Abnormal Tendon Primary and Secondary Stress Values

Question:

Hello,

In the attached model, the tendon Primary and Secondary stresses of three elements are being very very high compared to other members.
These members are located at the edges of the bridges above piles.

can you please check why these abnormal results are calculated? 

Thanks

Answer:

Hello ,

The E value for the dummy element is very low.  This E value should be equal to the actual material being used.  Only the weight density should be 0.  Kindly modify modulus of Elasticity.

Do let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS


Dear Nandeep,

Thanks for answer. But company says that it continues after revision. You can find the screenshots for high values.
It is the same secondary creep values as well.
Can you check again?


Hello,

Sorry about not addressing the complete issue first time around.  Basically what matters is the stiffness.  So, Elasticity as well as area & moment of inertia of the section matters.
In this case, the issue is happening purely due to incorrect tendon force transfer.  The tendons have been assigned to dummy elements with negligible properties.  Now, the relaxation and creep and shrinkage losses would come in accordance to definition for plates.  However, the immediate loss would be calculated in accordance to the section property to which the prestress has been assigned.  Since the section has negligible property, initial loss becomes a very high force.  This can be checked very easily in Results> Results Tables> Tendon> Tendon Loss.
In this particular case, I wouldn't recommend going for such modelling.  If feasible, kindly create a grillage model rather than using a dummy element approach.
On simply considering actual section property to which the tendons have been assigned, the results are better.

Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS


Dear Nandeep,

Thank you for your detailed answer. I have shared this with the user. However he said that using grillage type will not be suitable for this model since it is only plate type slab. There is no gap in the center as in grillage.
Do you know how to create a slab (without gaps) with tendons? They have many bridges similar to this, but they can not model the actual bridge. Additionaly, they can not get the hand calculation results in midas civil even if the hand calculation is correct.

Thanks.


Hello,

Personally, I wouldn't recommend civil for this, if it is necessary to provide prestress in this way.  We have tested the scenario for when the prestressing cable is concentric with the plates, but in this case, there is an eccentricity and that complicates things.  A sample file of how prestress could be assigned to plates is attached herewith.  The client would have to come up with a methodology to replicate this for their structure.
In this case, the final tendon forces were comparable to the tendon forces of the beam element.  However, the creep & shrinkage loss would be 0 in tendon loss table as these would be considered as elastic deformation loss for the tendon in plates.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 10/17/2018 10:00 PM (dklee@midasit.com)      Updated: 10/18/2018 7:31 AM (dklee@midasit.com)
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perilikaya_A1_3d_4.mcb
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perilikaya_A1_3d_4_20180825.mcb
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Tendon Loss.mcb
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