Residential found. wall design with extreme lateral load
Residential found. wall design with extreme lateral load
(OP)
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?
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?





RE: Residential found. wall design with extreme lateral load
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!
RE: Residential found. wall design with extreme lateral load
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)
RE: Residential found. wall design with extreme lateral load
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?
RE: Residential found. wall design with extreme lateral load
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)
www.SlideRuleEra.net
RE: Residential found. wall design with extreme lateral load
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.
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load
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.
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load
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.
RE: Residential found. wall design with extreme lateral load
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!
RE: Residential found. wall design with extreme lateral load
Good luck!
RE: Residential found. wall design with extreme lateral load
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.
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load
It sounds like you have a lot of jogs in he wall to help you out.
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral 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:/
Also, there are several FEM programs that allow good modeling of a wall like this that would give excellent output for design.
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load
That's why I suggested that it might be a factored load. Seems like a possible reason that it is so high.
I know the horizntal forces get high out near the nid'length of the wall but he said he wanted only conventional footings and I know, wide footings and the needed steel is asking alot.
I think the most effective solution will involve:
1. Use better analysis tools (either plate charts or FEM)
2. Get at least 10 inch thickness (12 if possible)
3. Detail steel placement in the wall to maximize the effectiveness.
I guess to answer the initial question: does bars at 6" oc in an 8 inch thick wall seem reasonable, I'd say NO. It might be what is required from the simple analysis but I think that analysis is not adequate and the wall thickness is not adequate (he hasn't said yet that he can not go thicker, only that he doesn't want a wide footing).
RE: Residential found. wall design with extreme lateral load
Before you go through all that work designing the wall, I would seriously look at the recommended equivalent fluid pressure of 81 psf!
This.. IS an awfully high equivalent fluid pressure! I would ask what soil values he used to determine this? Because you are dealing with a basement wall and thus it is free from rotation, he is likely using at rest earth pressures.
In order to get a efp of 81, the soil in question would need to have a internal angle of friction of 13 degrees?? (assuming a moist unit weight of 105pcf) This would be a TERRIBLY soft clay! I would really ask for clarification from the geotechnical engineer. You may be overdesigning. Or do you have shallow ground water?
Also, equivalent fluid pressure is commonly used for simple retaining walls for ease of calculation, but it has some limitations. Because simple active earth pressure calculations (without surcharge, slopes, loading) yields a trigular pressure distribution there is a similarity to a fluid pressure analysis with a given equivalent weight. But you cannot properly design surcharge loads, wall geometry, or live/dead loads with the efp approach.
Yeah, its high. I'd recommend backfilling with compacted, free draining sand, (internal angle of friction = 32-36 degrees) Then you will be using (at rest) efp's around 30 to 35pcf range.
Or you could always just use Rankine and calculate the actual pressure envelope.
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load
RE: Residential found. wall design with extreme lateral load