post tension equivalent frame

[timothyg6]

Would like to use generic stiffness analysis computer software to analize a post-tensioned beam - column frame for lateral loads. To include the post-tensioning as a load case for column and foundation design, are the secondary moments added as concentrated moments at the beam column joints or perhaps the unbalanced post-tension uniform load added to the beams?

 

[rapt]

timothyg6,

The secondary prestress moments are the result of Secondary Prestress Reactions.

Use the reactions.

 

[CoreyGuo]

Theoretically, you will need to apply secondary reactions to your lateral model. The secondary reactions, however, typically are very small and most times they are negligible. Sometimes we spot check the magnitude of the secondary reactions, but we never apply secondary reactions in the lateral model. Any more comments, Rapt?

 

[haynewp]

I have wondered this same exact thing. I haven't done PT in a while, but some other engineers I know that do it all the time also do not include secondary forces/moments in their lateral model. I questioned whether it should be included and was basically told it would not matter. I don't know about that one until I prove it for myself, I tend to not take someone's word when it comes to analysis.

 

[rapt]

CoreyGuo,

Yes, they are normally pretty small compared to the applied vertical load effects but not always depending on the prestress arrangement. I think most people ignore them unless they are significant.

 

[CoreyGuo]

Rapt,

I've designed several different types of PT structures and I did pay some attention to the secondary reactions. I never see any significant seacondary reactions in comparsion with the reactions from graviy or lateral forces. I wondering when these secondary reactions could be significant. We ceryainly need to take the significant reactions into consideration. Do you realize any situations that require your attention?

Thanks!

 

[rapt]

CoreyGuo,

It depends how close your tendon profile is to a concordant profile. For a concordant profile the secondary reactions are zero. The more removed the profiel is from concordant the larger the secondary reactions.