Hi,
I have two types of PSC I- Beams and both of them having Strands at Top. I have attached the sections. The Strand at the Top Flange are tensioned at 2.2 Kips for some of them and 35 Kips for few cases and they are PERMANENT Strands and not Temporary.
How do I model these strands?. Also, Is there a way I can use 2 different Jacking forces?
Please let me know your thoughts.
Thanks
Venket Tiruchi
| Attachment | Size |
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 Top Flange Strands.pdf | 221.25 KB |
Venket,
Sorry to see that peer users of PGSuper didn't respond to your question. The only way these free peer-to-peer support forums work are if everybody participates.
For your "Girder without Draped Strands" case, you can easily model this in PGSuper. You'll have to fake out the software a little. Make the geometry of the "harped" strands such that the strands are straight. The you can input one jacking force for the strands in the bottom bulb of the girder and a different jacking force for the "harped" strands that run along the top of the girder.
For your case of "Girder with Draped Strands", the current PGSuper does not support this case. However, version 3.0 of PGSuper can. While not officially released, beta versions of PGSuper 3.0 are available as part of WSDOT's BridgeLink application.
There is a new strand input feature that lets you completely ignore the strand locations defined in the girder library and simply input rows of strands anywhere you like in the cross section. Strands can start and end at any elevation in the cross section and can have up to two angle points for modeling harped strands. Each strand type can have its own Pjack.
Sorry it took so long to get you an answer. Hopefully you were able to figure this out on your own or take advantage of one of the commercial support options available.
Rick,
Using either PGsuper 3 or the previous version, I get an error when I use a prestress jacking stress less than 0.5 fpu (calls out Article 5.9.5.4.4b LRFD 3rd Edition 2004). In my case, I want to pull the top strands 10 kips each for a total of 40 kips. I can not bypass this error once it pops up. Can I modeled this situation in PGsuper another way?
-Donn Digamon, P.E.
There could be a couple different reasons why you would get this error message. The most common is that you are using Project Criteria that specifies relaxation losses prior to prestress transfer are considered in the effective prestress computations (e.g. WSDOT criteria, see WSDOT BDM 5.1.4.3D).
Prior to LRFD 3rd Edition 2005, computation of relaxation losses before transfer was required by LRFD. However this was removed with the 2005 interims when the new loss methods were adopted. WSDOT still computes the relaxation losses before transfer, and without guidance from AASHTO, we use the procedure from LRFD 2004.
The relaxation loss equation in LRFD 2004 requires that fpj > 0.5fpu or it isn't valid.
The only workarounds I can think of are to either model your top strands with fewer strands at a higher force to increase fpj (say 2 strands @ 20 kips or 1 strand at 40kips) or use Project Criteria that doesn't consider relaxation loss prior to transfer in the effective prestress computations.
What is Georgia DOT's policy regarding relaxation prior to prestress transfer?
Is using lightly stressed top strands common in Georgia?
PGSuper support for lightly stressed top strands could be better, and we would be collaborate with you folks at GDOT to improve the software (much like we collaborate with TxDOT and KDOT). Since PGSuper is open source software, there are was to modify the software to cover all of GDOT's policies and create PGSuper Configurations that encapsulate GDOT standard girders and design policies.
I have the same situation as Venket. I have a scenario where I have top flange strands that are straight, depressed (harped strands), and more straight strands in the bottom flange. The methodology described here of apply a Pjack to different strand types does not seem to solve the problem because the top flange straight strands need to have a Pjack of one value and the bottom flange straight strands need to have a Pjack of a different value. Is there a way to assign a Pjack to individual strands or does the program limit to assigning a Pjack to types only? The other way is to be able to assign a cross sectional area to the top flange straight strands that is different than the bottom flange straight strands. This would also accomplish the same result but I don't see that option.
Disregard this. We found a way to emulate the different jacking forces and mix/match the types. Basically we classified all the traditional strands as harped and just geometrically oriented the bottom straight strands as straight....Then classified the top flange straight strands as straight and gave them a different Pjack. this works. next item to consider is "how to make these strands as non effective for purposes of camber calculations" so that the fabricator can use these as optional and the haunch calculations will workout regardless if the fabricator includes the top flange straight strands or not.
Rick,
Thanks for the input. I am trying this out today.
Thanks
Venket