Concrete Grade Beam Ledge

[Serhiy2]

Good day,

I'm working on a project where I have concrete grade beam (12x24) spanning between piles and supporting the wood I-joist floor structure. I was exploring the option where I would provide a 2" wide ledge in the concrete beam. One thing I'm not sure about is that whether I need to reinforce this ledge or not. My loads are about 1800 plf. Going off of section 11.5 (CSA A23.3-04) it implies that I need to have steel shear reinforcement in this situation. My shear stresses are pretty low but I'm still hesitant to proceed without shear reinforcement. Please see sketch attached - would appreciate some thoughts on this situation.

Thanks

 

[jayrod12]

How are you getting away with such a shallow gradebeam? Surely your crawlspace has to be deeper than 9.5"

 

[jayrod12]

Think about the next poor engineer or plumber that needs to crawl under there. 20" clear is my bare minimum. And I like to have my gradebeam buried on the inside at least 6" preferably closer to a foot.

 

[txeng91]

I would go 4” wide with the ledge to allow for a 2x4 will. By the numbers, you should be able to get away with no additional reinforcement as the stirrups will take the load and distribute it through the depth of the beam, but I will typically put a “c” shaped stirrup with a #4 top and bottom of the ledge bump out. Jayrod makes a good point though. Code usually requires floor framing to be treated if it is less than 18” from grade. Also, why bring the concrete up to finished floor instead of dropping it below the joists?

 

[Ron247]

I doubt you will actually need shear reinforcement at the ledge but if the code states it is needed, I would put it in. I am really curious how this crawl space is supposed to function. I see others have already expressed concerns. There may be a code requirement that sill plates be a minimum 2x4 (R404.3 of 2015 IRC). I do not know what code you must meet but I would make sure I read up on crawl space access, dampness and lumber clearances from soil before I spend much more time on this.

How far below top of Grade Beam is the exterior soil? Since you do not have a traditional strip footing with a foundation wall on it, it appears to me exterior water can run under your grade beam into the crawl space. If so, you will have a future mold, mildew, wood destroying fungus problem. I know from experience, that for the same volume of water, shallow crawl spaces (12" or less) have far more intensity of mold than much taller ones (30" or more). You have more air in taller ones to deal with the same volume of water.

If you do have more access, you can ignore the following:
Is there only 9.5+1.5= 11" of clearance? Even if you meet the treated requirement noted by txeng91, 11" clear is considered inaccessible except for a 12 year old. If there is no good access, there is no termite treatment/bond, no maintenance such as plumbing/electrical etc. I know I have crawled in at least 6,000 crawl spaces by now and what I am seeing in your sketch would make me tell a potential buyer to beware. The first time anything has to be done in the crawl space, you will pay people to hand tunnel your crawl space. That gets real expensive. Laying flat and digging is very hard to do.

Also, allowing the concrete GB to be the same level as the wood joist means the subfloor will have to run across the concrete. I would hate to not be able to secure my perimeter subfloor and also hope they align good vertically.

 

[Serhiy2]

Thanks for valid points - I see that I didn't think crawlspace detail through. I was thinking that crawlspace top elevation will start at underside of grade beam and will slope down from there at slope 1:3 or so until it hits ~4' depth. This way my crawlspace would be 21.5" at a distance of 3ft away from grade beam. I will revise my perimeter grade beams to 36" and will still have my crawlspace sloped inside at 1:3 or so.

This is a redesign from concrete slab because budget doesn't allow to do it so keeping the grade beam as shallow as possible is appreciated. Regarding the ledge, I will avoid it and just proceed with 2x4 nailer board sitting on the top of grade beam as I found top mount hangers that will work.

Grade on the exterior will be around 4~5" below the top of grade beam. Crawlspace will be lined with poly and sand. Sump pits will be installed throughout. Building is a single story wood-framed structure 160'x80'.

All I was able to find in NBCC about clearances between wood and soil is in Part 9 of NBCC (Small housing etc.) that 6" clearance shall be maintained between grade and a member.

 

[jayrod12]

Serhiy,

Check out clause 9.18.4.1 for minimum clearance where access is required.

For what it's worth, my detail would look something like this:

And to perhaps get the contractor on board, I'd be looking at a 10" wide beam on the outside with a 6" upstand and a 2x4 ledge. The interior beam can be 8"x 24". The savings in width will come close to offsetting the increase in depth.