Initial prestress calculations

[Anonymous]

With reference to the attached screenshots, I was hoping someone could explain how ASBD is calculating the total initial prestressing force (without reductions). The sum of the top and bottom prestressing forces is 2874.3975kN (2432.1825 + 442.215) - so where is the value of 2855kN coming from?

Likewise, the calculation for the reduced value of the prestressing force (2756.466kN) does not appear to align with the requirements of EC2 Cl 5.10.3(2). By my calculation this should be:

f = min(k7 x fpk; k8 x fp0.1k)

= min (0.75 x 1860; 0.85 x 1640)

= 1394MPa

F = (f)1394 x (A)150 x (n)13

= 2718.3kN

Or with losses included:

f = 1395 - 11.7 - (-9.75) = 1393.05

F = 1393.05 x 150 x 13

= 2716.45kN

I have included the material properties for the prestressing tendons as a screenshot.

Finally, on a similar point, I have not yet found a way to directly amend the initial prestressing force applied to the tendons other than manually amending the values for k7 and k8.

[dave_geeves · Accepted solution]

Hi,

Thanks for an interesting question, and it is good to know that the printed output is being checked occasionally to verify the results.  Thanks also for including a couple of screen shots to help explain your query, but if you have any further questions on this it is better if you also zip up a copy of your data file and attach it to the post as it makes it easier to directly replicate your issues.

I must admit that the calculations are not as clear as they could be for this case, which I have raised with the developers before, but I do believe they are essentially correct.

Let me start with your last point in that there is no way to alter the initial prestress.  You mention that you could manually change k7and k8, but in fact it is more common to limit k1 and/or k2 (Cl 5.10.2.1), which is not exposed in the materials form.  To change these values, you will need to use the general NDP form from the options menu.  So, to limit the initial prestress (before any loss whatsoever) to 70% of fpk, you would set k1 to 0.7 (where the recommended value is 0.8).

The values of k7 and k8 are used to check the prestressing stresses at transfer after all immediate losses are considered and if these are exceeded then the initial prestressing needs to be reduced. However, the immediate losses are calculated as a function of the initial prestress, so this becomes an iterative procedure.  I believe the software does this until an appropriate convergence is reached and then reports a %age of the initial prestress that needs to be used (an information message pops up when the calcs are performed).  If all iterative calculations were shown in the output then there would be 10s of pages, so only the last converged value is used in the printed results.

The reduced value of prestress you are seeing (2756kN from 2855) is the reduced value of prestress before immediate losses.  The value of prestress that you have calculated, using k7 and k8, is the value that you would expect when all immediate losses have been considered.

When I was first introduced to Eurocodes, I found this quite difficult to fathom, especially when comparing it to the simpler approach we adopted in BS5400 but I have now got used to it.

I hope this has helped and if this has answered your query then please mark my reply as an answer so that others may benefit.  Thanks.

Kind regards

Dave Geeves

[Anonymous]

Hi Dave,

Thanks for your thorough response - much appreciated. 

Kind regards,

Liam