Secondary PT moment and Compression only springs

Question:

Hello,

I have a question on secondary PT momnet with compression-only springs. In this model, I have a cantilever beam with two compression-only springs restrained in the DZ- direction as shown.

The construction stages include:

1. Activate substructure, cantilever, and two COMPRESSION-ONLY springs at the tips.
2. Stresss cantilever tendons.

I understand that the PT force never oversedes the dead load, and therefore the overall reactions on the two springs are still under compression.
However, I saw tension (63.7 kip) at compression-only springs under tendon secondary component which, multiplying by the moment arm (64.5 ft), generates the secoment moment (4110 kip-ft).
I don’t understand why there’s secondary moment here. I would expect to have zero secondary moment. When PT gets stressed, compression-only supports have NO pulling forces on the structure, and there should NOT be any secondary PT moment.

Please correct me if I am wrong. Otherwise, please advise on how to proceed.






Answer:Hello,

Secondary stresses would be developed for an indeterminate structure. The pylon and the 2 spans could be considered to be acting as propped cantilever.






Question: Hello,

Thanks for the reply. Please see the system below. This is an indeterminate system, with two compression-only supports at the tips and pin in the middle. It has dead load, which is larger than the vertical component of the PT force. In this scenario, I still don’t think there will be any secondary moment due to the PT. What do you think?




Answer: Hello,

For the same system, lets consider prestress and dead load separately.
If the prestressing strands are passing through the beam, such that there is certain upward or downward reaction at the pin support. Since this pin support is restricting the free movement, secondary stresses would be generated due to prestress.
After this is done, simply the forces/stresses are calculated for other load cases and these are superimposed to obtain the final results.
Hence, if the prestressing force wouldn't cause any reaction at the pin support, only then there won't be tendon secondary. In all other cases, there will always be tendon secondary for an indeterminate structure.



Question: Hi,

Is there a way not to included in the secondary PT moment only at this stage, while at the same time, not changing the reactions and displacements at the supports?



Answer: Hello,
There is not way to mimic that situation. However, can I know why it is required to create this type of condition. I can't imagine how this is practically possible...





Question: Hello,

As the self-weight component on the supports is larger than the vertical component of the PT force, the cantilever would still sit on the support without being completely lifted off. I agree that the reactions on the supports would be reduced due to the vertical component of the PT force. I understand such a change in the support vertical reactions would create a moment and this moment is considered as secondary moment in the software. In my condition, the bridge cantilever tip is free to lift up and there is no pull-back force from the supports on the cantilever tip when the PT is stressed. Therefore, the bridge itself does not feel the pulling force from the supports, and there should not be any secondary effect felt by the bridge.

Also, in this scenario (dead load > PT vertical component), under the dead load, the compression only spring is under compression. Even though PT vertical component is trying to reduce the reactions, the compression-only spring is still under compression all the time. Therefore, a compression-only spring under this scenario would act the same way as a linear spring.
Therefore, I was thinking to do two methods to eliminate such secondary effects. The first one as I mentioned in the previous message, is to eliminate the PT secondary directly; and the second one is to activate the dead weight as an element load and to active the support, AFTER stressing the PT. In the second method, when PT is stressed and lift the cantilever tip off the springs, since supports are not activated yet, there is no secondary moment. Then in the next stage, the dead weight is applied as an element load, with the activation of the supports at the deformed condition (the supports could be a linear support). This would eliminate the secondary moment that goes into the CS: Tendon Secondary.


Please let me know what you think.





Answer: Hello,

I do agree that the tip supports won't pull the girder down in case of prestress. However, this is like expecting that whether there is a compression only support at tip or not, the overall results should be same. That won't be the case. The self weight of the structure causes a reaction at the cantilever tip and the tendon stressing would cause reduction in this reaction. Hence, there would be tendon secondary.
If the dead load is removed, then in that case, tendon secondary is not obtained, because the dead load, which is holding the girder down is non-existent now.
To summarize, its not the support that is causing tendon secondary, it is the already applied load that gives rise to tendon secondary. If the applied tendon force causes a force larger than reaction due to dead load, then there won't be tendon secondary moments or reactions.

The supports always do remain under compression, however, the overall structure remains indeterminate due to this and hence there would be secondary moments and forces. The reduction in reaction would in turn affect the bending in structure and this is reflected via the tendon secondary.

Logically, I do think that tendon secondary should exist. However, if it is to be removed then, the second suggestion as provided by you would work.
1) Activate the required elements. Do not activate support at tips.
2) Activate Prestress load
3) Activate supports with deformed option
4) Activate the self weight.

Hope this helps. Kindly let us know if further assistance would be required.
Creation date: 10/29/2017 1:18 AM ()      Updated: 10/29/2017 10:57 AM ()
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