Rebar Development

[bookowski]

This should be an easy one that comes up a lot but I can't find a good answer.

How do you deal with rebar that has very low demand and also a very small embedment length. In my case I need to leaves dowels up for a cantilevered CMU wall on a slab on metal deck. The tension forces in the rebar will be small, about 1k unfactored, but the embedment depth is obviously limited.

ACI sets a minimum Ldh for hooks at 6in., this wouldn't be possible even if I was sure the bars would line up with a flute which most likely they won't. So that rules out calculating Ldh. It also doesn't seem reasonable to use the Asprovided/Asrequired provision on Ld to such an extreme degree.

Chapter D is only for anchors, they direct you elsewhere for pullout of rebar (at least that's how I'm reading it). Intuitively I can't imagine that 1k is going to pull out a #4 hooked 3 or 4 in. into a slab but it would be nice to be sure.

 

[ztengguy]

can you embed a insert that gives you the tension capacity that you need, and thread in a rebar or hook, etc.

 

[JAE]

You have to meet the minimum values given - i.e. your 6" for hooks. The variation of concrete - its cracked vs. uncracked state, etc. prompts ACI to prescribe these minimums.

ztengguy has the right idea.

 

[bookowski]

I know the min. is 6" but that doesn't seem realistic unless no one is embedding rebar dowels on slab on deck jobs. I've never seen anyone using embeds for simple dowels up - say for every parapet on steel job. This seems overly conservative.

 

[PMR06]

Is this CMU wall over a steel beam? If so you could weld the bars to develop (A706 of course).

 

[JAE]

OK - why not 1" embedment? Where do you draw the line?
I think ACI just used 6" as a limit. I'm not sure they actually have testing/research where the variability of development strength goes haywire under 6" but they might.
We typically would weld the vertical dowels (or deformed bar anchors) to the supporting steel in cases like yours.

 

[bookowski]

Clearly 1" doesn't make sense. I would hope that as engineers we have the ability to make some judgements beyond simply reciting the code requirements. Are you also saying that 5.75" wouldn't work but 6" would? Seems too simple of an approach in my view to view the code so rigidly, to each his own though.

What if the cmu doesn't line with supporting steel? Just wondering if anyone had a rational approach that they have used for say 4" of embedment.

 

[PUEngineer]

I've skimped on the 6" embedment before. I think I used 5" on a job recently. It was a little different in that the curb was concrete, so I could dictate that they were placed in the flutes. Is epoxying a straight bar into the slab an option?

 

[dhengr]

Why not just weld #4's, or whatever, at spacing multiples of the CMU core spacing, to a piece of .25" x 1" bar stock. Stand this on top of the steel decking wherever you want, and screw it down. The flat bar should give you a few extra inches of effective development length, and actually help you space the vert. rebars in a straight line, where they should be.

 

[bookowski]

dhengr - I like the idea. The fact that it lays out the bar spacing at the same time is a nice freebie.

PU - I could epoxy, was thinking of that option. It just seems strange to epoxy a bar because I don't know what the capacity is if it's cast in.

More than trying to fix this one situation I was just wondering what people are typically specifying for dowels out of a slab on deck. Our typical detail just shows hooked dowels from the slab, I've thrown this on loads of steel jobs and only now decided to check it. My gut feeling is that a hooked bar in 4" or or 5" of concrete would have quite a bit of capacity.

 

[hokie66]

Use dhengr's solution. As for glueing the bars in, I never did understand why some think that is better than cast in bars of the same length. Sales pitch, I reckon.

 

[shobroco]

Depending on glue is especially wacky if you've seen anyone do it. There is great bond between the glue & the dust in the hole; not so much between the dust & the sides of the hole.

 

[DST148]

@bookowski - Getting 6" Ldh into concrete is difficult not only for masonry parapets on slab on metal deck but also in case of flat plates 7" or less in thickness, due to the presence of continuous band of reinforcing around the slab perimeter.
We are not in a high seismic / wind zone. This is what we typically do in case of masonry parapets not more than 3'-6" in height and bearing on slab on metal deck.
Min 8" thick. fully grouted parapet, minimum embedment of 8 times the diameter for the hooked dowels, restrict flexural tensile stress to less than 50% of code allowables for unreinforced masonry and a reinforced bond beam at the top of parapet. The varying spacing of reabrs is acceptable to suit masonry cores and the deck flutes.
As per ACI 530-05, the design of anchor bolts is based on physical testing. The min embedment requirement is considered a practical minimum based on typical masonry construction. Although not directly applicable, it may be used to get an idea of the tension that can be developed by a hooked rebar.