Why Is Jacking force for post-tension tendons the same?

[JDG3718]

I am asking this question because in the field I saw that the jacking is the same but elongation for each tendon, but doesn't each tendon needs it's own jacking force because in the design each tendon is subjected to different moment and load ?

 

[KootK]

The only appreciable force experienced by the tendons is tension. No appreciable moments. Since it generally takes a large number of tendons to reinforce structural elements, it typically doesn't make sense to introduce the field complexity of varying the force in individual tendons. Better to keep all the jacking forces identical and adjust the amount of prestress and load balance in increments that correspond to integer multiples of equally stressed tendons.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IDS]

All the tendons are stressed to the same high percentage of their yield stress. The additional stress due to working loads is a small proportion of the initial pre-stress, so adjusting the level of pre-stress to allow for this increment would make very little difference.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JDG3718]

So the prestressing force P_jacking is distributed in a number of strands ? for example if I have a slab of 3 ft of width and 6'' thick having 5 strands (7-wire 1/2'' strands 270 PSI) the jacking force P_jacking is transferred to each strand or all the 5 strands ?
I am having a hard time to understand that in the field they were jacking tendons with the same force but the pulled length for each tendon is different (they measured the length of the tendon that was pulled for each tendon is different as specified by the plan) but my question is the following :
In designing the strands the slab is divided into strips, but each strip is subjected to different load and moment, so it is reasonable that each tendon need different stressing force and hence different jacking force, how come that in the field all the tendons are jacked the same force ?

 

[cooperDBM]

The tendons are generally stressed to the same high level for efficiency in materials and to reduce the impact of initial losses. To vary the prestress in the slab to suit the loading you would more likely vary the tendon spacing or number. Of course the tendon profile is set to suit the loading. It's often worth minimizing variation in the design to reduce the risk of construction mistakes.

 

[JDG3718]

So I understood that the jacking force is kept the same but for example strip that is heavily loaded the spacing of the tendons is less than a strip that is lightly loaded.
But if spacing is the only criteria in this how come that each tendon is jacked with different length (pulled with different length but same jacking force) ? how we calculate the length of the wired that is pulled from the jacking ?

 

[dik]

Delta=P*L/(A*E) and it works well with post tensioning.

Dik

 

[JDG3718]

So according to this equation the only parameter that varies is the length of the tendon hence for different tendon different jacking elongation (since as said by the comments above the jacking force is the same) so if i have the same length I get same elongation when jacking even if the loading is more (the spacing and number changes but the elongation when jacking is the same because the same length) ?

 

[KootK]

Yeah, you should see similar elongation in tendons of similar length. It's never quite perfect because all the little things cause prestress losses will also have effects (usually subtle) on the elongation at a particular jacking force. Things like tendon profile and anchorage seating movements etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

OP... if you calculate the length of a straight tendon and one with a 6" (arbitrary) sag... you'll be surprised at the difference in length... splitting hairs again...

Dik

 

[rapt]

JDG3718,

No, P varies along the length of the tendon, depending on the tendon profile and tendon type. Friction varies for different prestress systems.

The P used in the calculation above is the value at each point along the tendon, not the jacking force.