Soil Bearing Capacity 8

[3DSigns]

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

 

[mike20793]

Only a geotechnical engineer that has properly reviewed the site is going to know if you are going to have settlement issues or shrink/swell issues with the clay. How deep is the footing and what is your frost depth? At first glance, your pressures seem like they will be okay, but we cannot comment with any amount of certainty.

 

[3DSigns]

Hi Mike, thanks
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.

 

[oldestguy]

Looking at your details and wondering about building the foundation, I wonder if you have thought of the complications a contractor will go through to do all the fancy underground details. No one will see it. Wouldn't a big ugly concrete block, no fancy sides, etc. with minimal reinforcing down 3 feet with more concrete be cheaper and less time consuming to build? Also less likely to tilt.

 

[3DSigns]

Not sure. It already going to weigh 43,755 lbs. The angled sides under the ground help distribute the weight with less concrete, but making them straight with the same footprint is going to add another 2,550 lbs to the thing. I will dig the footer with a small backhoe, but a square-point shovel will carve out those angles in less than an hour. I don't know, just trying to cut down on weight and distribute the weight because the thing is already very heavy and this soil gets wet and soggy sometimes.

 

[SlideRuleEra]

Not sure... I don't know...

Oldestguy knows, take his advice. When working in soil, what seems easy on paper (precise excavation) is often difficult in practice.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ron]

Ah...pet peeve..."footer"! If you're an engineer, don't use it!

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!

 

[XR250]

I was waiting for Ron to chime in about "footer"
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!

 

[3DSigns]

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

 

[BigH]

. . . and weight to the nearest pound especially since an increase or a decrease in the air content of the concrete will throw off the weight!

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.

 

[Ron]

Wayne....I forgot about the true clays in "lower Alabama". Taking another wild-ass guess, maybe in the Chipley area? Yes, those can be tough to dig in, but are sufficiently strong as BigH noted. Just keep the foundation simple. If you keep the bearing pressure at or below about 1000 psf, I don't see an issue at all. If I guessed right and you are in or near Washington county, then you probably are in a relatively well drained area, even with the clay.

"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!

 

[3DSigns]

Ron, you hit the nail on the head; we're based in Chipley and the property is on the east side of Holmes Creek in Washington County. We go to Dothan all the time (Lowes, Home Depot, Emfinger Steel etc). Thanks for all of the information, including the proper terminology. I was worried about the foundation but you all have certainly eased my mind. Y'all are the greatest!

 

[oldestguy]

Here is what I'd do. I'd have a back-hoe there and excavate a hole measuring about 4 ft. by a little over 17 feet, with SLOPING SIDES. Bottom width could be bucket width. It does not have to be straight sided. Three feet depth is plenty and less probably would be OK also. Make sure the sides don't have any significant loose stuff. No forms needed. Immediately fill with 5 bag (cement) mix concrete of stiff category (not soupy) to about a foot below final grade, level top. No re-bars within it, but very soon there after, as soon as you can, shove "U" shaped re-bars at least 18" down into it. leaving 8" of upside-down "U" exposed in the area where a formed thick slab footing will later be placed. Then with new forms in place, you can get going with reinforcing projecting up as needed for reinforcing your wall blocks, with bottom to be within the new footings with right angle bends on the bottom ends for anchorage. Your 3 ft. wide footing probably is OK. The stiffer you can keep the concrete the stronger it is. Once setting is well underway, keep the footing moist.

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.

 

[MacGruber22]

812.78 lbs/sqft

I hope you realize those are not appropriate significant figures.

"It is imperative Cunth doesn't get his hands on those codes."

 

[3DSigns]

812 psf ?

 

[Ron]

Wayne...800, 850.....bearing pressure and geotechnical parameters are not exact by any means. In engineering and construction we are guilty of calculating things to two or three decimal places, measuring the result with a micrometer, marking it with a crayon and then cutting it with an axe!

Good luck!

Ron

 

[BigH]

@Ron - well there is a huge difference, eh?, between 812 and 813! 800, 825, 850 . . . all about the same!

 

[oldestguy]

Again thanks Ron. I was waiting.

 

[Ron]

BigH....
OG...I learn from you and BigH every time!

 

[3DSigns]

.........I'd have a back-hoe there and excavate a hole measuring about 4 ft. by a little over 17 feet, with SLOPING SIDES. Bottom width could be bucket width........[/]

Is this what you mean oldestguy?