Post tensioned strength with no P/A
Post tensioned strength with no P/A
(OP)
I am currently setting up a spreadsheet to calculate strengths, stresses etc of prestressed beams ans slabs. I have been instructed to have the option of whether or not to include the P/A component, as sometimes axial deformation cannot occur when the slab is say tied into stiff walls or frames. How should I modify my limit state strength equations and stress equations to allow for this? Anyone know where I can look this up?
Cheers
Cheers






RE: Post tensioned strength with no P/A
RE: Post tensioned strength with no P/A
When you are applying a post-tensioning force, you will have an axial force induced in the slab. What do you mean by include or not include P/A?
RE: Post tensioned strength with no P/A
RE: Post tensioned strength with no P/A
RE: Post tensioned strength with no P/A
"Yes, but that part is already accounted for in the strength equations. It sounds like he is trying to base the design stresses as if the PT never gets into the concrete? "
I Dont necessarily want to base the design stresses as if the PT never gets into the concrete, but I want to include the option of whether or not to include the P/A component in calculating the stresses at transfer etc. In my understanding if the structure cannot physically shorten axially then there is no P/A component, but still uplift due to the tendon drape. So far I have set up a solver in excel that solves for the neutral axis of the section depending on whether its cracked etc, based on a triangular stress distribution and strains in the steel etc. Is it at simple as simply misusing P/A from my found stresses at the end or am I missing something? I have a few more questions but I think thats enough for now haha
Cheers
RE: Post tensioned strength with no P/A
RE: Post tensioned strength with no P/A
RE: Post tensioned strength with no P/A
Include the prestress force and apply an equal direct tension force (equal to the prestress force) at the centroid of the concrete (cracked centroid) to represent the restraint force
RE: Post tensioned strength with no P/A
Could you have a look at the attachments I sent and tell me if that is correct for the limit state at transfer? Your help is much appreciated. For the elastic stress analysis should I find the geometrical centroid or the neutral axis assuming a triangular stress distribution in the compression zone? Also does the fact that a tendon may be unbonded affect both the limit state transfer equations and the elastic stress equations besides the fact that if its bonded there will be more area of transformed concrete etc?
Any help is much appreciated
RE: Post tensioned strength with no P/A
that change the results as you go along a multiple span beam with varying widths and section modulus?
RE: Post tensioned strength with no P/A
RE: Post tensioned strength with no P/A
So to model this you put the P into the steel but you put a tension force into the concrete to counteract this.
(looks like too many people have been relying on computer programs and idiots guides to PT design rather than doing the real calculations and understanding what they are doing)
chillerz69
The tension force should be applied at the neutral axis of the cracked section.
Don't forget that there is no Pe term added to the applied moments, only Msec for the prestress component.
RE: Post tensioned strength with no P/A
RE: Post tensioned strength with no P/A
Yes, it is a laugh a minute here. Anyone is welcome as long as they take me out for a good lunch!
RE: Post tensioned strength with no P/A