×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Soil Bearing Capacity
8

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

RE: Soil Bearing Capacity

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.

RE: Soil Bearing Capacity

(OP)
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.

RE: Soil Bearing Capacity

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.

RE: Soil Bearing Capacity

(OP)
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.

RE: Soil Bearing Capacity

Quote (3DSigns)

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Soil Bearing Capacity

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!

RE: Soil Bearing Capacity

2
I was waiting for Ron to chime in about "footer" bigsmile
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!

RE: Soil Bearing Capacity

(OP)

Quote (Ron)


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

. . . 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.

RE: Soil Bearing Capacity

Wayne....I forgot about the true clays in "lower Alabama"lol. 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!lol

RE: Soil Bearing Capacity

(OP)
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!

RE: Soil Bearing Capacity

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.

RE: Soil Bearing Capacity

Quote (3DSigns)

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."

RE: Soil Bearing Capacity

(OP)

812 psf ?

RE: Soil Bearing Capacity

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

RE: Soil Bearing Capacity

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

RE: Soil Bearing Capacity

Again thanks Ron. I was waiting.

RE: Soil Bearing Capacity

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

RE: Soil Bearing Capacity

(OP)

Quote (oldestguy (Geotechnical))

.........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?

RE: Soil Bearing Capacity

This is a rough idea. You need some depth for resistance to leaning and your 3 feet seems OK. The re-bars are mainly to make use of the deeper footing for resisting that overturning. I figure a back-how wold dig this like a trench and getting exact smooth sides is easier that way, I think. If you can, ask the digger to place (tack weld or better) a steel plate to cover the teeth, leaving smother base, etc.

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.

RE: Soil Bearing Capacity

Wayne...it is not necessary to slope the sides. In fact, if the clay swells from water, it would be easier for it to push the sign up.

RE: Soil Bearing Capacity

(OP)
I was thinking of pouring it all at one time, monolithic, to save having to get the concrete truck out here twice. Would pouring it 4'' wider on each side, in the footing, push the concrete forms upwards with hydraulic pressure?

RE: Soil Bearing Capacity

Possibly. If you set your forms at the edge of the excavation you won't have that issue.

RE: Soil Bearing Capacity

(OP)
Only slightly sloped?

RE: Soil Bearing Capacity

I wouldn't slope the sides at all. Make them vertical. You don't save that much concrete and you complicate the construction. It also reduces your overturning resistance.

RE: Soil Bearing Capacity

(OP)
Ok, this is basically what I had from the beginning. I wanted a "stabilizer" jutting down further into the ground because we are in a 130 mph wind zone. But after calculating the weight of this thing, I wonder if hurricane windsor gusts would impact this thing at all? Maybe a direct hit from a tornado would.

RE: Soil Bearing Capacity

Guess I stirred things some. Side slope can be anything that stands. If vertical stands and doesn't cave, fine. Just don't leave significant caving there when placing concrete.

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

(OP)
I am going to drill little 1/8" holes in the 3/4'' plywood and use them to string brace wire between the two forms. Tighten them real tight with 38" 2x4's next to each. As the concrete is poured, I'll snatch the 2x4's out, leaving only the wires to keep the sides of the forms from bowing. That's the plan anyway. I also have some massive railroad-trestle timbers which I may lay up along the sides, stake them and wedge them in. That should also help to keep the forms from moving during pour and setup. I just need to get underway before the rainy season gets here.

RE: Soil Bearing Capacity

(OP)
Thanks Ron, looks like about the same amount of concrete too.

RE: Soil Bearing Capacity

No re-bars going up inside the blocks?

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

(OP)
Yes, see my design in my second post, bottom left of the drawing. I plan to measure and stub up about 18" where the block cavities will be, lay all the block, place rebar down through the corresponding cavities and pour full of concrete. Yes, 4.5 yards concrete inside the form will be about 18,600 lb worth, maybe even more with the water. Yes the dirt is a good idea I will make sure they won't move before pouring.

RE: Soil Bearing Capacity

A Google search of concrete forms might help. Commercial form ties might save you the grief of rusting monument faces where the wires remain. Also, some examples of different form bracing may help keep things in line.

RE: Soil Bearing Capacity

(OP)
Thanks oldestguy!
This site is the best! :)

RE: Soil Bearing Capacity

I'm a tad late to the latest - but I too do not see why the below ground excavation is wanted to be angled . . . a straight sided excavation would be more reasonable. Tomlinson has a good discussion of "trench footing" .. . hoping he is okay with it - see attached.

RE: Soil Bearing Capacity

(OP)

Quote (Tomlinson)

Uplift of these foundations causing hogging of the superstructure can result from adhesion of the clay to the sides of the deep strip, particularly where the excavator has produced a trench wider at the top than at the bottom.

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

More from OG. If possible, post a photo or two of your work. That is, if you aren't afraid of comments.

RE: Soil Bearing Capacity

(OP)
This is another one I plan to build for my CNC enterprise. Except for the graphic elements made of HDU (high density urethane), it will made entirely from EPS (polystyrene), will be 96''x65''x18'', weigh less than 200 lbs, and won't need a foundation. I will cut it out on the foam cutter I designed and built. The graphics will be cut on my CNC router.

https://www.bing.com/images/search?view=detailv2&a...

https://www.bing.com/images/search?view=detailv2&a...

RE: Soil Bearing Capacity

Hey, very exceptional work that. However, I was referring to the stages of construction for the current post. I'd bet the attention to detail will ba as good.

RE: Soil Bearing Capacity

(OP)
Thanks :)

RE: Soil Bearing Capacity

(OP)
We made a trip to Marianna, yesterday evening, and picked up the block, rebar, and some of the form materials, The blocks will be stored inside the shop to keep them dry until time to lay them. This is to help prevent the mortar joints from cracking later because concrete blocks expand slightly when wet (according to what I have read). An 18' x 4' area is staked out at left of the photo.

https://www.bing.com/images/search?view=detailv2&a...

RE: Soil Bearing Capacity

(OP)



RE: Soil Bearing Capacity

(OP)
I used 9 gauge galvanized fence brace wire, strung it between opposite studs on the forms top and bottom, then twisted the wires tight and placed a small rod in the twist-holes to keep them from unravelling when the pressure was applied. As you can see I also staked the thing from about 3' out and installed diagonal braces. My forms did not move or lift at all. :)
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

Nice job, Wayne!

RE: Soil Bearing Capacity

(OP)
Thanks Ron!

RE: Soil Bearing Capacity

I don't see where anybody has even thought about the wind loading on this sign!

RE: Soil Bearing Capacity

(OP)
Yes, We are in a 125mph wind zone, and this is a formula I used...


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

1. I don't see how the units work out in your formula. Moment = Force/Length ??
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'm late to the game, but this was my question exactly.

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

Ya'll have been assisting the OP for more than two weeks, and I'm not sure anyone can see the forest for the trees... Some ethereal equation that calculates cubic yards of concrete based on wind load? Are you kidding me?

RE: Soil Bearing Capacity

Quote (spats)

Ya'll have been assisting the OP for more than two weeks, and I'm not sure anyone can see the forest for the trees... Some ethereal equation that calculates cubic yards of concrete based on wind load? Are you kidding me?

This made me laugh smile

RE: Soil Bearing Capacity

(OP)
This is the book I found the formulas in. In it are 3 equations for figuring concrete with wind load as a factor. I will get the book out and review them for you when I get home. It also has a wind pressure formula. http://prodimage.images-bn.com/pimages/97809113806...

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

(OP)
Ok, I re-checked my book. According to it a 125mph wind, at 10-14 feet above ground, exerts a force of 30 psf. So the "29 psf" I posted above wasn't off by much. But even going by the 40 psf figure I found on the net, I have plenty enough concrete. The formula LxWxCxP/2000xD indicates less than 4 yards is needed in my excavation(for the 40 psf wind pressure) ; but my excavation actually has 4.25 yds. I realize there are many more factors to consider but I think it's going to take a heck of a lot of wind to knock this thing over and at over 23 tons, not to mention earth pressure, a lot of uplift too. Thanks all, I'll hire an engineer next time.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources