if I have the elongation for each cables, its correct that the average of the elongations will be considered as girder elongation value, then I can calculate the fps for the girder.
I know that each 100' give 8", so from E=stress / strain,
E, L, dL, Aps these will give the fps.
I presume, a tensioning plan was available for the 250 cables. This should take into consideration the deformation of the concrete while tensioning. If it is followed, then from elongation and Young's modulus you can get the stress.
and this will be considered as fe (the effective),
Question: how can I get the fps (permissible) without applied load?
in the lap the beam uploading a load up to collapses, a stress censor is used to measure the strand stress which will be considered as fps.
can i get the fps from the elongation, be noted fps= fe + dlta fps
see ACI coad eq 18-4,5
Dear Eng. Ehab,
You can only get an approximated value of stress in cables from measured elongation.
Theoretical elongation could be calculated as follows:
Theoretical elongation = ( P jack - P losses) L / EA
where:
P jack is jacking force
P losses is lost force due to immediate losses (friction, curvature, wobble,.. )
L, A, E are cable length, cross section area and elastic modulus
measured elongation should be slightly different from theoretical one because losses coefficients are always assumed ( or recommended by manufacture), So, there is some uncertainty in losses calculations ( P losses) that leads to uncertainty in theoretical elongation.
Hence, without measuring the actual losses you will get an approximated value of stress in cables.
regards
The elongations of the cables, is normal, but the question is if you stress the cable one by one or in groups, you will have a different elongation for each cable or group.
I know each should +- 6% from the deign elongation, so I believe for approval elongation, the average value will be the actual beam elongation.
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