PSC Design using Midas Civil

Question1 :

I have a some questions I would like to ask. Specifically, because the program doesn’t show how it has calculated the losses in the prestressing strands explicitly I am having trouble determining exactly how the program is coming up with the stress in the tendons (FDL1, FLL1). I was hoping you could point me in the direction of someone who has some expertise here who could help clear this up for me.

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Answer1 :

If you may refer the help menu, following is stated.



I have attached a model file, in which I would refer how these values are derived for tendon stress check.





Point 1 refers to FDL1 i.e the stress at tendons at anchorage
Point 2 , is little tricky.

You may note that these stresses do not exactly match.
This is mainly attributed to the, points in which midas gives output results.

If the maximum stress point does not fall within the 5 points ( i, 1/4,1/2,3/4,j ) of the elements, you can note the difference between the values in Loss table and stress table.

In the Loss table only at I & J end is provided , where as in tendon stress check, stresses are arrived at all the five points for each element and tabulated and hence accordingly the variation between both the tables. One way is to fine divide the elements, so that both the values match.




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Question 2:

Thank you for this explanation, it has been helpful to explain the difference in the check points. What I am more concerned about is learning exactly how this “Stress (After immediate Loss) A” is calculated (and any other of the losses calculations for that matter). I have looked through the manual but it does not provide any sort of equations or detailed information as to what the program is actually calculating behind the scenes. I am unable to open the file you provided because I am currently working using Midas Civil 2017. I have attached my file here which I created based on attempting to follow along with the attached tutorial in order to ensure my modelling is being done properly however the example fails to go in depth about the results (it would also be helpful to continue along with this example however cover the results and design features much further in depth). When I look at the values in column A I cannot match these with hand calculations based on AASHTO(also attached).

For an initial jacking force applied at 1 end of 202.5 ksi midas Civil Reports stress in tendons after instantaneous loss (losses due to elastic shortening) as 191.6538 ksi.
Hand calculations based on AASHTO simplified equation [C5.9.5.2.3a-1] produces a value for stress in tendons after elastic shortening of 197.144 ksi

This difference appears to be rather large and leads to vast differences in results. I am curious as to how midas is handling this because in all instances I have found in AASHTO a modulus of elasticity of concrete at transfer or time of load application, Eci, must be used. In midas I have not found where the release strength fci or Eci (function of fci) can be input. It should be noted I have verified my hand calculations with LEAP Bridge Conspan results and match exactly. It would be very helpful if midas could output these losses with detailed information of how they are calculated similar to how the moment capacity and shear design is done in the PSC design excel reports. I hope my questions and concerns are clear however if necessary I can provide more clarification and information as needed. Thank you for your help.

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Answer 2:

"Stress (After Immediate Loss): A" column gives the stress values due to the elastic shortening loss due to Prestress only (in case of pre-tensioning).
The Loss due to self-weight is included in "Elastic deform.Loss: B" column. So Elastic deformation loss includes the other type of elastic shortening losses which are caused by the subsequent loadings i.e. self-weight, live loads, creep, shrinkage etc after the prestressing force is applied.

As per the code, there are two equations given for the calculation of elastic shortening for the pre-tensioned members.
Eq. 5.9.5.2.3a-1 is used for the detailed analysis at each section for the various loading conditions. So this equation can be used for the individual load cases to calculate elastic shortening loss.
While Eq C5.9.5.2.3a-1 is used to calculate total elastic loss or gain taken as the sum of effects of prestress and applied loads.

So the hand calculations (done by using Eq C5.9.5.2.3a-1) are inclusive of the self-weight, so the values are different than the values obtained from software. (As software values for elastic shortening (Instantaneous) are exclusive of self-weight)
You can refer the help manual as well: http://manual.midasuser.com/EN_Common/Civil/860/Start/07_Results/12_Result_Tables/09_Tendon/Tendon_Loss_(Tendon_Group).htm

Kindly find the verification file attached. The verification is done using CEB-FIP90 code. It will help you to learn about how the software is calculating the losses.