Residential Basement Wall Reinforcing 4

[jandlo]

I have a contractor suggesting my basement wall reinforcing is excessive. I'm sure it is in terms of strength but for temp and shrinkage crack control and meeting code I'm not so sure. It's a 12" thick wall (mainly for a 2x6 wall with a brick veneer. It's 11'-0" from top of wall to top of basement slab and retaining a maximum of 8 feet of soil. I"m showing 2 layers of #4 bars @ 18" OC each way. The contractor wants to use a single layer of an unspecified bar size @ 24" OC each way.

My concern is controlling cracks over time and the threat of water infiltration.

Correct me if I'm wrong but because the wall is greater than 10', the prescriptive design in IRC is not applicable which means that the max 18" OC spacing from ACI applies.

I also wonder about the ACI provision about walls 10" thick and greater requiring 2 layers of reinforcing (except basement walls). Why the exception for basement walls. And is it reasonable to use 2 layers in my case -- a residential basement wall 12" thick?

 

[skeletron]

The exception is because a basement wall will typically be supported at the top. So the bending can be controlled based on backfill procedures and installation of the floor.

 

[XR250]

Can you cram all that on a 10" wall? You could let the 2x6's overhang into the basement a tad bit if needed.

 

[Canuck65]

Basement walls, or retaining walls don't require 2 layers of reinforcement because the soil pressure results in tension only ever in the basement (interior) side of the wall. Specifying 2 layers reinforcement, in my area, is not typical. Based upon the prescriptive residential code in my area, a 12" thick wall that is 11'-0" high, and laterally supported top and bottom, retaining 8' of soil would warrant reinforcement. As long as your calcs support it, I would stick with the 18" maximum spacing, certainly for the vertical reinforcement.

I have specified many times, for 8" and 10" thick walls, not quite as high as you have, 15mm rebar @ 16" 0/c each way on the interior face, which is what the contractors are used to installing. In your case, between an 18" and 24" spacing, what is the cost difference ... quite small compared to the total cost of a new build.

 

[jandlo]

I understand the mechanics of a basement wall vs retaining wall, but that doesn't explain the exception in ACI. I don't think ACI is referring to either basement or retaining walls in that provision. I'm just trying to understand if it makes sense to use only one layer of steel for a 12" thick, 11 foot tall basement wall, or if that created a potential for large cracks.

 

[kostast88]

If you stick with your main reinforcement @ 18'' spacing and you simply provide straight bars for crack control between the main (without anchorage), wouldn't you be able to satisfy the creep/shrinkage requirements?

Stick with 1 layer for buildability reasons if possible and increase bar size if necessary. Deep basemets tend to have 2 layers of reinforcement but I wouldn't say your basement is that deep.

I have worked on a 20m deep basement which required 2 layers of reinforcement and the contractor still complained about it preferring bundled bars...

 

[jandlo]

The current section is shown above. The dimensions were set by the arch, but the contractor days they can do a 4" deep brick ledge and I don't see why they can't use a 6" upper stem for a total of 10" thick.

I feel better about a single layer of reinforcing with that thickness(I'm thinking #6@18"oc), but where would it be placed in this wall, where the brick ledge is in some cases about 5 or 6 feet from top of wall?

I would be concerned about cracks at the brick ledge thickness transition with only one layer at the center.

 

[jandlo]

This is what I'm thinking as an alternate. Does that seem reasonable from a constructibility and crack control standpoint? I'm not sure how necessary the the bent bar at the brick ledge is. It feels right to me.

 

[gte447f]

jandlo:
Forgive me, but this exact scenario sounds all too familiar to me. The contractor is complaining because your specified reinforcing probably exceeds the prescriptive amount of reinforcing in the residential code. The problem is, he has already bid the job based on his experience with the prescriptive code, and now your engineered design is blowing his budget. That was a mistake between him and the owner. There are innumerable disconnects between the residential prescriptive code and the real building code that is more thoroughly grounded on engineering principles. The residential code is prescriptive, and no engineering is required, and if any engineering is provided , there are bound to be conflicts between the engineered design and the prescriptive code, resulting in these types of disputes with the contractor. That is why, in my opinion, if a residential project falls within the restrictions of the prescriptive code, it should be built without any engineering. If engineering is required, the contractor should sit quietly by for the engineer to complete the design and drawings, and then he should bid it, then build it in accordance with the drawings.

 

[jandlo]

Thanks for the response. Although I have experienced that scenario many times, this is a bit different. Contractors are bidding on the work right now. A particular contractor is trying to be a hero and save the owner money by claiming he can place this 10'-12' wall with #4 @ 24" OC EW because that's what he does all the time with no problem. He points to the IRC but fails to understand that is doesn't apply in the case because of the wall height (basement slab is 11'-0" from top of wall). He also doesn't understand other considerations, like lateral forces due to wind that could factor in since the grade slopes down around the house, and T&S crack control.

So basically he's trying to get the owner to believe that I am drastically over designing and wasting his money. He went as far as to personally attack me for the school I attended.

Anyhow, I had a frank conversation with the owner. I told him that I would recommend not using less than the ACI minims. I would not use the prescriptive designs for basement walls if it was my own house, even if it did fit the criteria. I think the additional steel is insurance against water infiltration from cracks due to temperature and shrinkage as well as differential settlement. Maybe I'm overly conservative, but that would be my approach.

The basement wall crack repair industry is gigantic. I suspect that inadequate steel is a factor in that.

 

[Rabbit12]

jandlo, aren't some contractor's just lovely?

Seriously, how much money are they saving here?

 

[jandlo]

This is not my typical experience with contractors, but yes occasionally I come across a gem like this guy.

It's a pretty large home. I think between my final revision and what he wants to install, maybe $5-8k. Well worth it in my opinion.

 

[jayrod12]

5-8k on a what 500k house? Tell the contractor to get bent.

 

[skeletron]

*tangential post* I had a contractor look at my footing (for a 60ft telecom tower) and say:
"I've never seen this amount of bar in a footer ever. And I built my own house on footers with no bar. Didn't need it..."

I had 0.2% rebar in the footing.

 

[jandlo]

It's nice to hear that I'm not the only one dealing with this. It's not great to hear it's so common. My experience is mostly in commercial and industrial. Residential contractors apparently want to use the bare minimum.

 

[Settingsun]

If it isn't acceptable, explain why. Many builders don't understand how structures work judging by some of the 'fixes' proposed when they do something wrong.

In my experience (not residential), it's usual for contractor alternatives to be submitted as an additional bid, ie they have to price the conforming design so you can compare apples:apples with other bids. So this contractor should price your reo, then give a deduction for the lesser reo alternative.

 

[jandlo]

This particular contractor is a piece of work. He had a low bid and it stated that it wouldn't be constructed per drawings but it would work. When the owner asked for clarifications on what he would provide (per my suggestion) he said bars at 24 in on center each way. Didn't even give a bar size.

When I politely explained why I believe more steel is required for reasons other than simply Earth retention, he chose to ignore my reasoning and instead focus on what he's done in the past. Keep in mind this was not a cookie cut house. I don't know what he was so hung up on. He's going to be competing against companies that are using the same design drawings. I think sometimes these guys have a chip on their shoulder and a strong disdain for engineers.

I guess I need to learn to let it roll off my back. Unfortunately I end up going to great lengths to defend my approach and it usually causes me to go down a rabbit hole and second guess myself. and as we all know there is no definitive answer for any of this.

 

[Eric C.]

If I were you I would stand by my design. You have, apparently, taken the loading conditions, site conditions and constructability issues into consideration. You have performed a reasonable analysis and have provided what you believe to be an acceptable design based on the factors involved. You have no reason to backtrack.

You might could entertain the contractor's "value engineering" request, however, if he would submit his own design sealed by his own engineer for your review, consideration and comment (not necessarily approval and change).

 

[bones206]

My concern is controlling cracks over time and the threat of water infiltration.

I'd invest in a quality waterproofing membrane, drainage mat and perimeter drain system. Regardless of how much the concrete cracks, water will find it's way in eventually if it doesn't drain well. The typical bituminous stuff they paint on basement walls gets brittle over time and has minimal crack-bridging ability.

I see cracks (probably shrinkage cracks) quite often propagating from the corners of those little sliding windows at the top of basement walls. Adding diagonal bars and an extra horizontal there would seem worthwhile, although it's not standard practice and it would be another thing for the contractor to complain about.

 

[jandlo]

Waterproofing is definitely a very important factor. It's outside my scope, but I agree with your recommendations.