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Old 12-22-2008, 01:02 AM   #1
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concrete grade/bond beams under slab?


This may be beyond the scope of this forum, but.... Is there a simple (ha ha) way to decide on whether or not I need grade beams under my slab, and if so, where would one go to get design dimensions? Probably an engineer, but I figgered if I could do it myself, then... The house will be slab on grade, good soil, good footers and foundation wall between footers and slab, etc, but it will be 96' x 44'. One story, ranch. Is this just beyond a DIY guy? I was guessing that a couple of 10" W x 18" H, reinforced w/ 3-#5 rebar, blocks running the length of the slab would help avoid slab cracks a great deal. Dunno, tho.

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Old 12-22-2008, 06:35 AM   #2
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concrete grade/bond beams under slab?


according to your post, you don't need them as rqd steel in the footer would be sufficient,,, IF you're concerned about the slab cracking, be assured there ain't a damn'd thing you can do - it WILL crack,,, no matter what you do, if the slab's sufficiently oversided for the characteristics of the conc mix, you'll have random cracking.

however, that's random cracking,,, by sawing ' CONTROL JOINTS ' into the slab at the proper time AND location, you'll encourage/convince/order/direct the conc to crk where it SHOULD,,, remember, timing AND location of control jnts're more important than anything else to prevent random cracking,,, even if it means staying up all nite & using a light plant

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Old 12-22-2008, 07:31 AM   #3
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concrete grade/bond beams under slab?


The way we do it is the structure's designer consults the soils report and talks to the engineer, and a design on the foundation/slab/grade beams is produced. After that it's just a matter of building what's on the drawing.

But more to your question, from experience it can go either way. It depends on the soil bearing capacity, slab design, footing design, whether the foundation crosses over a soft spot, a hard spot, fill, sand, mud, etc. 96x44 covers a big area. If you're DIY'ing your footings, slab, etc, that's fantastic. But I'm a pro, and I wouldn't guess at the design. I'd hate to get a call back when the slabs settle into some weird pattern, and bugs start crawling up through the cracks (it happens). Do it right and talk to a local engineer. It may not be needed, but it's safer to check than to guess.
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Old 12-22-2008, 11:21 AM   #4
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concrete grade/bond beams under slab?


Thanks for the replies. I better just bite the bullet and see an engineer. In the grand scheme of things, that does not appreciably increase the cost of the house, and slab problems aren't exactly easy to correct. Man, bugs crawling in through the cracks? Those are some cracks.
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Old 12-22-2008, 12:51 PM   #5
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concrete grade/bond beams under slab?


Bugs won't be an issue if you install a proper vapor barrier in accordance with the code (6 mil poly with all seams overlapped and sealed).

Always best to have an engineer take a look. Obviously you'd need a grade beam or thickened slab at locations of interior bearing walls, if any.
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Old 12-22-2008, 12:51 PM   #6
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concrete grade/bond beams under slab?


We were hired to repair a commercial building that was constructed without grade beams running in a grid - just one direction. After a few years of fork lift traffic, the place looked like a fun house. And it was a food grade facility, so the insects were a huge issue. Very expensive repair. Not fun, either. Big PIA.

An engineer shouldn't charge you that much. The phone call should be free, first of all. The way I do it is the call is free, as is the initial consult. If we both agree that there's not enough info in either of our hands, I let you know what we need to collect, and what it will take to get from that point to a design. If a soils test is required, can't really get around that unless the engineer wants to stick his neck out and guess. But it isn't that involved, except when the soil is really poor. I witnessed a test last fall where we started drilling cores with the goal of doing some soil quality and bearing tests every 4 feet, and hit a void/cavity 5 feet in depth, at a depth of 11 feet, filled with water. You never know what's under your feet until you test.
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Old 12-22-2008, 01:09 PM   #7
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concrete grade/bond beams under slab?


don't forget you'll probably have to pre-pour spray for bugs.

not to disagree w/either ag or 'mite as my post was bas'd on YOUR information ! ! ! engineers don 't like to guess as it makes their insurers grimace & raise rates,,, we contractors will guess when someone else's liable so we can shift blame the engineers/architects/owners/anyone else w/deep pockets

always wondered when, if there wasn't time to do it right the 1st time, anyone'd ever find time to do it over correctly,,, then there's the soils guy who based our site's report on the lot next door upon which he'd previously tested,,, VERY unhappy when our steel fell down as i recall
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Old 12-22-2008, 01:43 PM   #8
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concrete grade/bond beams under slab?


It's always good to hear from the folks who have been at this a while and seen some spooky things happen. Core drilling is mandatory, and will happen in a few days. Though the soil/vegetation LOOKS good, one never knows if an ice lens is hiding somewhere. Unlikely, but I've seen the results of having some uncovered and thawing, and an old boss used to throw any dead musk ox into a columnar one on the farm; she said you could hear the carcass roll till it faded away. Thanks again.

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