended strands are considered to be fully developed at the face of the girder. The force at ultimate in non-extended strands is taken to be 0 at the face of the girder and gradually reaches 100% at the end of the development length. By comparison, there isn't a force reduction due to lack of full development for extended strands. The effect of extended strands are seen in the computations for shear capacity, longitudinal reinforcement for shear, and ultimate moment capacity.
PGSuper takes creep and shrinkage into account when computing effective prestress. This is not influenced by extended strands.
Section 5.1.3D.2 of the WSDOT BDM gives a procedure for determing the number of extended strands. This may be of interest to you.
Best regards,
Rick
Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office
Yes, I did look at 5.1.3D.3. It mainly shows how to calculate required moment for Extreme Event 1 limit state but not for creep and shrinkage effects.
Thnaks!
WSDOT doesn't explicitly design for moments due to creep and shrinkage effects because we use a simple span design philosophy. Here are a couple of references for computing restraint moments that might help you.
NCHRP Report 322 - Design of Prestressed Bridge Girders Made Continuous
"Resolving Restraint Moments and Designing for Continuity in Precast Prestressed Concrete Bridges", PCI Journal, July-August 2003
Thanks!