Question for a concrete expert

[Bigrhamr]

Next spring I'll be putting in some concrete piers for a large engineered steel truss building with fabric cover. The piers are like an upside down mushroom, 4'x4'x1' tall at the bottom with a 2'x2'x3' tall section coming up to grade level. I've got the rebar schedule and a plan for embeded anchors.
So the question is on how to best pour and form it. It seems like to do it in one piece you would have to form the whole thing including the top part of the bottom 4x4 pad. Or in 2 parts seems much easier, just pour the bottom
and after it sets put a form on top of it for the 2x2x3 and pour it. The rebar detail shows extra steel passing through where the joint would be, is the 2 part pour acceptable or do you loose too much strength not having a monolithic pier? How do the pros do this?

 

[rieferman]

I'm definitely not an expert, but something like this http://www.bigfootsystems.com/ is what I saw used by the This Old House guys at one point

 

[bimmer1980]

I would recommend checking with the manufacturer of the building.... If they do allow two pours, I would recommend a keyway in footing in addition to the rebar. THat would help the column portion to bond with the footing.

 

[Mike83]

A construction joint at the top of the footing is fine. As long as the rebar (should be L-shaped dowels) extend from the footing into the column you'll be ok. Finish the footing flat everwhere except where the column will be built (make sure the area where the column forms go is level and smooth). Leave the area where the bottom of the column goes rough (don't trowel). Use a beveled piece of 2-by to form the keyway.

Pouring monolithically would just about guarantee voids in the concrete. The construction joint will work fine especially for a footing column that sees primarily axial load, but the keyway and rough surface will take care of shear forces.

 

[sungrove]

My 2 cents:
I'm not a pro, but I have done a fair amount of concrete work while rebuilding a small house. I made and hand poured a couple of foundation walls and also used a couple of the Bigfoot forms mentioned above.
My thought is that given that you plan a four foot square bottom area and a 2 foot square column, you will not have that much trouble pushing a 2 x 4 down into your concrete to get rid of air holes.
What gets a bit more involved is building your forms. I imagine you will have to build a lower box, then create a way to support the upper column on top of it and then seal the area around the base of the column so that you can control dirt getting in or concrete coming out. ( possible a concrete guy would say no need for this) The Bigfoot forms are great for all this because they have flanges on top to install the concrete tube forms from the hardware store.
In either case, whether I was making may own forms or using the Bigfoot forms, I would want to back fill with dirt to hold it in place snugly and not move. I'm realizing the bolts for the steel columns really require accurate placement.
One drawback to pouring it all at once might be that if there is a lot of rebar in there it could be much harder to get your concrete in without voids because it's hard to push in your board or whatever. Also, unless Bigfoot forms have changed, I don't think they are large enough for your needs. I don't think they come four feet wide.

Neil

 

[pcmeiners]

If it is a very large building, I would think Engineers would want a monolith pour. A concrete ******** would negate any voids, 2x4s ain't going to do it. Setup piers for large building, and lining up the anchors for the erected steel needs to be exact, stressful as hell... what I would do is create a small slab below the required 4'x4'x1' support just to mount the rebar structure. Anchor and align the rebar structure to the wet slab, let it set, then pour the required structure after it sets, in one pour of un-watered down crete, (we used to use steel plate welded to the rebar to align the erection anchors)... single pour is easy enough to do if your rebar/erection anchors can not move.


Throw the question out to Engineers at the following Engineering site....

http://www.eng-tips.com/threadminder.cfm?pid=256

http://www.eng-tips.com/threadminder.cfm?pid=167

 

[sungrove]

To ask the above forum a question you need to sign up like you do here. Problem is it is only for professionals. Or I guess one can just pick a kind of engineer you are when signing up. rocket scientist! Neil

 

[tcianci]

Where did you get the specifications for the footings to begin with? If they were engineered, then there should be some information on how they are to be constructed. That being said, you shouldn't have any problem doing a 2 piece pour and the 4 x 4 section could probably be formed by just the excavation itself. As has been said you want the re-bar to come out of your footing and into the column. In general there will be no bonding of concrete to concrete no matter what type of finish you leave on the footer all of your attachment will be through the re-bar, that will take care of the shear concerns, once you're backfilled, just about all of your load is compressive.

I am curious about the size of the footer, was this specified because of the soil conditions? A fabric covered building weighs next to nothing compare to a conventional structure. The footing size makes it sound like you're building this thing in a swamp.

If you really require a 4 x 4 footing be sure to use sufficient re-bar throughout the footing because the 1 foot thickness otherwise woudn't be sufficient to allow the load to be distributed throughout the whole pad and the 4 x 4 pad may crack.

 

[Mike83]

I bet when the wind really blows there's a fair amount of uplift in the foundation. The 4x4 size would take care of that to really grab into the soil.

 

[Bigrhamr]

The building is 70' wide x 120' long and about 35' tall at the peak so the big footer is for wind loads mostly I believe. Still seems like overkill but that's okay with me cause it's gonna make one hell of a kite if it ever gets loose!They engineered it for snow, wind and seismic for the area but did not do any soil testing so the footing specs are copied from another building of the same configuration with worst case soil conditions for the area.
Thanks for all the good ideas everybody. I like the idea of placing a little concrete at the bottom to anchor the rebar structure and anchor bolts. That would take the worries out of of them getting moved during the pour.

 

[sungrove]

You know, bigrhamr, I think you may be building a spaceship in there ; )

 

[Daniel Dudley]

I am not a big fan of sauna tube columns because I have seen too many shift. If you poured a rough footer in the base of the hole as previously stated, you could drop a few rebar uprights in it and put a bigfoot type form over it. It is easy to get these right where you want them and then back fill around them so they don't shift when you pour.

I like the solid one piece plastic type. Also easy to laser level and cut to an even hieght.

 

[Daniel Dudley]

The original bigfoot type footers do have a large base, and do use a sauna tube. They work well, and don't require two pours which is nice. I think they are more like three by three though. Forming for a tapered monolithic pour by hand is a real pain, and time consuming. You also have to break down the forms. Burying wood is against code AFAIK.

 

[pcmeiners]

" Problem is it is only for professionals. "
It may be for professional but they answer anyway, make sure you tell them your not an Engineer or they hit you with super technical info and references you do not have access to.
Good luck

 

[buening]

In my opinion, there should not be an issue doing two pours and it is done frequently. The dowel bars extending from the footing into the column base tie the column to the spread footing. I would suggest contacting whoever did the detailing of the piers to confirm.

 

[Daniel Dudley]

" Problem is it is only for professionals. "
It may be for professional but they answer anyway, make sure you tell them your not an Engineer or they hit you with super technical info and references you do not have access to.
Good luck

I do this a lot, and I know what works for me. I like easy and fool proof. Overbuilt never bothers me either, if it is simple and fool proof.

Location of these needs to be spot on, as does height. After this, putting in footers and cones is an after thought. If you have any doubts about your ability to do this, get help. It is not rocket science, and it is pretty easy to do especially after you see someone do it.

Not so easy to redo, and a real pain if you are ready to start building and things don't line up. Layout is critical here. You can cheat wood. Steel doesn't lie.