I’m curious as to whether anyone on here has dealt with a situation inherent in pier and grade beam foundations. That is, how, when dealing with 18″ piers, do you bring the steel from the cages up into an eight inch thick grade beam? Seems to me there’s a problem there.
Tom
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this is more of a bump than professional advice, others will post as well. If I had to guess, I'd think a cage within a cage, with the inside cage being taller so it goes in the 8" beam. both cages tied together with shorter horizontal rebar.
I'm thinking how much work that would be, heh, heh. Also, the stirrups are pre-bent and they return upon themselves, protruding into the interior space. I don't think they allow for much clearance inside a typical 18" pier setup. In other words, I don't really see getting another cage in there.
It does seem there must be a standard solution to the problem. I just don't know what it would be. We'll see if someone knows the answer.
Take a look at the following link and see if we can get our definitions straight first.
Grade beams are not on top of piers.
Tell us what you are building to get the best advice.
http://www.raisedfloorliving.com/footings.shtml
Grade Beam FootingA grade beam footing is a continuous reinforced-concrete member used to support loads with minimal bending. Grade beams are capable of spanning across non-load bearing areas, and are commonly supported by soil or pilings. A continuous grade beam is frequently constructed by digging a trench at least 8" wide to the depth needed to span the distance between supports. Grade beam footings differ from continuous spread footings in how they distribute loads. The depth of a grade beam footing is designed to distribute loads to bearing points, while the width of a continuous spread footing is designed to transfer loads to the ground.
I believe the posters use of piers is correct and synonimous with pile, piling, micropile, caison ect.
the way the message reads, it's as if he is describing a concrete beam on top of piers, not piers on top of beam:"do you bring the steel from the cages up into an eight inch thick grade beam?" from post #1, "up into"
A grade beam is a concrete beam built on the surface of the ground with piers going deeper into the ground under the beam.
Edited 2/9/2008 8:24 pm by Jason99
got it, so the beam sits on pier, than house sits on beam, or additional piers above? Just saw this old house, New Orleans, 30 foot pilings driven into soil with concrete beam above and block piers above that. pretty cool.
You can have any type of foundation on top. Basement foundation walls surface piers ect.
The whole point is to help unify the piers. You use piers in expansive soil or on hill sides to get better bearing. Then the grade beam locks them together to decrease differential settling.
A good visible example would be concrete or steel pilings on a freeway overpass with a pile cap connecting them. In a dwelling the pile cap is the grade beam on the ground and the piers go to bedrock or a better bearing strata.
Jason
OK - here's a Q from a southern boy... Are grade beams used up north? I'm guessing not... Frost heave would do 'en in.... again, guessing...
Yes, grade beams are used up north. Not so much in residential but in commercial and industrial
That's the ticket. It's a concrete beam reinforced with steel that sits on piers, at grade. Hence the name "grade" beam. The piers extend down to bedrock. (Or however far the soils engineer makes you go, heh, heh.)
So anybody know how narrowing down the steel situation is handled when you have 18" peirs and an 8" thick grade beam? Somebody out there must have dealt with this circumstance before. I'm sure it's very common throughout California.
Oh-oh I think we got another Googler. Like that guy with all the degrees who gives all that electrical advice - no dirty finger nails on his hands... :-)
That's not a grade beam foundation Ralph.
The engineering drawings should show the connections. Each engineer seems to have his own criteria.
John
I can't visualize too many options John. I keep thinking somebody that gets on here must have dealt with it before.
Not sure I understand the confusion. I have built many in Calif. Generally there are four or six bars coming out of the pier narrower than the grade beam and bent 90 degrees. Some guys stick them up straight and bend after the concrete pour. Usually they tie to the upper bars in the grade beam. Yes it is often a big cluster f--- to get them all to tie into the grade beam steel and still get proper concrete clearances. After they are tied the grade beam stirrups can be slid back over them to get their spacing.Piers are poured first, then the grade beams formed and poured. The more accurately the tie bars are placed according to the building line(no wet setting in Calif.) the less hickey work there is later to align them to the grade beam rebar.When I started, we dropped one bar into the pier. Then it went to two or three bundled and everyone screamed at the extra cost. Now six number fives with #3 stirrups at 18" seems to be the minimum. Have done larger. We also tie on 3" dobies to maintain side clearances. It's a lot of fun standing up those 20 footers and dropping them down the hole.Hope this helps, John
You sound like the guy to talk to about this John.
So it's not a monolithic pour...? What do the engineers have to say about moisture seeping between the pier and the beam? It seems like that might be an issue with flat topped piers.
I'm not exactly clear on how the steel leaves the top of the pier, spaced, (I'm assuming) 12" apart for the 18" piers and narrows down to 2" apart, (accounting for 3" clearance to outside of concrete), within an 18" distance, (distance from last stirrup to top of pier). Are they just bent and tied up in a bundle, (almost)? I was thinking it would almost be easier to form the grade beam before pouring the piers, then muscle the rebar into place. I bet doing the alignment on all that is a pain without real reference points. Is what I'm saying making sense?
Tom
I did a couple of those in California. On hillsides you have a machine come in and drill the holes. Then you have a crane drop in rebar cages. You have the cages built off site. It's cheaper and faster. I'm not sure who does it as I was just a carpenter at the time.
After the piers/caissons sp?/friction piles are poured you build rebar cages pier to pier with forms around them.
The grade beams really aren't designed to carry vertical loads. They are there to tie all the friction piles together into a unit. The piles go down into soil/rock that is stable but the tops are in soil that is unstable. The grade beams also stabilize that top layer of soil.
Needless to say, the grade beams weren't 8". Much larger.
Maybe your grade beam needs to be bigger just to get the steel in it?
Who designed it so that an 18" pier with steel in it is going into an 8" grade beam?
How many have they done?
Everything was engineered.
Edited 2/10/2008 12:07 am ET by popawheelie
I'm remodeling a house right now that has 8" thick grade beams. I can't tell how they ran the steel though. Having thicker beams would certainly make life easier, although I'd prefer not to spend a whole lot more on concrete than was necessary. The engineer I'm using first brought to mind how much of a pain it would be to go from 18" to 8" and after thinking about it I had to agree. He's 75 years old so I'm sure he's been involved in designing quite a few foundations. It's just that he's not the guy who has to make the steel work so I'm wondering on here how people deal with it.
I think I'll make the cages on site and drop them in myself. I only have to go eight feet deep and I've done that before with just plain piers. It wasn't too difficult.
sounds like you are about to accept a lot of liability.
the drawings i have sitting on my desk right now describe something similar with a 9' wide spread footing with a mix of 18"x18" and 30"x30" concrete piers bearing on the footing.
the engineer clearly detailed in several pages of drawings all of the rebar hook connections between the spread footings and the columns.
your guy really should do the same for you.
carpenter in transition
I agree. You shouldn't be making assumptions. Get engineered drawings with all the details. They will clarify a lot of your concerns. I have never done this type of foundation without them. Where are you located?John
I must not be communicating very clearly, lol! I'm just wondering how the process goes in advance of doing it. My engineer hasn't drawn anything yet. I just gave him my preliminary drawings last week, lining out what I had in mind for a foundation. The soils engineer has specified 18" piers and I assumed I'd go with an 8" grade beam, (which I've seen done before). The grade beam basically serves as a stem wall. (No piers on top of it, just wood and steel framing from that point on). All I'm really curious about is how the steel does the transition into the grade beam, since there's the spacing issue. The guys who did the foundation on the house I'm currently remodelling made it happen; unfortunately everything is encased in concrete so I can't tell exactly how, heh, heh.
However, thinking about several of the descriptions of the sequence described by others in this thread has led me to a pretty solid idea as to how I'll get it done.
Thanks,
Tom
One thing I thought of. The steel in the pier is bent 90* and tied to the steel in the grade beam. If the steel isn't bent in a real tight bend it might get closed to the edge of the concrete at the inside corner. This is at the bottom of the grade beam so you ease the corner right there so you get enough concrete around the rebar. It would make the area stronger if anything and it won't show. I have seen so much steel in concrete that it seemed bunched up. Ideally it should have space between the bars but we all know that sometimes it just doesn't happen that way.
I did some grade beams for a contractor/builder and the home owner was involved somehow. He might have been an engineer or a close friend of one because the amount of steel we put in them was nuts. We didn't mind so much but he kept adding more after the cages were already in the hole. None of us could fit except one little Mexican guy. I had to keep sending him into the steel cages because this guy couldn't make up his mind. He would come out bleeding. The contractor/architect/builder wouldn't stand up to the guy.
Geez, that sounds ridiculous. At some point adding steel will weaken the thing. I believe the concrete functions as glue.
There's supposed to be minimum of 3" of clearance between any steel and the exterior surface of the concrete, at all points. That leaves a 2" window in the center of an 8" beam where all the steel is supposed to go. If you think about it, you can understand why I'm questioning how this would properly happen, especially considering that the stirrups on 18" piers are 12" on the diagnol. And of course, the vertical steel in the piers are at the outermost corners of the stirrups. So, something's got to give. I'm sure once I get physically working on it the whole process will become clear. I'm just trying to think ahead, as is my habit.
""There's supposed to be minimum of 3" of clearance between any steel and the exterior surface of the concrete, at all points."" Where did you get that information from? No code that I am aware of says that . IIRC the distance to the surface of the concrete is not that large. I have poured engineered walls 8" thick that had two 5/8" mats of bar placed within the wall. Minimum distance to outside of concrete was 1"
Edited 2/11/2008 12:37 am by dovetail97128
I have had a lot of expierance in concrete construction. You are correct about rebar coverage. The max is 2" min is 1" no more no less. The bridge I am currently building is for the feds', and they are VERY particular about coverage 1" and they really hold us to it.
Well what struck me was that if you only had an 8" depth and needed 3" on both top and bottom that left the bar dead center in the grade beam. In that location one might as well not even put it in because it's only effect is to hold the concrete together . It is contributing almost nothing to the strength of the beam. It is acting neither in tension , nor compression.
They can't get your Goat if you don't tell them where it is hidden.
Very true, I usually don't respond to posts' like the original one, despite my experience in concrete structures I am still just a carpenter. These questions should be left to be answered by engineers.
Me too, just a dumb carp. But I like to ask engineers questions. Helps make me just a bit less dumb some days.
They can't get your Goat if you don't tell them where it is hidden.
Better yet, when an engineer asks a dumb carpenter a question. Hey I just drew this up but how would you build it. I deal with lots and lots of them. They drive me crazy somedays.
Yea, know what you mean. I had a project once, custom house. There was a post that had a number of beams run into it at various levels. I called the archy up and asked for details on the connections. He told me he would get back to me within the day . Called him late in the day and asked what the answer was. "I don't know , you figure it out and then I will draw it, I saw to it you got hired for things like this."
They can't get your Goat if you don't tell them where it is hidden.
Typical, but that's sometimes how things get done it's a collective effort. Although archys' don't like to share share the glory. Man I do miss building houses, maybe I will go back to it after we finish this bridge.
The grade beam itself is 8" thick, not 8" deep. I'm sure the engineer will specify something at least 18" in depth.
The grade beam should mostly be above grade like a stem wall. It is not completely buried, maybe 6 - 8" depending upon engineer or soil engineer. 2" clearance should be correct above grade. Make your stirrups to those specs and cram the steel in. Sometimes you have to bundle some of the pieces to get clearance. It is a lot easier to pour 3/8 rather than 3/4 rock to get concrete in, even with the vibrator.John3" clearance is at the bottom to grade. 2" on vertical surfaces. Most retaining wall specs show steel 2" from the earth side of the wall.Best of luck with your project.
Edited 2/11/2008 9:46 pm ET by JohnCujie
The issue I've been trying to understand is what the connection will look like at the pier/grade beam cold joint. That area will definitely be buried. I had an inspection today on another project and ran all this by the building inspector. He laid out several methods engineers have for dealing with the issue I've raised so I think I've got it covered, depending on what my engineer comes up with. I appreciate all the input though.
Tom
""The grade beam itself is 8" thick, not 8" deep. I'm sure the engineer will specify something at least 18" in depth."' Aha, I use "Thick" or "Depth " to define the vertical dimension and I use "Wide" to define the horizontal dimension. Helps when we are using the understandable terms in discussions. If your BI is calling for 3" then that is that . I don't understand it because no footing I have ever poured was required to have that clearance.
Footings are in constant contact with the ground so it must be a local issue . Maybe salt in the soils?
They can't get your Goat if you don't tell them where it is hidden.
Perhaps I should have been a little more specific. A 3" clearance between the steel and the exterior surface of the concrete is required when the concrete is to be permanently surrounded and in contact with earth, which is the case with the grade beam situation I'm referring to. Bridges are a different deal. That's why I'm curious as to how the steel connections are handled in the transition from pier to grade beam.
Your requirements may be different I don't know. Bridges are no different, footings are still under ground. A bridge ftg is the same princapal as a grade beam, just bigger. First we drive piling to bed rock, then we form up the cube. Its not the soil that takes the load, it's the piling. The ftg just spreads the weight onto the piles. Coverage is still 1"-2" even though its buried. A deck on the other hand is 2"-2 1/4" because of wear caused from traffic. I have never heard of 3" cover even in high rise towers.
Anything that's buried requires 3" cover, of that I'm sure. Don't know if it's different in your locale but I'm building in the San Francisco bay area.
I spoke with a building inspector today and he lined out several ways engineers spec the situation I've got. It's pretty straightforward.
OK, Not going to argue with that. Like I said I don't know your local codes.
Check out this image I found via Google images
http://images.google.com/imgres?imgurl=http://searchwarp.com/UserImages/Author-67279-img(26).jpg&imgrefurl=http://searchwarp.com/swa210615.htm&h=479&w=529&sz=79&hl=en&start=4&tbnid=aqAxtlR30vTclM:&tbnh=120&tbnw=132&prev=/images%3Fq%3Dpier%2Band%2Bgrade%2Bbeam%2Bfoundation%26gbv%3D2%26hl%3Den%26sa%3DG
It might help. I guess it really depends on how much rebar the engineer specs, more rebar, obviously more difficult to make fit.
Hey thanks dockelly. I'll have a look at that.
no problem, if that doesn't help, go to google images and type in variations of what you think your looking for and something might make it all clear. As they say, "a picture is worth a thousand beers, I mean words"
I don't see the problem.
You have an 18" pier (either sq. or round) and sitting on that you have the 8" thick by ? wide grade beam.
Take a say 5' length of bar. Bend the bar at 2'6' at 90 deg. At the top of the pier block the bar up to match the grade beam bar in elevation, tie it off to the vertical bar in the pier, tie it off the grade beam bar.
Or prefigure the elevations and tie it up before hand and drop the assembly into the hole.
Best to use either pre-bent bar or a bar bender as opposed to a hicky as you need a tight radius in the bend.
Grade beams I have done had the beam bar run continuous over the top of the pier and the pier bar tied to the grade bar.
Edited 2/10/2008 8:53 pm by dovetail97128