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]

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.

 

[Ron]

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.

 

[3DSigns]

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?

 

[Ron]

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

 

[3DSigns]

Only slightly sloped?

 

[Ron]

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.

 

[3DSigns]

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.

 

[oldestguy]

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.

 

[3DSigns]

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.

 

[Ron]

Here's the way I would configure it....see attached

 

[3DSigns]

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

 

[oldestguy]

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.

 

[3DSigns]

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.

 

[oldestguy]

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.

 

[3DSigns]

Thanks oldestguy!
This site is the best!

 

[BigH]

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.

 

[3DSigns]

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!

 

[oldestguy]

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

 

[3DSigns]

Yes,
Willdo
Here is some past work. I will post pics of the new project as it progresses.

https://www.bing.com/images/search?...784e866&oe=57E7C287&purl=&iss=sbi&adlt=strict

https://www.bing.com/images/search?...1264614861868_o.jpg&purl=&iss=sbi&adlt=strict

https://www.bing.com/images/search?...0199dd4&oe=57CD2C21&purl=&iss=sbi&adlt=strict

 

[3DSigns]

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?...4978648551969_o.jpg&purl=&iss=sbi&adlt=strict

https://www.bing.com/images/search?...1508120641612_o.jpg&purl=&iss=sbi&adlt=strict