Post-tensioning losses
Post-tensioning losses
(OP)
Hi guys,
I am building a rectangular beam which will be post-tensioned and bonded, i have calculated my theoretical losses due to Friction, anchorage draw-in, etc. My question is this, is there any way of measuring the losses experimentally ? I know that you probably can't measure them individually but what about the total loss? Is there some type of equipment i can use which will allow me to measure the loss in the tendon?
I am building a rectangular beam which will be post-tensioned and bonded, i have calculated my theoretical losses due to Friction, anchorage draw-in, etc. My question is this, is there any way of measuring the losses experimentally ? I know that you probably can't measure them individually but what about the total loss? Is there some type of equipment i can use which will allow me to measure the loss in the tendon?






RE: Post-tensioning losses
Wedge draw-in can be easily measured on site - it is an instantaneous loss so measure right off the back of the stressing ram.
If you have double live-end tendons then you can measure the friction between each two stressing ends by checking lift-off loads. If you have one dead-end and one live-end setup, then unless you install a load cell at the dead-end you will not be able to verify the force at this end.
I assume your bonded tendons are MULTI-strand tendons, or are they MONO strand stressed?
Any particular reason you wish to experimentally verify the PT losses? There is a lot of research from the 60's and 70' that covered PT losses.
RE: Post-tensioning losses
Thanks again
RE: Post-tensioning losses
Lift-off testing is simply applying load to previously stressed tendon via the same stressing jack/ram and monitoring the pressure gauge when the wedges of the tendon "pop" and load transfers from the anchorage wedges to the tendon-proper.
In your case, I would recommend load cells (vibrating wire or electrical resistance type) at each end anchorage, and hooked up to a data logger you can continuously monitor actual force at each end of the tendon over the 7 day duration at a time interval of your choosing.
Or, if you wish to reduce wedge draw-in losses inherent with strand systems, use high-tensile PT threaded PT bar where bearing plates and compatible spherical washers and nuts are used to secure the PT force. However, PT bar will not work in conditions where the tendon has drape.
Are your multi-strand bonded anchorages flat type, or circular-section type? How many strands make up the tendon/s?
RE: Post-tensioning losses
RE: Post-tensioning losses
Flat anchorage tendons are not normally Multistrand stressed. They are Monostrand. Each strand is stressed individually using a single strand jack.
Never seen a 7 strand flat anchorage personally, normal limit is 5 and I have heard of 6 but never seen one.
RE: Post-tensioning losses
The anchorage system i will be using is very similar to "PTSE Flat Anchorage" system available on tensa post-tensioning manual, and the tendon is made out of 7 wire strands. Sorry i was wrong in saying multi-strand, the strand will be mono-stressed as my beam will only have 1.
RE: Post-tensioning losses
I agree with rapt, flat-type bonded anchorages typically cannot accommodate 7 strands. From Tensa's website, their flat anchorages accommodate max of 5 strands in a flat-type anchorage, whereby each strand is singularly stressed:
This compares to a true multi-strand anchorage where all strands are stressed at the same time:
With regards load cells to monitor the dead-end and live-end PT forces (and hence determine losses between each end) the typical load cell is annular in shape, with different center hole diameter available from 25mm to 250mm, or larger.
With a flat-type anchorage, and its unique anchor-head shape, you maybe best to use a small diameter annular load cell and monitor one (1) strand only of the tendon, and use a barrel-chuck for the selected strand that is monitored (preferably the center strand).
RE: Post-tensioning losses
Yes i see it now, the person that will help me with the actual stressing(due to safety reasons) showed me the "PTSE flat anchorage" system and said that it will be a 7-wire strand, he must have made a mistake, meaning it will be a 5-wire strand. Will try to get my hands on a load cell, thank you so much for your time and fast replies it really helped as i was a little confused.
RE: Post-tensioning losses
You are welcome.
The terminology needs some explanation:
For your PTSE flat-type anchorage there can be max of 5 STRANDS, each consisting of 7-WIRES. Note the subtle terminology use in BOLD.
If you want some info on load cells this manufacturer has good products, including very economical data loggers: Link
What is the proposed PT beam section size? To accommodate a flat-type anchorage you are going to need some decent beam width, and if you are limited my testing load capacity you may be best to explore options on how many strands you wish to place in your tendon to achieve the desired test results.
RE: Post-tensioning losses
RE: Post-tensioning losses
What is the objective of the testing? Purely a 'build-it' and 'bust-it' learning experience, or testing a specific objective?
Also, check with your PT supplier, Tensa, they probably have load cells in their inventory to measure the PT forces described earlier.
RE: Post-tensioning losses
RE: Post-tensioning losses
You may be interested (maybe not?) but I dug out my 30 year old undergrad work. However, of no real relevance to your proposed testing. We used a 1 x 15.2mm dia. strand tendon (not grouted) that was continuously stressed until failure occurred via compression failure at midspan. Failure occurred at a prestressing force of 155 kN - in a violent manner that we captured via a 'trip toggle' to a camera away from the danger zone.
RE: Post-tensioning losses
RE: Post-tensioning losses
Yes, a parabolic profile, with zero end eccentricities.
We were verifying the calculated 'strength at transfer' of prestressed elements so that entailed increasing the applied prestress jacking force in the strand tendon until the concrete compression strength capacity (not stress) was exceeded at the bottom section of the beam at midspan, closest to the maximum drape.
We purposely narrowed the beam section at midspan to only 95mm in width to make sure that as we increased the jacking force to maintain our desired compression failure, we did not get close to the tensile breaking capacity of the 15.2mm strand.
If we had of used a wider section, or stronger concrete, 1 x 15.2mm strand would not have had enough tensile capacity to achieve our objective mode of failure.
Good luck with your testing program.