Min reinforcement for bonded PT beam

[struggle66]

Hi everyone
For bonded PT beam, do I need to provide minimum reinforcement? If so, can I count PT strands as reinforcement?

Thanks

 

[BAretired]

You need to provide minimum reinforcement and yes, you can count prestressed reinforcement.

BA

 

[aaronPTeng]

Struggle,

You do count the PT as reinforcement, Also as for minimum reinforcement they can be designed with minimal to no conventional reinforcement (depending on P/A etc). however for bands of depth 300 or greater, you need to provided minimum safety reinforcement for concrete placement,

regards,

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[rapt]

Struggle,

There are several areas to consider.

Most codes require a minimum ultimate capacity for PT members (and RC members but often for RC they simplify it to a minimum area of reinforcement). This is normally an ultimate capacity in the order of 1.2 * Cracking Moment. This includes any PT or Reinforcing in the section.

For service conditions, codes also require minimum crack control reinforcement near the tension face if the tension stress is greater than a certain limit. I think the ACI code limit is too high for this. It should be about .25 root Fc. This reinforcement can include any PT that is near the tension face.

For transfer, obviously the PT cannot be included as it is at the compression face, but the stress limits should also be much lower as there is often no bonded steel near the tension face. I think the limit in this case should be 0 tension!

 

[struggle66]

Thanks everyone for your replies

Rapt
I just started using rapt software. Dose it cover up the requirements you mentioned above. At transfer so if the tensile stress is more than zero, minimum rebar need to be provided at compression zone. I think the purpose of minimum rebar is to prevent sudden brittle failure. So what is reason of putting min bar in compression zone.

Sorry for my lack of knowledge as I am 24 and graduate from under developed country.

Two more things

Many designers reduce column stiffness to reduce moment transfer to column but I think we should not reduce column stiffness unless it is going crack either it is one way or two way. Is it correct? However if we do not reduce the column stiffness, it is hard to accommodate high moment in the column.

And my company is a PT specialist but when they do design, they do not design with middle strip and column strip. They include the slab away from the drop panel or band in one design strip. I think the stresses on the slab away from drop panel are not the same as stresses on drop panel. But they are doing like that and they put half tendons inside drop panel and half in the slab. Please guide me how to run and distribute the tendons properly.


Can you suggest me a good PT book to study? Thanks

 

[rapt]

Struggle,

As I said earlier, there are several reasons for minimum reinforcement. One is to prevent brittle failure which is the Mcr rule. It is also required to provide crack control at service and transfer.

Who said anything about putting reinforcement in the compression zone? At Transfer, put it in the face that is in tension at transfer for crack control. This will normally be the face that is in compression under ultimate conditions.

RE 0 tension, no RAPT currently puts it in at about .4 of the normal cracking moment. This level can be justified by calculation if there is no restraint to shortening. If there is significant restraint to shortening, 0 should be used or the restraint stresses calculated.

I agree about column stiffness. You cannot reduce it if the column will not crack. If it will not crack, it should not be difficult to accommodate the moment, but it cannot be redistributed away if the column will not crack!

The design approach you mentioned is definitely incorrect and not allowed by the Australian code or any sensible design engineer. It results in significant under design of the slab. It is used quite often in Asia by designers influenced by the PTI design information but it is definitely not correct and should not be used. You will find several references to this in other threads on Eng-tips.

Prestressed Concrete by Gilbert and Mickleborough.

 

[struggle66]

Thanks
Engineers in this forum are really nice.

 

[aaronPTeng]

struggle what part of the world are you from?

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[struggle66]

I was born in Myanmar a country which you will not know but now working in Singapore 9 months ago.

 

[aaronPTeng]

Struggle,

I know where that is, Good luck with your development and work in Singapore,

Regards,

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976