Truss calculation and post-tensioning

Hello, I am a college student and I am doing a research project for my physics class about the collapse of the FIU bridge. So far I learned about statics and how to calculate a truss. However, I do not have enough knowledge about the post-tensioning process. I want to include equations and calculations to show the tension and compression forces before and after placing the bridge on the piers. I would appreciate if you can give me some guidance on how to achieve this.  

Thank you

@Talin fard  I am not familiar with the subject but you can have look at these I found about Truss post-tensioning 

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=18280&context=rtd

http://borjournals.com/a/index.php/jecas/article/viewFile/110/pdf

Hope you will atleast get some idea.