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

 

[oldestguy]

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.

 

[3DSigns]

Thanks

 

[3DSigns]

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?...2bc0f0d&oe=57E3D5A2&purl=&iss=sbi&adlt=strict

 

[3DSigns]

 

[3DSigns]

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.

 

[Ron]

Nice job, Wayne!

 

[3DSigns]

Thanks Ron!

 

[spats]

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

 

[3DSigns]

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.

 

[MacGruber22]

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."

 

[IFRs]

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?

 

[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?

 

[sticksandtriangles]

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

 

[3DSigns]

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/9780911380651_p0_v1_s192x300.jpg

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.

 

[3DSigns]

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.