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# Pre-tensioning the cables in a cable stayed bridge by using SAP2000 problem

## Pre-tensioning the cables in a cable stayed bridge by using SAP2000 problem

(OP)
Hi all, I need an urgent help please

I'm using SAP2000 to model a cable stayed bridge to investigate the relationship between the cable pattern and the cable stayed bridge stiffness ( the deck stiffness). After analysing the bridge under the self-weight loads I got the displacements, then I applied the pre-stress forces as a temperature (thermal) effect by using the following equations:
delta= alph x deltaT x L >> deltaT = delta/ (alpha*L).
but unfortunately after running the model I found out the the displacements in the cable/deck joints got INCREASED !

I have seen in somewhere that someone have done 5 iteration to get smaller values in the displacements and he started getting them decreasing gradually form the first time, however I have done three iteration and the values are just increasing !!

After finishing from this stage I have to apply live loads from the vehicles, so
1- Am I working in a right pace ?
2-Shall I use the Load-optimizer instead or applying the pretension loads is right like this by using the temperature?
3- Is applying the temperature effect should be applied just on the cables or on the whole frame group (the cables and parts of the deck) ?
4- When I'm applying the temperature effect, shall I make it just once or more than once, and shall I add the deltaT every time by "replace the existing loads" or by "add to the existing loads" option ?

Replies continue below

### RE: Pre-tensioning the cables in a cable stayed bridge by using SAP2000 problem

(OP)
Correction:
3- Shall the temperature effect be applied just on the cables or on the whole frame group (the cables and parts of the deck) ?

### RE: Pre-tensioning the cables in a cable stayed bridge by using SAP2000 problem

(OP)
Although I didn't get a reply from any one, I will put the answers that I have got from my supervisor so the other people who are facing the same problems can benefit from them.
1- Yes, the work was going in a right pace.
2- Using the load-optimizer is the correct solution in my case (first try to apply the pre-tensioning by the temperature load, if you got higher values for the displacements, here you can use the load-optimizer as it will make automatic iteration to reach the sought goals).
3- Apply the temperature effect just on the cables.
4- This question I'm not sure about it, as it is not my case any more so I will apologize of answering it as I don't wanna write something that I'm not sure about.

Hope this could help :D

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