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Existing basement wall - can it span horizontally based on arch action?

Existing basement wall - can it span horizontally based on arch action?

Existing basement wall - can it span horizontally based on arch action?

(OP)
An existing basement wall is horizontally reinforced only with #3@24" each face. The 2 level underground parking structure will have the intermediate parking level broken out in some cases immediately adjacent to this wall, to carry out repairs, and then poured back. The shoring engineer has indicated temporary braces to the wall, at 3 feet on centres, about 2 feet below the intermediate floor level that is to be broken out. When I check the #3@24 in the wall, it is grossly inadequate amount of steel, based on the earth pressure and the wall acting as a horizontal beam, with 2 foot as the assumed width of wall (i.e. just 1#3 as being effective; even if I took 4 feet of wall and 2#3 effective it is still grossly inadequate steel). It seems to me that this requires a continuous horizontal waler in addition the the braces, or the braces at much closer centres. However, I wonder if there is any possibility that the wall can be relied upon to act as an arch, in which case no horizontal rebar need be relied upon. What do you think?

RE: Existing basement wall - can it span horizontally based on arch action?

Without further calculation, the section looks too shallow to consider arching.

RE: Existing basement wall - can it span horizontally based on arch action?

For what it is worth, I think your arch approach works. With props at 3' on centre, I can't comprehend the wall failing in horizontal bending, despite the small amount of reinforcement. I would be more concerned about the stiffness of the props, the connections to the wall and the floor, and the ability of the slab on grade below to take the prop force.

This building you are working on seems to have lots of problems, and the provision of such a small amount of horizontal reinforcement in the basement walls is another telling symptom.

RE: Existing basement wall - can it span horizontally based on arch action?

I'd be comfortable assuming arching action. Your shear span to depth ratio here would only be 1.5. That's strut and tie territory for sure. And strut and tie in this kind of application is really just arching. Of course, just because arching is appropriate doesn't mean that there isn't still work to do such as:

1) you'll want a high degree of confidence in these braces.
2) your nodes need to check out where the braces connect (variant of a bearing check).
3) at the end spans, the rebar may need to work as properly anchored ties for the arches/struts.

On a simply intuitive level, I'd have a hard time accepting that lateral support at 3' o/c isn't adequate. That's pretty darn near continuous support in my book.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Existing basement wall - can it span horizontally based on arch action?

@hokie: we simul-posted. I didn't mean to repeat your advice as though you hadn't given it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Existing basement wall - can it span horizontally based on arch action?

(OP)
To hokie66: I seem to have left the impression that the building as not well designed. Actually the building appears generally well designed to conform to the Code of the day (1956) and the drawings are exquisitely well detailed. The exception seems to be the relatively light horizontal reinforcement in the walls. The current repairs are due to deicing chemical corrosion of rebar, which must be expected after 60 years. I know that is not an issue in Australia, but it is the biggest parking garage issue here almost in almost all parts of Canada and northern U.S.

The issues I have are with the temporary shoring. Yes you are right about the slab-on-grade being inadequate to resist the wall bracing load...in fact it is grossly inadequate. I don't know what the shoring engineer is thinking. I have already brought that to the shoring engineer's attention.

Ron - On the possibility of the wall arching: The span to depth ratio is 3:1. What ratio would you want to be comfortable with arching? I believe bridge engineers don't put rebar in the slab if the span between the supporting beams is short enough although I don't know how they determine that.

RE: Existing basement wall - can it span horizontally based on arch action?

(OP)
To Kootk - ok.

Sorry, I posted my previous post before I received your post.
Unless the end span is at the end of the wall, the continuing wall can provide the force to resist the arch thrust, and the reinforcing is not required.
Yes agree the analysis has to be carried thru to include the node stress checks.

This however still leaves the problem that the force in the 3 foot spaced diagonal braces is much greater than what the 5" slab-on- grade can sustain, when analyzed by the Westergaard equation. The vertical component of the force in the wall brace is 21,000 pounds (unfactored) and I don't see how a 5" slab on grade can take that. The unfactored bending stress in the slab on grade is much greater than the modulus of rupture. Modulus of rupture = 570 psi for 4000 psi concrete, if one uses 9 root f'c. Calculated flexural stress = 828 psi, unfactored, for load applied away from joints. Unless the Westergaard analysis is wrong. So would have to halve the spacing of the wall braces to get a 1.35 safety factor for the slab on grade. Or put a member at the bottom of the braces to distribute the load to the slab on grade.

RE: Existing basement wall - can it span horizontally based on arch action?

Have you considered installing ground anchors? Provided access issues can be resolved, that might possibly be a better solution.

RE: Existing basement wall - can it span horizontally based on arch action?

Have you bored any holes in the existing SOG or how did you determine the subgrade modulus? Is the shoring contractor using any timber cribs?

RE: Existing basement wall - can it span horizontally based on arch action?

(OP)
I assumed an optimum subgrade modulus of 300 pounds per cubic inch. I would not expect it to be more than that and it may be less. But the results are so far overstressed that tinkering with the subgrade modulus will not bring the stress into the acceptable range. No timber cribs; there is just a 10x12x1/2" steel bearing plate at the each end of the diagonal brace.

What do you mean by ground anchors? This is a 60 year old building in downtown Toronto facing one of the busiest streets with lots of buried services, so if you mean anchors thru the wall that would be very difficult and expensive. Would likely be simpler to halve the spacing of the braces. There are only about 20 of them. So if they have to use 40 braces, not a big deal...just want to be sure that is necessary before I insist that the shoring engineer is mistaken.

RE: Existing basement wall - can it span horizontally based on arch action?

Yes, I did mean anchors into what you have clarified is the street. Maybe it is not permissible or practical, although the anchors may be below the services. All that propping will be difficult to work around with the shoring for your new floor.

RE: Existing basement wall - can it span horizontally based on arch action?

(OP)
To hokie66: yes that is a good point about it may be below the services, since the braces ill be down in the second parking level below grade. It was the contractor who suggested at a site meeting the idea of halving the spacing of the braces, so I hope that he has considered the point you raise about being difficult to work around the braces, but I will double check with him. The locations where the braces are required are limited in their extent, so it may not be so hard to work around them. Thanks as usual for your good thoughts. Much appreciated.

RE: Existing basement wall - can it span horizontally based on arch action?

I would have no problem assuming 'arching' for what you describe. But the loads are large enough that temporary braces with connections to the wall and slab-on-grade seem like too much trouble.

Could you use wales and install some larger more horizontal braces at larger spacing close to the underside of the slab (1 or 2-ft below)? The brace could slope up to eventually reach the underside of the suspended slab. That gets rid of the slab-on-grade problem and puts the load back into the diaphragm that has always resisted that soil load.

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