Residential found. wall design with extreme lateral load

[cobadger]

I am engineering a house with a 10' tall, poured, basement walls. The soils engineer has indicated a equivalent lateral fluid pressure load of 81psf. The bearing pressure is given as 3100psf. Expansive clay has not been found...according to the soils report. It seems odd to have such a strong soil vertically (3100psf) with such a high lateral load...but I am not a geo. engineer, merely structural.

My question is simply that my math indicates vertical reinforcing of #5 bars 6" o.c. (I have assumed a pin-pin connection for the wall and the main floor as a diaphragm.) The contrator is very unhappy to say the least, as he has never done a foundation with rebar of this magnitude. I'm not asking for an exact answer, but does #5 bars, 6" on center seem reasonable or overkill from anyone else's experience?

 

[PMR06]

81psf lateral does sound high. Can you backfill with free draining gravel and lower it?

Are you designing an 8" wall? If so, perhaps you could use a 10" wall and get more depth out of your rebar? Of course you'll have two mats then.

More importantly though, and not to pour gasoline on your fire, but check out thread 507-166096. Here the discussion is the fact that the top diaphragm and connection are usually more difficult to make work.

A counterfort (see thread 256-166662) or cantilever retaining wall may give you another options as well?

Good luck with the "I've been doing it this way for 25 years" attitude!

 

[COEngineeer]

Follow what PMR said:

1. You can assume the fixed fixed (both sides) and fixed at the bottom and free at the top. However, if you assume the bottom is fixed, that means you need to make the toe and heel bigger (and thicker) to make sure it has 1.5 X overturning moment (design it as a cantilever retaining wall). By saying the bottom is fixed, you can transfer some of the moment to the vertical steel.

2. Break the span of the wall with a counter fort or a buttress.

How long is your wall? (horizontally)

 

[cobadger]

The wall is 8" thick and is broken into smaller segments because of the floorplan layout. In other words there are several bumpouts in the wall to accomodate sun rooms, porches, etc. The longest wall span is 24'. I have been designing the walls as a one-directional slab or vertical "beam." I undertand length of wall is a factor, but designed as a oneway slab or vertical beam, I think strength should be sufficient and if anything I feel I would be erroring on the conservative side. Please advise otherwise.

The contractor has explicitly stated he wants traditional footings if possible...no giant ones under the wall or keys. He's actually a nice guy, it's just that excavation in the Rocky Mountains can be, well, rocky, and expensive.

Overall the floor diaphragm is 59 wide x 34 deep. It's a big house (7000sq.ft.)

The soil load is what is, so I can't reduce that by bringing in fill.

Given all this, does #5, 6"o.c. sound reasonable?

 

[SlideRuleEra]

If you compare the area of reinforcing steel to the area of concrete the ratio will give you a nice indication. For 8" thick, one linear foot of wall will contain the following:

2 each #5 bars (total crossectional area of 0.62 sq. in.)
8" x 12" of concrete (total crossectional area of 96 sq. in.)

Steel/Concrete Ratio = 0.62 / 96 = 0.0065

IMHO, that's a significant amount, but not unreasonable for structural concrete (especially considering there is only one rebar mat)

http://www.SlideRuleEra.net

 

[COEngineeer]

cobadger, i work in colorado also. Anyway, when I design foundation I like to use PCA tank design book. It will give you the moment on x and y direction on the wall. It is a lot faster than modeling it on the computer.

The problem designing it your way, you use too much steel in the middle. Plus you dont take the consideration the vertical steel taking the lateral load. Also, you might want to take advantage of using more dowells and reduce the horizontal steel, instead of designing the whole wall based on the maximum moment(at the fixed end). My guess would be #5 @6 dowells and maybe #5 @ 12" for horizontal. Because the moment at the side is very large so you can use the dowell as the As. Just make sure the steel can take the moment up the devopment length.

I'll do a little calc tomorrow. I dont have my pca table with me but I think just guessing the design you probably need:

10 inches wall, 24" wide footing, 12" thick footing, #5 horizontal dowell @6" about 3' each leg maybe, #5 @ 12" horiz, #5 @ 16 vertical dowell and #5 @ 16 vertical.

If you do keep the footing small (16" wide) and keep the wall 8", you probably need #5 @6" like u designed it. I highly suggest increasing the thickness of the wall and widen the footings.

 

[aggman]

I think you should check your numbers again. Using a 8" x 10' wall, 3000psi/ 60000psi, triangular distribution, no surcharge, 2" cover, and one way design I am getting less reinforcement required. I also looked at designing it as two-way and it helps some, but not a great deal using an aspect ratio of 2.5. I don't think anyone is going to tell you exactly what their numbers generate, including myself, but I think you need to check your numbers.

 

[COEngineeer]

aggman.. its common practice here in colorado to put the steel in the middle. so d is only 4". on tank design u will have possitive and negative moment. Its a good idea to keep in the middle.

 

[cobadger]

Gentlemen,

Thanks for your input. I too get less rebar needed as I stated the situation. I should have mentioned that the wall is carrying about a 5000/ft axial load, which changes the design slightly, i.e. combined axial load with bending. Maybe that's ignored in residential foundation design, but I can't imagine that's the case.

COengineer, thanks for your input as well, but I don't understand why placing the rebar in the center of the wall would be a good idea. I understand in tank design there will be pos./neg. moments, but this is a foundation...and in theory their is only positive moment on the wall. If that changes, something else...like a mudslide, has played a role. Why not place the rebar on the tension side and maximize it's effeciency?

Again, thanks for your help, but further comments are appreciated.

 

[COEngineeer]

A foundation wall is fixed on both sides. If you have fixe fixed beam loaded evenly, what kind of moment diagram do u have? You can use only the dead load of the axial load to help resist the overturning moment when you design it as cantilever retaining wall. But really, thats the only time I use the axial load (plus to figure out the pressure at the toe).

 

[COEngineeer]

Ok, based on fixed fixed fixed free scenario with 8" thick, I must use at least #6 @ 6" for horizontal dowels (about 3' each leg) and #5 @ 12" horizontal. And on the top of that, you must fix the bottom. Meaning, you have to increase the size of the footing. There is no way you can make it work without increasing the size of footing or adding counterfort.

If I use 10" wall i can use #5 @ 6" horiz dowels(3' ea leg) and #5 @ 12 horiz. #5 vertical dowell @ 8" (2.5' vertical leg), with #5 @ 12# vetical. you do need to make the footing bigger also and thicker.

 

[PMR06]

cobadger- I'm glad to hear the contractor is a nice guy... I didn't mean to imply he wasn't. I just get frustrated when clients react as if engineers caused the problem, when really we just provide solutions.

If you have assumed and designed the wall to span vertically with pin-pin ends, then you are correct to detail the rebar as such. If you had assumed a tank model as COEngineer is advocating, then you would have to detail the bars accordingly. In your simple span case though, the inside face will be in tension, and placing the rebar closer to the tension face is more efficient. It is more common to place the bars centered in an 8" wall though, so this unique detail must be inspected more rigorously. In addition, axial loads can be used in the calculation of a concrete section flexural capacity. I wouldn't rely on more than 0.9D + 1.6H load case will allow though. Actually, given the high loads and all this concern, perhaps a P-delta analysis of the wall section (see ACI 14.8.3) would be justified.

Now, assuming the concrete wall works, there is still an enormous top reaction to be dealt with!

 

[COEngineeer]

I agree what PMR said, I dont know if its a good idea to assume its pinned on the sides and at the bottom. Because I wouldnt assume any concrete pinned unless it is only sitting on something and there is no angled dowels embeded into it. Plus you will have a lot of reaction on the top of the wall that you will be acting againts your floor diagphram. I am not recommending to change the size all of the foundation wall. I just think you should increase the size of the footing and the thickness at critical walls(i.e. 17 - 25' long wall, just a guess).

Good luck!

 

[DRC1]

Most residential basement walls I have seen have no reinforcement except at the top. Generally the footings are poured below the bottom of the floor slab, and the soil is generally 18" below top of wall. Assuming the slab and the diaphragm pin it, the stresses in the wall are low. Even so it defies conventional analysis, but in general seems to work. Tisis not to say there are not cracked walls here and there that could have been prevented by a little bar. My recomendations are:

Talk to local buildng officer and find out what the standard details are - no sense reinventing the wheel

Be sure to add a width of crushed stone and underdrain with positive draiage away from the house. It does not cost a lot to do a first class job when building the house. Subsequent work can be very expensive.

If you want to add steel, consider using mesh. The material cost is higher, but you will save time and money on the installation.

Be careful backfilling. I believe premature backfilling, i.e. backfilling before the floor is in place, causes much of the cracking. If it must be backfilled early, be sure it is well braced wall to wall.

 

[COEngineeer]

DRC1, in colorado, almost all the house have basement. So you need a lot more steel than that. Usually the walls are 10 ft or higher and laterally loaded only on one side.

 

[jhoulette]

I'm an engineer in Colorado. I would design the foundation wall to span horizontal, break the spans up to no more than 16ft using a counterfort/buttress. The wall steel would be vertical #5 at 16" and horizontal #5 at 12" to 16" with additional #5 bars on the outside face of the foundation at negative moments areas created by the counterforts/butresses.
It sounds like you have a lot of jogs in he wall to help you out.

 

[COEngineeer]

jhoulette, so what do you guys do at the corners? Do you put the angled reinforcement at the outside face also?

 

[jhoulette]

yes, we basically have 2 or 3 additional rows of the bottom 2-#5 bars. This way you have bars on each face to handle neg or pos moments

 

[HouseBoy]

81 psf per ft of depth seems rather hing but that is what the soils consultant is paid to determine. Perhaps you can find out if that is a factored load.
Also, for 10 ft. wall heights, I would never use 8 inch thickness. 10 or 12 inch thickness would be ordinary.
The problem with assuming fixed at the corners is that you need to make sure the horizontal bars at the corners are positioned to resist bending moment. Many common practices (for residential concrete contractors) woudl not provide this capability. You will need to detail the steel bars at the corners for both "inside" and "outside" corners.
Keep in mind that for the loading you indicate, the horizontal reaction at the top of the wall is 1350 pounds per foot of wall. There are not anycommon framing details that are capable of developing this capacity, let alone having a diaphram that can take it. Also, with all the ins and outs you indicate, how would you detail the chords of your diahram?
With all that said, I would design the wall as pinned at the base, pinned or free at the top and fixed at the sides using at least 10 inch thickness and preferably 12 inch thickness. With the 24 ft max length, ther's a good chance that you can get it to work reasonably. This reference may be handy:

http://www.usbr.gov/pmts/hydraulics_lab/pubs/EM/EM27.pdf

Also, there are several FEM programs that allow good modeling of a wall like this that would give excellent output for design.

 

[COEngineeer]

Keep in mind that for the loading you indicate, the horizontal reaction at the top of the wall is 1350 pounds per foot of wall. There are not anycommon framing details that are capable of developing this capacity, let alone having a diaphram that can take it. Also, with all the ins and outs you indicate, how would you detail the chords of your diahram?
With all that said, I would design the wall as pinned at the base, pinned or free at the top and fixed at the sides using at least 10 inch thickness and preferably 12 inch thickness. With the 24 ft max length, ther's a good chance that you can get it to work reasonably. This reference may be handy:

http://www.usbr.gov/pmts/hydraulics_lab/pubs/EM/EM27.pdf

If you design it as pinned at the bottom, you will still get a lot of horizontal reaction at the top especially a the mid point. Thats why I fixed it at the bottom using a larger footing. But yeah 81 psf seems kind of high. Usually theyll give you a round number but here in colorado we have so much bad soil, ive never seen 81psf.