PT Anchorage inside column and Multi Jack Vs Mono Jack

[struggle66]

Hi,

Good Day Everyone,

Long time no question

(1) If there is an PT anchorage inside the column, does it weaken the column depending on the compressive strength of the anchorage material.

(2) What are the differences between mono jack and multi jack other than elastic shortening losses.

Thanks

 

[rapt]

Struggle

The column should no be weakened as long as the recess is not within the column and the anchorage is fully grouted and column concrete strength is the same as grout strength, so about 40MPa! The main problem is the reinforcement placement and fitting it all together.

RE Multi/Mono strand, how many strands are we talking about and what duct and anchorage type.

PS I am getting sick and tired of incomplete questions on this site. We are all engineers, how about we put our brains into gear. We cannot answer questions sensibly without adequate information! If you do not know how much information you need to supply, you should not be working in the area and asking the question!


Single strand stressing with round duct does not work. The strands become interwoven and you have no idea how much friction you will get, so you have no idea how much force you will get into the strands. The first will be ok, the last will get very little for large tendons. In fact the last strands will put extra load into the first strands!

As an example of what happens we once stressed 42 strand tendons as multi-strand to 82.5% load and some strands started failing (idiot concrete contractors fault fortunately) so we had to de-tension then, and you have to do that strand by strand. These tendons were 100m long so expected extension of about 600-700mm. When we released the load on the first strand, it pulled in about 10mm and stopped, with no load at the end! It was purely held at nearly fill load by friction with the other strands!

 

[struggle66]

Hi rapt,

Thank you

For recess, we patch back with non-shrink grout with same compressive strength as column so should it be a problem?

Sorry for the lack of complete information in my question. I forget that you don't have mind power to automatically understand my question . But your reply is exactly what I wanted (Stressing of multi strand round duct using mono jack). We have done it one time for 19s because contractor didn't see the stressing pocket on drawings and cast it. According to my site team and third party witness, the elongation fell within the tolerance and I didn't know if the last strands were pulled in.

And please don't be sick. Most of the things I learned about PT are here from this forum especially from you. Engineers like me will be struggling without people like you, KootK, ingenuity,...

 

[rapt]

Struggle

I doubt that the rebuilt recess will work as well as the original concrete under high compressive stresses. But there is possibly reduced compressive stress over the depth of the beam due to some spread over the depth especially at mid depth. But do not quote me on that, I am on holidays and my brain might not be thinking clearly!

 

[struggle66]

rapt no worry!

Enjoy your holidays.