Soil Bearing Capacity
Soil Bearing Capacity
(OP)
On a monument sign I'm going to build, I just calculated the weight of 8 yards concrete, 300' of #4 rebar, 93- 12x8x16 block, 24- 8x8x16 block, concrete in the block cavities containing vertical rebar, stone-veneer facing, mortar, EIFS caps, and two sandblasted and CNC machined high-density urethane sign units. I ended up with 43,755lbs. My permit has already been issued, but since I had forgotten to calculate weight and they never asked, I am concerned about the size of the footer, its pressure on the soil, and wonder if I should "overbuild".
The footer I have in the submitted drawing would be 3'2''x 17''which gives it a footprint of only 53.833> sqft. That comes to a direct vertical pressure of 812.78 lbs/sqft and it is in clay soil. Both the county planning departmenand the building inspector have already approved as is but if I widen that footer by just a foot on each side making it 5'2''x17' (99.875 sqft), that will distribute the weight more, lowering the pressure to 438.1 lbs/sqft. and may help me sleep better knowing this ginormous sign won't sink after a few heavy rains. Do you think this is necessary? Thanks! Note* This is NOT for a client. It is my own sign, on my own property, and I will notify the building department before doing this. Thanks for your advice. Wayne
The footer I have in the submitted drawing would be 3'2''x 17''which gives it a footprint of only 53.833> sqft. That comes to a direct vertical pressure of 812.78 lbs/sqft and it is in clay soil. Both the county planning departmenand the building inspector have already approved as is but if I widen that footer by just a foot on each side making it 5'2''x17' (99.875 sqft), that will distribute the weight more, lowering the pressure to 438.1 lbs/sqft. and may help me sleep better knowing this ginormous sign won't sink after a few heavy rains. Do you think this is necessary? Thanks! Note* This is NOT for a client. It is my own sign, on my own property, and I will notify the building department before doing this. Thanks for your advice. Wayne






RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
There is no frost depth as I am in Florida. The footing in the plans is to be 12" deep with a "stabilizer" going down to 36". I have raised the 'pedestal' above grade and added the alternative footer, which is almost 86% wider, to the right side of the drawing below.
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
Oldestguy knows, take his advice. When working in soil, what seems easy on paper (precise excavation) is often difficult in practice.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Soil Bearing Capacity
Agree with Oldestguy and SlideRuleEra.....your detail is a lot more complex than it needs to be. Your sign is a simple monument sign.
You're in Florida. If my research/guess is correct, you are in the Tampa area. You mention that the soil is clay. Are you sure it is clay? That would be unusual to find true clay at the surface in that area, though there are some clay areas/pockets to the northeast of the Tampa area. It might be clayey sand. In any case, your bearing pressures are fairly low, particularly your wider detail. Unless you have some really crappy soil conditions, most anywhere in Florida you can get an allowable bearing pressure of 1500 to 2000 psf. Look at the buildings around where the sign will go. Is there anything extraordinary about the way they are built? Most likely they are on simple shallow foundations and performing well. There are plenty of good geotechnical engineers around there who could give you an off-the-cuff synopsis.
I've designed several similar monument sign foundations. Mine are simple, cast-in-place concrete blocks with minimal reinforcement just as OG described. Most of the time, for these signs, we need ballast for overturning moreso than other reasons!
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Also if you're an engineer, don't compute soil bearing pressure to 2 decimal places or area to 3 decimal places. Significant figures is one of my pet peeves!
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No, I'm not an engineer; I'm a 5th generation backwoods Florida Cracker sign builder with a bad habit of using hick words, :). I'm in NW Florida on the "coastal plain", but over 55 miles inland from the Gulf of Mexico at about 11' above mean sea level, approximately 200 yards from a creek bottom and I'm dead sure it's clay directly below the black topsoil. Sticks to your shovel or hole diggers so tenaciously that you'll expend 6 times the energy just trying to get the dang stuff off. I've seen Grandaddy use motor oil on his shovel; probably used hog lard back in the old days. Anyway, you're right: the closer one gets to the coast from here, the more sand exists in the soil indeed. My apologies, I'll call it a "foundation" or "footing" from now on. ;) Thanks all. From what I'm hearing, the clay should support my foundation just fine and I just need to make the sides of my hole straight and vertical. I can't wait to start :)
RE: Soil Bearing Capacity
First off - 900 psf bearing is not very large at all. In a bearing capacity situation, your clay needs only to have an undrained shear strength of about 450 psf - this is in the "soft" range (N values (rough idea) of about 4 plus/minus) - this is "bad" soil. From a settlement point of view, you have already removed weight of soil so the added net weight will be only that "above" the weight of the soil removed. Would you then have a settlement problem. Doubtful. As Ron, SRE and oldest guy point out - keep things simple!
As a case in point - a mate of mine had a problem like this: High mast pole - footing to be 2.6 m by 2.6 m about 800 mm thick. Pedestal about 1.1 by 1.1 m up to ground level. He wanted to keep his excavation very "small" - so my mate suggested just to put in his mud mat (blinding concrete), then build his rebar configuration on this - and just fill up the excavation with concrete - - simple and having the added benefit of the foundation base being placed against the undisturbed soil - a no brainer for me. But, no, he had the formwork already built and wanted to put it in. Fine. But, the kicker was that in some places after removal of the formwork, he only had something like 150 mm of space between his foundation base and sidewall. How can you compact anything in this space. In the end, he filled up the space between the footing base and the excavation sidewall with concrete - meaning all that formwork time and money for nothing.
RE: Soil Bearing Capacity
"Footer" is not a hick word. It is commonly used by contractors all over. It's just that, as a licensed engineer, a licensed contractor and a part-time professor of construction management, I try to push proper terminology so that all are speaking the same language....cuts down on confusion and there's certainly enough of that in both engineering and construction! I certainly didn't mean it as a slight of any sort...hell, I was born in Dothan, so I only speak one language....southern!
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
Concrete is interesting stuff. If you make it too soupy, the resulting strength is lower than if it is stiff. However, then once setting starts, you don't let it dry out because it needs water to develop the "glue" that holds things together. Given a continuous moisture environment concrete will continue to gain strength at a reduced rate for years. However, 30 days of moist conditions is plenty and even a week is OK in some circumstances.
I'll add that for the purpose of holding the monument sign, settlement should not be of concern, since it won't be noticed. However, local information indicated no problem there anyhow.
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I hope you realize those are not appropriate significant figures.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Soil Bearing Capacity
812 psf ?
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Good luck!
Ron
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
OG...I learn from you and BigH every time!
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
The FORMED footing on top can be something like you had originally planned (apparently some grade raising there). It needs to be thick enough for tension transfer from "hair pins" to the vertical bars. I didn't show other vertical bars that you will need to go up into your higher sigh places. Ideally they need to get resistance from the upper formed footing one way or the other, perhaps by lapping bars as much as you can.
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RE: Soil Bearing Capacity
I'd not ask a contractor to do the last figure narrowed "fin" without a heck of a lot of work.
That higher formed block may be more trouble than a short "footings" . Holding that higher form there won't be easy. Once it starts to move you really gotta move also.
If this stays rather complicated, the only way I'd do it (if I was a contractor) would be cost plus.
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
I'd be overly conservative in holding those forms there. Maybe even mound up earth around them in addition to sturdy stakes. If they rise up, your job is a gonner. That's a damn heavy liquid.
Staking next to an excavation is poor practice and may cause cave in.
I'd re-think the whole thing and do it in steps. Saving an extra ready-mix trip is not worth it.
Keep every ste0p simple.
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
This site is the best! :)
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
I see what you mean. Thanks Big H. Straight sides they will be. I plan to pick up the blocks and form material this afternoon. The block will be stored inside until ready to use and I'll go ahead and make my forms. Then I plan to rent an excavator on Friday and hopefully we can get a concrete truck out here by Tuesday. Thanks!
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
Willdo
Here is some past work. I will post pics of the new project as it progresses.
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https://www.bing.com/images/search?view=detailv2&a...
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https://www.bing.com/images/search?view=detailv2&a...
https://www.bing.com/images/search?view=detailv2&a...
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https://www.bing.com/images/search?view=detailv2&a...
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
I will be keeping the concrete wet for a couple of weeks to make sure it's cured before going to the next phase. Thanks for the help y'all.
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
W = width of face in feet
H = height of face in feet
C = distance in feet from grade to center of face
P= windload pressure (29psf@125mph)
D = depth of hole in feet
(W x H x C x P)/(2000 x D) =
(16x7x3.5x29)/(2000x2) = 2.84..... 3 yds concrete
My footing (below grade) is almost 4 yards and will actually have another foot of fill sloping away from the base which will give it even more lateral earth pressure. In addition to that,the other 5 yards of concrete in the pedestal gives it more ballast. There are a total of 9 yards of concrete in the monolithic footing/base shown above.
RE: Soil Bearing Capacity
2. 29 psf seems a bit low. I suspect that the pressure coefficient of the sign has not been used in deriving the average pressure on the sign.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Soil Bearing Capacity
I don't think that your formula calculates a downward toe pressure.
Using "D" it seems that it calculates a sideward pressure at the bottom of the trench.
The moment is W x H x C x P = 11,855 ft-lbs.
Divide this by the length of the foundation and half the width of the foundation to get toe pressure which adds to the dead weight.
Might also check uplift on the windward side, resisted by foundation weight.
The dimension C is half the height of the sign - this suggests that the sign is at grade - is this true?
RE: Soil Bearing Capacity
RE: Soil Bearing Capacity
This made me laugh
RE: Soil Bearing Capacity
Meanwhile, here is another equation for determining wind load...
F = A x P x Cd
Where F is the force or windload
A is the area
P is the pressure
and Cd is the drag coefficient
I also just found this equation on the net, for determining wind pressure; P = 0.00256 x V2, Based on that, a 125 mph wind exerts a pressure of 40 psf instead of "29" like I had above. I will check that with what my book says too.
RE: Soil Bearing Capacity