Reinforcing Tall Residential Concrete Basement Walls 2

[MILRAD]

I often have to design residential basement concrete walls that are taller than 10 feet.

The way I read the IRC and ACI 332, those codes only apply to walls less than 10 feet and ACI 318 has to be used if the wall is taller.

ACI 318 has minimum reinforcement requirements for both vertical and horizontal bars and maximum spacings.

So if a 10" wall is 9'-11 7/8" I can use plain concrete according to ACI 332 Table 8.2.1.3b with 30 pcf lateral but if the wall is 10'-0 1/8" I have to use at least #4's @ 16" o.c. vertical and #4's @ 10" o.c. horizontal based on ACI 318 14.3.2 and 14.3.3.

ACI 318 14.2.7 says minimum reinforcement in 14.3 can be waived if structural analysis shows adequate strength and stability. How do I do that? Is there a maximum spacing that can be used like the 6t in ACI 530? Do I have to worry about crack control? If I have bars spaced 48" would the reinforced concrete shear strength apply or would I need to use the plain concrete strength (a coefficient of 2 vs 4/3)? Do I just wave my hand at it and say it will work?

Any help or ideas will be appreciated. The contractors I work with don't like it when I call out vertical and horizontal bars.

Thanks

 

[minorchord2000]

I always design the walls using standard methods in accordance with ACI 318. First I design the walls assuming a cantilevered condition with the outside soil causing tension on the outside face. I design the footing assuming a cantilevered condition but sliding may be resisted by the basement floor slab (if present). Sometimes the addition of the floor at the top of the wall can change the boundary condition to pinned at the top, in which case the moment diagram will change with some tension required on the inside of the wall. Therefore the second condition to be checked is fixed base and pinned top. Both conditions should be checked. The resulting reinforcing steel required should be checked against the minimums as prescribed by the code. The larger requirement will control. I would never design a basement wall as unreinforced since the lateral pressure on the wall can change especially if the outside soil becomes saturated. I always use a geo-membrane and a slotted PVC pipe on the outside of the wall and along the bottom of the wall to capture and divert any water that may accumulate behind the wall. I always check for crack control.

As far as the contractors are concerned who might complain about the addition of reinforcing steel, I would respectfully remind them that you are only using standard wall design methods based on sound engineering judgement and normal design practices, and in accordance with the ACI code. If they still complain...kindly suggest that they get an engineering degree and design the walls themselves. That usually cools their rhetoric.

 

[dauwerda]

While minorchord2000's practices are certainly acceptable and will result in a very well constructed wall, I would say they are definitely not the norm. I don't believe I have ever seen a residential house built with a footing sized large enough for the basement wall to act as a cantilever (except in very special circumstances).
Unfortunately, residential work is notorious for having builders balk at anything that is actually engineered. However, I believe you are interpreting the code correctly MILRAD, anything over 10ft requires traditionally designed walls. Frankly, if it were my house I would want the ACI 318 minimum reinforcement whether I was over 10ft or not. I have seen too many residential walls "built to code" that just don't perform that well, and putting more reinforcing in when the wall is built is way cheaper than dealing with any problems that might arise down the road. I think this is one of those things where the construction culture around unreinforced and lightly reinforced basement walls needs to be changed in the residential building industry.

 

[r13]

If the basement wall is not restrained at the bottom, and the slab is floating above the footing, then it is a simply supported beam. Do the calculation, if the result is less than required, then provide code specified minimum, otherwise reinforce accordingly.