Jimmy Hoffa, perhaps? – – a friend is building a shop off a hillside – 32′ deep by 42′ wide – stepped footers are poured, the plan being to lay 12″ concrete blocks up to 32″ above grade (rebarred and the webs filled with concrete) – the back wall will have 6′ of block containing the fill on which the slab will be poured. The block man has recommended three ‘deadmen’ to help keep the wall from buckling sometime in the future –
the current thought is to auger 16″ holes underneath the future slab, fill them with concrete with 5/8 rebar sticking out. This rebar will then be bent and welded to rebar protruding from the block wall. This assembly in turn will disappear within the fill that will support the slab –
not very elegant – anybody got any suggestions on a method to secure this wall against movement?
thanks, DOUD
Replies
David, as far as I can tell/read, you just did, ie suggest the method. Jim J, ps, all depends on how much an individual wants to gamble, and who's money is involved. For there are structural engineers, as I'am sure you are aware of. good Luck
you just did, ie suggest the method
ya, but there have to be other techniques/strategies - - my personal opinion is that a rebarred 12" block wall with webs filled is not going to buckle - I don't think there is a structural engineer in the county, but admittedly good advice - thanks
Back in the 70's, if my memory serves me right , on the taller garage stems, we would do something very similiar to what you are suggesting,, except we would incorporate all 4 walls, all coming together to the Deadman. It was all tied together at the time of grouting. So, I'd suggest 4000lb grout mix, and generous bond beam courses, again David , Good Luck, and whatever you do, I'd let that wall cure, before backfilling. Jim J
Why not bend a bar from the top of the wall into the slab? Or, as 44 suggested, hire a structural engineer.
...that's not a mistake, it's rustic
Why not bend a bar from the top of the wall into the slab?
a good idea, I'm sure that will happen - -I think the concern is that this 6' of block wall containing the fill will buckle outward along a horizontal line a couple of feet up - - I've seen several block retaining walls fail similarly, but never one that was rebarred and the webs filled with concrete - we are looking for elegant overkill here - thanks - -
David -
This isn't part of your question but in my view, I'd be as concerned about putting the slab on that much fill unless it's an engineered fill of good granular material compacted to optimum density. That, too, will stress your block wall. Horizontal rebar along with the verticals grounted in the cells will be important. I would strongly urge you and your friend to consult a structural engineer. You're dealing with some pretty scary stuff here!
Dennis in Bellevue WA
[email protected]
engineered fill of good granular material compacted to optimum density.
agreed: the pros are up to it -
You're dealing with some pretty scary stuff here!
oh come on, :-) this isn't a trailer in a tornado - -risk of catastropic failure is very low, - just looking for elegant methods of overbuilding - when Kraig talked to me about it, the first thing I flashed on was the buttresses that you see associated with masonary walls, in compression rather than tension like the suggested detail -
it'd be nice to have an engineer, I'll agree - - wish there was someone local -
thanks - -
OK, David, so "scary" was a bit too,... emotional (hehe) - my intent was that I sleep more soundly when I have good assurance that 1) my concrete walls aren't going to crack {I didn't say fail and come crashing down} and 2)my slabs don't become challenges for skateboard clubs!
Simply grouting the vertical cores of the CMU won't do much good unless they (the cores) are reinforced. Which you know of course. Foundations taller than this have been and are built on a regular basis. But we don't know each other, you and me. I don't know what your experience is, probably greater than mine, but I don't know that. The question posed, in its simplest terms, would a CMU wall be OK for supporting six feet of backfill. That would be like me asking you if I could get away with building my lumber storage racks out of 1" bar stock angle welded with a little 110 wire feed welder. You'd have to make a few assumptions about my talent and experience welding.
As the discussion grows, you obviously aren't a shade tree mason and I mean/meant no disrespect. Indeed, the block wall will do fine with the addition of rebar, both horizontal and vertical. Concrete, either in the block cores or in the form of the dry-pressed CMU (I'd suggest using Concrete Masonry Units rather than the lighter pumice blocks) neither of them have much if any strength in tension (which I'm sure you know) The rebar supplies this tensile strength. Which you probably know as well. So - just envision where the wall will be mostly in tension and place your reinforcing as far as practical from the face of the wall that's subject to the force causing the tension while at the same time providing adequate cover to prevent the bar from being subject to moisture (rain or ground water) thus rust and consequent spalling of the block or concrete/mortar cover.
I still think the biggest concern will be settlement of the fill and the threat of the slab cracking up. Don't be afraid of the overkill approach to reinforcing the slab, either. #4 @ 12" both ways minimum would be my suggestion. It still may settle on you but at least it will stay in one piece.
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Dennis in Bellevue WA
[email protected]
> ....the first thing I flashed on was the buttresses that you see associated with masonary walls, in compression rather than
tension like the suggested detail -
Absolutely .... I agree - you're further ahead using any kind of masonry in compression than in tension. When you can. The buttresses should be on the outside of the wall if the fill is on the inside. If I'm seeing the picture right.
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Dennis in Bellevue WA
[email protected]
thanks Dennis, - you have itemized the conerns exactly - if you were to assume much experience on my part you would be wrong - I try to observe and learn - this forum is such a resource! - and the comraderie of people who actually 'do' - - an engineer would be nice, but the question I have is about the technique he would use beyond the rebarred/concrete filled webs to strengthen the wall - I suspect that the strategy would involve getting some right angles off the wall, but don't know the specifics of how to accomplish it -
I'm going to try to get over to the site and get a couple of pictures - I haven't seen it yet, and I'll ask Kraig tomorrow (today) about the fill/compaction plans - I agree with your concern about settling, it would be a shame, this is gonna be a nice shop -
David, back in the mid eighties I worked for an outfit that built Winn Dixie groceries stores. One in particular I remember had a 180' CMU wall 22' tall with 4' of compacted fill inside. If mempry serves me correctly, the buttress' were 20' oc. The vertical steel was no. 5 rebar 16"oc with fullf grouted cells to 4' above the footing where the first bond beam was place. At slab level there were angle iron clips welded to rebar bent and tied into the bond beam steel. The angle iron was drilled to have no. 5 bar bent and inserted through the it. This bar was centered in the interior slab pour and tied to the 6/6 10/10 wire mesh. I don't remember the centers on the slab steel, but was most likely 2' both ways to at least 6' out from the wall.
That particular job had the building pad constructed and compacted before we dug for the footings. The pad was oversized and the footings dug in the compacted area. After the footings were poured the outside compacted fill was removed to finish grade and to facilitate the wall construction. The interior fill was roughly 4' deep and 2' wide and compacted in 6-8" lifts with DGA. There was never a problem with wall buckleing then or years later.
Your fall away lot presents a similar challenge, and I would think a similar solution. You may want to consider building the pad and compacting it before you dig footing and lay up the wall. This would reduce the amount of loose fill you need to add inside , after the wall is built.
Dave
another thing, you cannot weld rebar
"another thing, you cannot weld rebar"
Why is that? Never heard that before...........Women who seek to be equal with men lack ambition.
As a metal it can be welded, as a structral device for concrete, it cannot be welded due to it will weaken the srtuctral value of the rebar. Since rebar is in tension this will make the bar brittle. These are stated in the ACI manuals and the steel book.
It has always been a no-no in heavy constuction to weld rebar
Thanks to all of you for your replies - I'll get back with a report and some pictures as soon as possible - title maybe "farm boys build wall" or something - - hey y'all have a good, safe day - DOUD
David -
I'll go out on a limb here since I don't have any information regarding the type or quality of your backfill (under slab) material and so forth, but ....
Re-reading some of the suggestions I see where the term 'deadman' is coming from. Or at least I think I do. That plus the 'grade beam' idea presents the picture of a beam, at slab level, under the slab running from the back to the front of the shop. I think that:
1- provide at least #4 footing wall dowels into the block at 16"oc max. Dowels should be at least 2' out of the concrete footing. Layout is kinda important since these have to fit into the cells of the block. These need to be bent at 90 degrees at the bottom and the bends alternating in direction being perpendicular to the length of the footing. Make sure the ends have at least 1" clearance from the inside of the footing form.
2- Provide (2) #3 bars horizontally at 16"oc. #3 since the block joints are typically 3/8". My personal opinion of the truss-type horizontal masonry reinforcing is ... well, kinda OK but prefer rebar.
3- Provide #4 vertical reinforcing at each footing wall dowel cell. Grout solid with peag gravel aggregate grout ... not the mortar mix.
4- at the top of the wall or at the place where the slab intersects the wall, provide #4 slab dowels extending at least 12" into the (grouted) block cells and at least 16" out into the slab ('L' shaped bent bars). Theree should be at least 15 bar diameter laps between these dowels and the slab reinforcing. Use these kinds of dowels both at the front wall and the back wall of the slab.
5- tie the slab reinforcing to these dowels front and back. Now you have, quite literally, a diaphragm, the slab, that ties the entire length of the two walls, front and back, together.
If you really want to go for 'elegant' overkill, provide at least one bond beam in the back wall at mid ht. between the footing and the slab. Reinforce this with a couple continuous #4. Continuous means laps are at least 15 bar diameters. Don't bother welding any of this stuff as long as you get sufficient lap lenghts.
Standard disclaimer .... The preceding advice is offered solely as a suggestion to be reviewed by a competent engineering entity familiar with local design requirements and jobsite soil conditions. It's worth more or less what you paid for it (grin).
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Dennis in Bellevue WA
[email protected]
"...it cannot be welded due to it will weaken the srtuctral value of the rebar."
If that's the case, does it mean rebar is hardened? I always thought it was just mild steel..........
Not trying to argue the point, just curious.A really hard laugh is like sex—one of the ultimate diversions of existence. [Jerry Seinfeld]
Its what the book says
Rebar is welded all of the time. These are reinforced with angles. Standard practice for bondingrebar mats in tunnelsa and other underground structures. Most DOT's have standard details for it.
Field bending grade 60 rebar is also done all of the time. But that assumes proper equipment. The only "no field bending" requirement is for epoxy-coated bars.
...that's not a mistake, it's rustic
Edited 8/7/2003 12:01:14 AM ET by Bungalow Jeff
Edited 8/7/2003 12:02:35 AM ET by Bungalow Jeff
i had a job a million yrs ago doing just that.the company i worked for built the concrete barriers for road constructin,3 pieces of #5 welded to hooks on either end so they could all lock together
Edited 8/6/2003 9:28:38 AM ET by bud
> ...another thing, you cannot weld rebar
Of course you can. Most rebar spec'd in commercial use today (around my neck o' the woods) is grade 60. No, this shouldn't be welded or field bent. But using grade 40 it can be both bent and welded. But there's really no need to weld rebar in the first place since lapping at least 15 bar diameters provides enough development to qualify as continuous length in most cases.
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Dennis in Bellevue WA
[email protected]
Most rebar spec'd in commercial use today (around my neck o' the woods) is grade 60. No, this shouldn't be welded or field bent
I give up. All I use is grade 60. No reason to weld it for concrete use, but we bend it all the time. #6 gets to be a challenge. No hydraulic benders around here. Why not field bend it?PAHS Designer/Builder- Bury it!
Tom -
I can't say with absolute certainty, but from what I recall what engineers have told me is that grade 60 tensile strength is such that most manual bending machines aren't set up for the proper radius bend of bars over a certain diameter. Thus it's kinda like (I guess) bending a piece of pipe - down to a certain radius it's OK but beyond that it's likely to crack the bar. That's only from memory so please don't quote me ... yet. We're in the process of moving and all my reference material is packed and either here getting loaded for the move or already up at the other place.
Where the "no field bending" thing comes into play mostly is with slab dowels on the face of a wall. Lots of contractors like to use bend-out bars but they're a no-no with grade 60. Mainly, in this case, because that would be the second bend in the bar at the same place. In fact, that may well be what I'm remembering - that grade 60 bars should only be bent once at any one location.
Grade 60 can also be field welded provided it's stress relieved to retain it's tensile strength. But since field welding processes are far less than ideal, it's easier just to spec "no welding" for it in the first place............
Dennis in Bellevue WA
[email protected]
Dennis: Thus it's kinda like (I guess) bending a piece of pipe - down to a certain radius it's OK but beyond that it's likely to crack the bar.
That will happen. Our approach is to be careful what radius we're trying for, rarely very tight. Using our normal hub cutter/bender we can crack #4 (grade 60) but for those tight column surrounds #3, that the engineer spec'ed, works fine. When we get to #6 it's usually with a loader bucket and a generous radius. OSHA what?
Our only welding is anchor J's to flat plates used for welding the bar joists down. With 300 psf total load (300 tons) we don't worry much about uplift.
Common sense does need to enter into it. Cracking at the bend clearly would be a defect.
Thanks. PAHS Designer/Builder- Bury it!
I was always told, and I practice it today, Lap = 30 times the diameter of rebar used. Jim J. Heads up around that rebar, but you all know that ,
I ask the engineers at work about welding rebar. They replied that ACI and AWS allowed welding but ACI is only on #6 bar and bigger and AWS on #7 bar and bigger. The reason was that 30 times diameter of these big bars would be alot of waisted bar due to weight, But the bars can only be in a compression footing and that the weld must meet 125 % of yeild of the bar do if a footing was design with miniimum steel, more steel or bigger bars would have to be added to increase yield. And then comes the subject of corrision and testing for the 125 % yield. So although its legal most designer just say no welding or bending of bars in field. The correct term is "In Field"
Around here we just do not do it, not many people have welder on site
ACI 318 AWS/ANSI D1.4
Edited 8/7/2003 8:38:20 AM ET by BROWNBAGG
All the rebar I've ever handled, has been #3,#4,#5, once in a blue moon, will I ever get around #6. Most all the work I do, is around residential construction, it does, kindof, makes some sense, welding the larger bars. Brownbagg, oh, how I enjoy these boards. Be safe out there, Jim J
Reinforced pilasters tied into the reinforced wall would be a way to go. But if you are interested in overkill................
You might also look into MSE (Mechanically Stabilized Earth), more commonly used in large projects such as the vertical retaining walls you see on the highway on overpass approaches, where it seem that just the wall is holding back tons of earth and the traffic on the roadway.
Look also at T-Wall Retaining Wall System for the idea.
Essentially, the fill is compacted in lifts around T shaped forms that are intergral to the wall and extend back into the fill, providing a friction resistance to horizontal movement. These forms are concrete with the rebar encased, preventing rust out.
i agree with the previous posters, what you are trying to do is a different approach to a structural concrete wall and grade beam to support a slab. it should be calculated out to see which method is stronger, and what it will cost. i would build the grade beam and structural concrete wall, it may be more expensive, but i could come up with a design that an engineer would sign off on, it is something i am familiar and comfortable with, so i have a good idea of how long it would take as well as cost. the new methods, while maybe cheaper(materials) and strong enough, will probably take longer if you are not familiar with the technique. the block wall will go fast enough, but the strengthening with welding of rebar(which requires a special certification not covered by a normal structural steel cert) may be time consuming. what about waterproofing to prevent deterioration of rebar that is in the fill, it should be encased in concrete. the formwork for a structural concrete wall is simple and fast. the rebar needs only to be tied as it is encased in concrete. you will use lots, but it is cheap and also fast, and concrete is very fast, splash wiggle and dash.
it should be calculated out to see which method is stronger, and what it will cost.
well, one of the considerations is that the block man owes Kraig ;-) please tell me more about the grade beam - - I share your concerns with the exposed rebar in the fill - this is an location that inspections/certification is not an issue - Kraig (electrician) is meticulous and learned early on that doing it 'right' is a lot better than 'fixing' it - and 'right' is more than doing the minimum - I think this is why he is searching for something a little more elegant than un-encased rebar in tension... thanks -
An alternative would be to lay up piers inside the 12" block (integral with the wall and on the footing) with #5 rebar vertical in them. The pier rebar can be left long (6 to 8') sticking up out of the pier. Bend it over and into a hairpin shape and embed it in the slab. This pretty normal for a poured concrete wall with unequal inside fill. Horizontal reinforcement for a block wall is done with DuraWall and placement of bond beams, but with only four courses high a bond beam at the top couse might be all you can do.
Dave
...lay up piers inside the 12" block (integral with the wall and on the footing)
this is something like came to my mind - blocks turned 90* from the wall, creating butresses every so often - I don't know enough of the detail tho - -
...with only four courses high...
well, there will be app. 4' of wall below ground level, 6' above ground to the slab level and then 4 courses to the start of the framing, the concern is the long term integrety of 10' from footer to slab on the back wall that retains the fill - -
Kraig is meeting me here tomorrow to review my current project (3 Hp of 3ph refrigeration) - any further strategies/details/something ya saw once are welcome - thanks to you all for the discussion - -
In So. Cal there are a lot of homes over the side of the hill and the approach is to use floor trusses with concrete over them. The trusses and the concrete work together to support the weight on them. Nobody wants fill.