Cut an existing prestressed beam

[rt06]

I have an existing multi-span prestressed (bonded) concrete beam. I need cut a big opening in the interior span, which means I will lose one entire span. Does anybody have some experience in cutting existing prestressed beam? I think it is doable since the PT tendons are bonded to the concrete along its length. But I want to check with your opinion whether I miss something here.

Thanks a lot.

 

[csd72]

This will effect the moment distribution of the span.

 

[shin25]

You certainly need to check the strand layout in the beams. Since, you are loosing one entire span moment distribution will be affected. At the affected supports, the negative moment will be reduced and the midspan positive moment will increase. Without proper check, you may have service and ultimate level design problems. Also, this will affect the development length of the strands.

 

[csd72]

A few issues that need to be looked at (among many):

1. The deflection in the adjacent spans will increase.
2. The shape of the moment diagram will change, will the tendon profile still be compatible?
3. The development of the tendon at the cut end.

Minimise the effects of 1 and 2 by specifying a cantilever that gives a similar moment over the columns. Then check existing moment versus proposed moment and existing deflection versus proposed deflection.

 

[rt06]

Thank you all for the suggestions. I will check the forces/deflection for the remaining spans, the development length of the tendon, etc. I might need to come up with some strengthening scheme for the remaining spans, I might have to design them as Class C instead of Class U members. Thanks.

 

[kelowna]

Are you going to cut the actual tendon? Even if the tendon is bonded to the concrete, if you cut through, isn't there the risk of 'explosive' failure? Sudden change on the tendon tension might be enough to break the bond with the concrete.

 

[rt06]

Yes, it is what I am going to do, and is what I am worring about. For unbonded tendons we are surely not able to do this. But for bonded tendons I am hoping it is fine and there will be no "explosive" failure. But I am not sure of this. Is there anybody who has done it before? and is there any reference?

 

[lkjh345]

I recently went to a seminar where a rep from PTI presented material concerning making openings in existing post-tensioned slabs. According to the speaker, there should not be an 'explosive' failure of the tendons if they are cut in a bonded system, as the force transfer between concrete and tendon is continous.

PTI has examples of cutting of tendons in bonded systems for new openings. The actual cutting process is not particularly complicated.

The analysis of what is left is complicated.

I would contact PTI for any examples, literature etc that they may have.

 

[rt06]

Ikjh345, thanks for the help!

 

[kelowna]

I have some experience with PT, but always installing it, never cutting it. I have always heard that you do not cut PT tendons EVER and that demolition is to be considered very carefully.

I did a quick google search (I am sure you did too) and I fount the following:


Contrary to popular opinion, PT slabs are no more difficult to alter or demolish than other structural forms. As for any construction method, the Health and Safety file must contain all structural design and construction information to enable future alteration and demolition. With PT, small holes can generally be accommodated between tendons, and larger holes can be accommodated away from the column strips as with RC slabs. With bonded systems, tendons can be cut without the risk of uncontrolled release of the pre-stess. More care is required for unbonded systems because cutting the tendon will distress the element over its full length.

http://www.concretecentre.com/main.asp?page=1089

I am afraid I stand by the 'popular opinion'. I would suggest you talk to some expert on the matter. VSL or somebody with experience in demolition. Certainly the decision should be taken by somebody with ample experience on the subject