Prestressing force in strand

[Edvinas]

Hello People, i have a question i would like to ask, at this point it may sound stupid, but i don't want to make any mistakes. OK, so, a guy came in to my college to stress my beams (for my final year project), and the stress that he put in was 535, that's what he said, without given the units, and as per usual i forgot to ask him. The gauge that he used i will attach below, so can anyone just confirm how do i convert that into tensile strength of the strand ? or the breaking load ? Thank you very much.

 

[rapt]

Ask him! How would we know the relationship between gauge pressure and stress on his stressing equipment.

And student questions are not allowed on this site! Read the rules.

 

[Ingenuity]

...and when you call him back tell him his equipment has expired calibration.

If you knew the gauge pressure at max stress, and the jack ram area you could multiply pressure X area to get an approximate value for force. Looks like the stressing equipment is PJM from Australia.

 

[Ingenuity]

I am going to hazard a guess and say that the units to your "535" is 'bar', where 10 bar = 1 MPa. So 535 bar is 53.5 MPa.

PJM Industrial manufacture two sizes of monostrand jacks/rams. One is 25t and the other is 30t capacity, with different effective ram areas, as follows:

If it was the 25t capacity ram that was used, your approx jacking force would be as follows:

P_j= 53.5 MPa * 3,388.6 mm2 = 181 kN

I am going to assume this is NOT 12.7mm 7-wire strand and probably 15.2mm diameter 7-wire strand, with a P_pu of 256 kN, so P_j= 70% P_pu.

Needs verification. Lots of 'ifs'.

 

[rapt]

But remember every ram is slightly different and you need the calibration certificate (hopefully up to date) to work out the real answer. So call them!

 

[Ingenuity]

Agree.

 

[Edvinas]

Thank you guys so much, i just saw all the posts now, which were very helpful, again thank you for taking your time. Ingenuity, rapt i spoke to the guy, i think he was a little ashamed, and then said, the calibration could have only been of by a very small amount, if any, and i should mention that when writing my conclusion as there is nothing they can do now. Yes it is a 15.2mm cable, yes it's a 25T jack.

Thanks.

 

[Ingenuity]

as there is nothing they can do now.

Edvinas:

Not correct, unless you have cut off the stressing tails or grouted the tendon? If NOT, you can re-stress them.

If this was a real project, the PT company would be required to obtain a valid calibration certificate (or get the equipment re-calibrated) and possibly re-stress the tendons.

From previous posts, this is part of a research project you are doing with relatively short beams and checking losses. Did your elongations check out okay, based upon the beam and stressing configuration?

 

[Edvinas]

Sorry, should have mentioned it before, yes the tendons are cut off. The elongations are ok, i would think so. Basically one beam we stressed to 35%fpu, and the other to 70%. The elongations for the side of initial prestress is 8mm for both beams, and for the other side, 70%fpu beam has 20mm, 35%fpu beam has 12mm. Beam length 3.5m.

 

[Ron]

Re-calibrate the jack and see if there's a difference. If there is, apply the ratio of the difference to correct the values. If within tolerance, no worries. If not, have them restress the tendons or replace them if they can't restress them.