Hello everyone,
For my thesis, I am investigating post-tensioned slabs and am using the Tendon add-on to model the tendons. I understand that the add-on is still under development, and I have found several useful discussions on the forum regarding its current capabilities.
As I understand from previous discussions on the forum, prestressing can be modelled using axial strain, which is particularly useful for defining a varying prestress profile along the tendon to account for prestress losses.
Since prestress losses are not yet available in the Tendon add-on, I currently calculate the losses (friction, wedge set, and long-term losses) separately and then plan to apply the resulting prestress as a varying axial strain at several locations along the tendon based on the final prestress profile. Although for now I only apply a uniform axial strain.
However, when I tried to verify the resulting axial force in the tendon against the axial strain that I applied, I noticed some differences. When axial strain is used as the load input (compared to End Prestress for example), the resulting axial force in the tendons decreases towards the middle of the tendon and increases again towards both ends. This seems similar to friction losses (if prestress is applied at both ends of the tendon) and thus I believe it could be a related.
I tried comparing this behaviour with my own calculation of the friction losses, but the difference between the two is quite significant. Therefore, I am not sure whether the reduction in axial force in the tendon is actually caused by friction losses or whether there is another interaction or mechanism that I am not taking into account. This leads to a couple of questions.
- Is this reduction in normal force caused by friction and elastic deformation effects?
- Is it possible to apply prestressing as axial strain from only one end of the tendon? Currently, it seems that both ends are affected.
- If the reduction is caused by friction, is there a way to disable this effect when using axial strain? I have already tried setting the friction factors of the tendons to zero, as well as using a pre-tensioned tendon, but this did not resolve the issue.
Any insights into how RFEM handles axial strain in the Tendon add-on would be greatly appreciated.
Thank you in advance!
Hello @ibrahim_sinan 
Thank you for your question and participation in our Dlubal Community! 
The solution for prestressed concrete implemented in RFEM 6 is a finite element approach that uses ‘Tendon’ elements. These member elements are automatically coupled to the „master“ object – which may be a concrete slab or a beam member element – provided they are located within its volume. ‘Tendon’ elements have only axial stiffness and can be positioned eccentrically with respect to the elastic axis or the plane of the “master” object.
At the current stage of development, the programme does not take into account losses due to friction or wedge penetration. The phenomena you describe are due exclusively to the stiffness of the ‘Tendon’ element and its position relative to the main element or surface.
Tendon eccentricity introduces additional stiffness into the finite element system and, of course, depending on the tendon’s position relative to the main object, generates an additional internal force that influences the result. If the tendon element and the main object have the same axial position or lie in the plane of the surface — for example, a centric tendon — then this effect does not occur.
The ‘axial strain’ load type in a centric tendon, taking into account the tendon’s stiffness, results in an internal force corresponding to the axial strain minus the elastic shortening. If the tendon is also eccentric, due to the bending moment, it will shorten or lengthen in addition to this elastic shortening. Therefore, in a parabolic tendon with constant axial strain load, a result diagram of internal forces similar to that of simultaneous tension on both sides is obtained due to the eccentricity, but this has nothing to do with friction losses or wedge penetration.
If you want to neglect elastic shortening or eccentricity effect you should neglect Tendon Stiffness. In the actual state of development, simply reduce the stiffness of the prestressing material by a factor of 1/10,000 and multiply the axial strain by 10,000 by means of Structural modification. Have a look to the attached file.
We wish you all the best with your degree thesis. 
slab pt.rf6 (1.1 MB)
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Hi Manuel,
Thank you very much for clarifying how the tendon elements integrate into the master object. It is now clear to me what was causing the differences I saw.
Unfortunately, I cannot open the file you shared, as I am using an older version of RFEM 6 (6.13). Instead, I created a new file with a slab and a centric tendon layout. I was able to replicate the reduction in axial force in the tendons due to elastic deformation using a hand calculation based on an approach for pre-tensioned tendons (only looking into the results of the prestressing load case).
This, however, raises some further questions.
Since I am working with post-tensioned tendons with a parabolic curvature, I assume that a suitable approach would be to apply the tendon stiffness reduction using a Structural Modification (also increasing axial strain) in order to neglect the effect of the elastic shortening, although I am not entirely certain whether this would be the correct approach.
Furthermore, I have some doubts regarding how I plan to account for the prestress losses in my model. I am planning to apply the resulting end prestress as a varying axial strain at several locations along the tendon (reflecting my prestress loss graph). Would this still be a suitable approach for accounting for the prestress losses if the Structural Modification is also applied?
Thank you again for your explanation and for taking the time to look into this.
Best regards,
Ibrahim
Hello @ibrahim_sinan
Thank you for your positive feedback 
. You should be able to download the latest version and use it in the same way as you are using version 6.13. I’ve included the download link below.
https://cdn.dlubal.com/download/RFEM_6.15.0007.exe
As for your first question: in the case of post-tensioned tendons, if they are tensioned On site simultaneously, no elastic shortening occurs, as the prestressing jack controls the force and the shortening of the beam takes place gradually until the target force is reached. In other words, by the time the post-tensioning jack has reached the specified force, the beam has already been shortened. Elastic shortening occurs only in prestressed precast Tendons or in sequentially tensioned post-tensioned Tendons. You will therefore need to apply an axial strain whilst at the same time reducing the stiffness of the tendons by the same order of magnitude as you have indicated in your question in order to neglect elastic shortening and stiffness-eccentricity effects.
Regarding the second question, yes, this is a suitable approach, you will indeed need to calculate the losses due to friction and wedge penetration manually and include them as variable axial strain diagram along the tendon length scaled by a factor x10,000 and at the same time reduce Tendon Material stiffness by a factor of 1/10,000 in order to neglect elastic shortening and stiffness-eccentricity effects.
Normally, during the preliminary design phase, we make certain simplifications regarding losses and therefore apply a constant axial strain and model single resulting tendon with a parametric and iterative layout. Once we have designed the tendon layout and determined the amount of prestressing steel, we reconfigure a realistic construction arrangement, with more Tendons members and a specifying real prestressing losses depending on where and how Tendons will be Tensioned.
Keep tuned, as we expect to release the prestressing losses for prestressed tendons and a solution to neglect Tendon stiffness soon 
.
I hope I have been able to help you with your interesting questions , and please do not hesitate to submit your thesis via our website so that your work can be recognised and you will receive a 6 month professional license after your graduation.
https://www.dlubal.com/en/education/free-structural-analysis-software-for-students/submit-your-thesis
Wishing you a great day 
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Hi Manuel,
Thank you very much for the detailed explanation and for answering my questions. Everything is clear to me now, and I have a much better understanding of how to approach the prestressing in my model.
Thanks again for your time and help!
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