Creating Opening in Reinforced Concrete Wall

[ToadJones]

Excuse the vagueness in advance...

I have a potential job which will require creating an opening in a reinforced concrete wall.

The wall is part of what is essentially a long machine pit and also acts as somewhat of a retaining wall.

The client is asking for an opening in the wall for conduits to be run through the wall.

I haven't had a chance to look over drawings just yet, but I was hoping to be warned of any pitfalls I might encounter here.
My biggest concerns are cutting the existing wall reinforcing and making the wall function the same after the opening is created.
I was also worried about creating a sharp-corned or square opening and the problems that might cause.

My intention is to reinforce the opening to provide support to everything above the wall, including the wall itself.

I'd appreciate any input.
Thanks

TJ

 

[Ron]

TJ...if the hole is for conduit, why not use a round hole? Coring can be done in relatively large diameters.

Brings to mind the question...if development length is already developed, and you cut a hole through the cross section, does the newly exposed cut end of the rebar get subjected to a different development condition? Expect some interesting comments on that one.

 

[ToadJones]

Ron,
It is for many many conduits.
I'm hoping for interesting comments

 

[Ron]

TJ...I'm surprised by the lack of comments on this one. Here are my thoughts...


If the wall is laterally restrained top and bottom, then try to leave the middle third intact. If the wall is not restrained at the top, then make the cuts in the middle third.

Rather than sharp corners, cut four large core holes in the wall and connect them with tangent cut lines. This will potentially reduce the re-entrant corner cracking.

Reinforce the opening. Hard way is to chip the concrete and make a rebar cage for the opening, then back-grout. Easy way is to use channel or angle.

 

[csd72]

If you do know the actual reinforcement details then these things are often easy to justify as most members have around 5% more reinforcement than they actually need. then it is generally a case of making sure that the averaged moment capacity is greater than that supplied.

Depending on the size of the hole then cracking may not really be an issue and it is often a case of making the client aware of the consequences and getting an okay from them in writing.

Rons suggestion of coring the corners is a very good one (he beat me to it).

 

[ToadJones]

Ron...I thought there would be more comments as well. I like your core drilling the corners idea (reminds me of cutting a counter top receive a rectangular sink).

The other concern here is that the wall receives a series of rather high concentrated load about 8'above the location of the hole.

 

[Ron]

Assuming the loads are vertical in the wall plane, then dissipation is rather quick. I would check a deep beam section above the cutout, considering the deep beam supports to be fixed. I don't know how large the loads are, but I doubt that shear would control.

 

[csd72]

even plain concrete could arch across an opening if there is enough lateral resistance on either side.

 

[hawkaz]

Depending on the size of the opening and load from the retaining condition, you may need to strongback the jambs. Make sure the existing horizontal bar can span between the jambs also.

 

[a2mfk]

Toad
What we have done in the past with tilt walls is conservatively assume the opening and adjacent concrete is not there and then add steel members like HSS or angles bolted to the wall to span around the opening and then carry the loads to where you need them. Very conservative but not difficult or expensive.

Works great with building walls, but maybe not with what you have described.

I've also been wanting to use carbon fiber on a project but have not needed it yet. This can replace tensile reinforcement, so if you don't have as-built info you could treat the concrete as unreinforced and design new reinforcement for it. I believe you give the carbon fiber guys your tensile reinforcement needs and they take care of the fiber and attachment design.

Have fun!

 

[rscassar]

If you can't get the wall to arch over the opening by strut and tie model, I would drill and epoxy fix a channel to the wall. Core the wall so no reentrant corners are created and I would perform an analysis if it is a highly stressed shear wall to make sure the structure is not compromised

 

[TXStructural]

If the interior face will be purely in tension, you can replace the cut reinforcement with steel flat bars attached to the interior face with post-installed anchors. Extend the bars far enough to engage the concrete reinforcement (a full development length beyond the opening.) While carbon fiber would work, I wouldn't recommend it if there could be fire or chemical exposure, or the area is subject to mechanical damage.

I have also designed small concrete pilasters for either side of openings where the span is vertical. You could consider this if the opening is large enough to warrant it, and where handling large steel members is impractical.

If you decide to use a steel shape (for flexure rather than tension), be sure to match stiffness in proportion to loads between old and new members. If the steel member isn't stiff enough, it will not take any load.