Large Hole in RC wall 1

[MegaStructures]

General theory question that is not related to a specific project.

What are the expected consequences of cutting a large hole greater than 3' in diameter in a reinforced concrete wall and are there fairly simple methods to avoid these consequences by strengthening the opening? For example I expect the consequence of cutting through a RC wall would be

1) Flexural rebar would be interrupted and would lessen the buckling capacity of the wall at that section due to the reduction in flexural capacity of the wall at that section.
2) If a square, or rectangular hole, is cut, there would be cracking at re-entrant corners, which could spall and expose rebar.
3) Cutting a hole in the middle of the wall would make the area of the wall directly above the cut act as a deep beam, which introduces a new set of expected behavior with a tie forces forming directly above the opening and introducing a possible tension failure mode right above the opening. This is possibly avoided by making the opening circular?

Are there additional concerns that would need to be taken into consideration and could the opening simply be strengthened with a steel collar welded to the existing rebar that has sufficient strength to transfer required moment and shear forces rather than performing a full analysis of the wall. Would there be a large concern that a steel collar would be to flexible to transfer the required forces.

This is a pretty open ended question, but the theory is interesting and hopefully somebody can help me add to my thought.

 

[SlideRuleEra]

The "wild card" information you have not mentioned is loading on the wall, both magnitude and direction. Different answers if the wall has, say, full height horizontal hydrostatic loading as opposed to light vertical loading. For many walls even a large hole is not big deal... a case-by-case problem based on all info.

http://www.SlideRuleEra.net

 

[MegaStructures]

SlideRuleEra that's true it's just a thought I had and not a specific project, so i don't have any loading condition to consider, but let's say the possibility exists for high lateral (acting towards the wall face) and high vertical forces acting on the wall and they can act independently. It may be generally true that a large hole in a wall is not a big deal, especially in the case of vertical loading, since the compression force can redistribute around the opening, but as engineer how can that be quantified? If there are high lateral loads and a large portion of the flexural bar is cut then how can that wall be certified to be code compliant without a full up reanalysis of the building? If local reinforcement could be made to continue the rebar strength that seems like it would be ideal for retrofit of existing structures. This seems like it could almost be an interesting research project if there isn't already ample research on the same topic.

 

[BridgeSmith]

If there are high out-of-plane lateral forces, development of the interrupted vertical reinforcement would have to be considered.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[MegaStructures]

BridgeSmithAgreed, but would it be possible to replace the flexural strength with steel reinforcing around the hole, rather than having to reevaluate the entire structure?

 

[BridgeSmith]

In similar situations, we have distributed the loading and resistance using assumed distribution patterns at 45^o angle.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Without further analysis, I probably would provide reinforcement at least equal to the lost concrete section and reinforcing, similar to the new design, we often provide clustered bars around opening to compensate the loss of stiffness. Circular cut usually is less critical than rectangular cut, the 90° corners are tough to handle, unless you leave a fillet there. In any case, I still prefer to have a quick check though.

 

[SlideRuleEra]

NetTension - Well, you have to start somewhere. I would begin by looking at the rebar that remains. Perhaps three loading cases:

Case 1: Hole at, or near the bottom of a wall. Wall bending load will have to be resisted by horizontal rebar above the cut and rebar embedded in the wall's foundation (rebar shown in red).

Case 2: Hole more or less in the middle (vertically) of the wall. Bending load resisted by horizontal rebar both above and below the hole (rebar shown in blue).

Case 3: Hole near the top of the wall. Bending load resisted by horizontal rebar below the hole (rebar shown in green).

For vertical loads, consider the wall to be masonry; use loading assumptions from The Brick Industry Association's Technical Note on Structural Steel Lintels.

Of course, this is not an "answer" and what to look at next depends on the above results. At least I'm now off of dead-center, no longer wringing my hands, and past wondering how to start.

http://www.SlideRuleEra.net

 

[r13]

This is a rare occasion to see long writing from SRE

 

[MegaStructures]

It seems to me that a simple solution to the problem would be adding a steel collar that can transfer the flexural capacity of the sum of the rebar that was cut. Seems that if the hole was circular the steel wouldn't necessarily have to replace the full compressive strength of the concrete, because the compressive forces in the section would flow around the circular opening. My original thought was really to think of a way to certify a hole cut in a reinforced concrete wall without completing a full analysis of the structure.

 

[msquared48]

For small holes, yes. But longer holes are problematical depending on the vertical distance to the roof or floor diaphragm and the proximity of other viable shear war wall sections. No general rule here...

Mike McCann, PE, SE (WA, HI)