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overcutting corners in an opening

overcutting corners in an opening

overcutting corners in an opening

(OP)
I am a concrete cutting professional and I have been wondering about the strength of cut openings.  Specifically, is there a structural / strength difference between a square cut corner and a overcut corner.  Are there any codes that specify this?

I am asking because of the growing job requirements for square corners (with NO overcut).

Thanks,

isocutter

isoworks@home.com

RE: overcutting corners in an opening

By overcut do you mean that you are extending the cut lines beyond the corners? If so there will be a loss of strength at the corners as tension cannot be transmitted across the cut lines. If the overcut cuts through steel reinforcement that will make it worse. As far as the concrete is concerned it will "see" the size of the opening as the distance to the ends of the cuts. This is not quite true but without a detailed finite element analysis I would assume this if I had to do a structural check on a concrete wall with a cut opening.

Carl Bauer CEng MICE
Bauer Consult
PO BOX 2224
Gaborone
Botswana

RE: overcutting corners in an opening

First, there is a reduction in strength with any cutting and more with overcutting.  It depends on the 'fineness' of the design if this is an issue.

If no overcutting is specified, then you should price the work based on providing the extra labour, and discuss this with the engineer after you have the job.

Reinforcing steel can be located within the slab with a good quality stud detector or device made for this purpose and depending on the hole size and location it may be possible to reduce the number of bars cut.  If no additional bars are cut, the engineer may be amenable to accepting this.

With post-tensioned stuff, usually specify that the tendons be located and that a chipping hammer be used in proximity.

In occupied spaces, often see contractors cut square holes using a circular coring drill; this approach tends to be less messy and dusty than using saws.

I've also seen coring drills used for the corners and the edges sawn; this provides a smooth corner and reduces any stress riser.

RE: overcutting corners in an opening

I would think that the overcuts would essentially create a larger "effective" opening than the raw opening you intended to create.  By overcutting beyond the opening corners, the concrete between the overcuts becomes non-structural and you end up with less undesturbed wall/slab than you would have without the overcuts.  

In addition, the overcuts may damage reinforcing bars which extend just beyond the opening edge, further reducing the wall/slab strength.  

RE: overcutting corners in an opening

(OP)
Thanks for the replies.

It seems that the several of you that have responded are in agreement that the corner would stronger with no overcutting.

Does anyone know of any codes that exist that specify square corners or is it just up to each engineer?

Thanks again.

isocutter

isoworks@home.com

RE: overcutting corners in an opening

No code issue that I'm aware of.  Simply a good rationale.

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