Pouring footers

[JB4310]

I remember a few concrete jobs I worked on as a laborer when I was young, some of these were complex jobs that took weeks to form. I worked for an old off the boat Italian guy, a hard worker and a great work ethic. But some of these jobs were way over his head, even with engineers on the job interpreting the plans, it was a real struggle.

One job we started forming 30 feet down below a factory floor, when we started the pour, the tension was high, when a quadruple reinforced steel form blew out about half way up, and spilled 20 yards of 8,000 psi concrete down into the pit, the boss almost blew up The Italian slang and swears were flying between the older workers and Foreman, I luckily was not dragged into it, but will never forget it.

Made me appreciate how much stress concrete forms are under, especially when using vibrators, before the concrete sets.

JB

 

[CurlyDave]

You are never supposed to do that.

Pour no more than 4'depth, vibrate it thoroughly and wait an hour before pouring another 4' at which point the upper 4' can be vibrated.

Usually there are enough linear feet of forms that if you start at one end and move around the outside of the form, you will not get back to your starting point in an hour.

 

[JB4310]

You are never supposed to do that.

Pour no more than 4'depth, vibrate it thoroughly and wait an hour before pouring another 4' at which point the upper 4' can be vibrated.

Usually there are enough linear feet of forms that if you start at one end and move around the outside of the form, you will not get back to your starting point in an hour.

I'll take your word for it, since I do little concrete form work. I do place alot of concrete, but just over our narrow work trenches.

That job was a massive machine footing/ foundation that had more steel in it than concrete. The factory was built on a filled swamp, we found wood pilings 30 feet down. I don't remember doing it in lifts like you mention, but I do remember how hot that monolith was for over a week after we stripped the forms.

Even as massive as it was I remember they had several thru passages for mud jacking, should it of ever started settling. It was an aluminum extrusion machine that was to be installed on it.

JB

 

[bironacad]

Anyone ever mis-calculate or receive the wrong amount of concrete when pouring a footer? Ordered concrete today to pour my moms footer and we came up short, we were the last delivery for the day so we could not have the truck go to the yard and get more. So now it is setting up overnight and we will try to pour the rest in the morning.

I can't believe I was off by about 2 yards, I ordered 5 yards. Boards bowed out a little is some spots. average depth is 20", 18" wide by 43 feet in length for that width. then a smaller run at 8" wide by 21 feet 20" deep.

Will the new concrete set up strong enough to the stuff that was poured today or are we scre*** and have to dig it up (little seat time if that is the case)? We poured 3500# with fiberglass for the footer. When we ran out, I made a key in the top of what was poured and had my brother get some rebar to put in vertically. We have one side left, about 14" by 21 feet by 18" wide left to pour.

We are building a metal building by American buildings.

With my footing this past summer I chose to do my footing in sections. I first called in the building inspector to ask his opinions. I was down to rock and it was irregular which required steps and deep spots so to save my back and lumber in framing I did it in three sections. The inspector just said to tie in the spots with rebar. Rebar was cheap so I put it all the way through here are some pictures.

 

[EddieWalker]

Great thread. Lots of good points!!!!

Eddie

 

[flusher]

Did you dig and dump in a trailer or truck. There is no evidence you even dug down for 18" depth. Nice clean pour.

Just dug the footing trenches by hand. Filled the FEL bucket on my Mahindra 5525 and moved the load to the dump area on my place (I have 10 acres flat pasture). I always need fill dirt for this and that. I didn't want to hire the trenching done with a TLB since I didn't want the gravel driveway messed up. Took a few days to dig the trenches and set the forms, install rebar, etc.

The depth was 18" at the high point and about 24" at the low point on the footings (the driveway slopes down toward the road) so the average depth was somewhere between the two. IIRC, I used 22" depth to calculate the yards of concrete needed.

 

[Deere Dude]

formed work is about the only place one can really challenge if your shorted,

My brother-in-law ordered one yard of concrete to be dumped in a TV antennae hole but he told the driver he was shorted by 1/4 of a yard. The driver argued with him about it so the boss drove out from 10 miles away to check it out.
My BIL is meticulous and had dug an almost perfect 36"x36"x36" hole. That is one yard exactly. The boss stuck a hard ruler down to verify and told the driver to go and get 1/4 of a yard extra and bring it out. They do come up short now and then.

I found that I have to be as accurate as possible or I will come up short every time. I don't buy into just averaging the depth out. Almost every foot along the form has to be measured. Skilled people can probably do it without measuring and if I don't I can guarantee I will be short.

 

[JB4310]

Just remember how overbuilt our foundations are, concrete has incredibly compression strength, but is equally week in tensile strength.

Theoretically a 1 inch wide foundation, would hold up not only the average house but you could stack several of your neighbors houses on top of that.

The problem is with it's almost non existent tensile strength, that skinny little foundation would fail and buckle during back fill, or any other lateral force or movement. That's why we have 8 - 12 inch thick foundations.

It is not often you hear about a catastrophic foundation failure, I deal with foundation problems in my waterproofing/ drainage business. Often times people panic when they find a crack in their foundations, thinking that their house is settling or is gonna collapse. Some unscrupulous contractors will play on those fears, but I try to calm them by explaining first of all, that crack is not from settling, they are from stress relief, and have been there since within a month of the foundation being built.
I can't fix a cracked foundation, I can control any water that may come from it.

I mentioned earlier how overkill concrete foundations are, so just remember all this concrete you are placing is going to be around for a long long time, even long after we are gone, should that ever happen.

I used the term tensile strength in this post, I'm no engineer, there may be a better term for what I was trying to get across, maybe flexural strength??

JB

 

[CurlyDave]

In a lot of cases, the limit is not the strength of the concrete, but the load bearing capacity of the soil underneath the concrete.

The design of the foundation should have enough square footage of concrete so that the soil underneath it is capable of holding up the concrete foundation and the structure on top of it.

The theoretical 1 inch wide foundation you mention would sink into the soil creating huge settling problems.

 

[JB4310]

In a lot of cases, the limit is not the strength of the concrete, but the load bearing capacity of the soil underneath the concrete.

The design of the foundation should have enough square footage of concrete so that the soil underneath it is capable of holding up the concrete foundation and the structure on top of it.

The theoretical 1 inch wide foundation you mention would sink into the soil creating huge settling problems.

Agreed, that is the critical factor. In my example a suitable footing was assumed. But in very poor, unstable conditions any reasonably sized footing would be inadequate and in the most favorable conditions no footing would be required to meet the concrete's capacities.

I've worked in small ranch houses with 12 inch thick foundations with 12" x 24" footings. A linear foot of that footing and foundation weigh more the weight bearing on it.
Then I have a customer that thinks their house is going to collapse cause they have a stress crack in the foundation. Of course it looks bad cause it may have water leaking from it. I have to assure them it's nothing to worry about structurally. That's when I get into how overbuilt/ overkill our foundations are.

JB

 

[KennyG]

I used the term tensile strength in this post, I'm no engineer, there may be a better term for what I was trying to get across, maybe flexural strength??

JB

Tensile strength is the correct term, but for foundations, bending forces are the cause of the tensile loading. If you can imagine a beam that is being bent by a load, the top part of the beam is in compression because it has to shorten and the bottom part is in tension because it is lengthened.

The two general approaches we use for concrete is to reinforce it so the steel will take the tension (which works as long as the steel is oriented along the direction that is in tension) or making the concrete section so massive that the compressive strength will prevent it from deflecting (bending) enough to cause significant tension loads.

More sophisticated designs load the concrete in compression after it sets up so that during the initial deformation, it stays in compression and doesn't experience tension until it is past it's design load.