Conduit in Beams

[pmblair]

My contractor has placed 4" electrical conduit running in my grade beams in the same directions as span for a decent length at 2 locations.

At 1 location it is about 2" above the bottom steel. At the other it actually rests on the bottom steel. I am having them move the pipe higher up in the beams.

My question is is there a ACI reference on how much concrete i need above the bottom bars to transfer the load into the bars?

I am thinking 2"-3" but not sure how i would calculate or if there is a code reference.

Thanks

 

[BAretired]

Don't know if there is a code reference or not, but I would ask him to provide a clearance of at least the diameter of the conduit.

BA

 

[JedClampett]

I think you want to assure that the reinforcing is bonded. And the code doesn't give any specific guidance on spacing between the bars and the conduit. There’s a whole bunch of other requirements in ACI 318, Section 6.3. Since the code says the conduits must be spaced three diameters center to center, that seems to make sense. The center of the conduit would be 12 inches above the center of the bar.

 

[BAretired]

Three diameter c/c would be two diameters clear. I will revise my recommendation from one diameter to two to be consistent with Jed's post.

BA

 

[pmblair]

Thanks Gentleman.

I am familiar with the ACI 318 Section 6 requirements.

But i feel that applies conduit to conduit, not conduit to rebar.

The reason i am making the distinction is, what if we were talking about a 12" x 24" deep concrete beam, and we blocked out a large section in center of the beam for a penetration. If we provided say 1 1/2" cover each side of the stirrups, made sure the penetration wasnt in the compression block, and there was adequate concrete for horizontal shear transfer, i dont think we could come up with a reason why not.

I guess the question i am asking is how much concrete is required to fully bond and transfer the load into the rebar?

How would i calculate that?

Thanks you again, i value your opinions and help.

 

[JAE]

Well, a corner bar in the bottom of a beam has 1 1/2" clear concrete, add the stirrup diameter and a bit more for the stirrup bend and you have a set amount of concrete that all bottom rebar in concrete beams typically have on their edges, even without interior conduit. That at least offers a rough number to go by I would think.

 

[BAretired]

In a situation like that, it is usually best to ask for a bit more than you need in the expectation that you will probably end up with less than you asked for.

BA

 

[msquared48]

I trust the conduit is PVC and not Aluminum?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[pmblair]

Thanks Everyone.

Yes it is PVC.

I think the key to calculating it is the development length. The requirement for development length is to assure bond.

I am looking through ACI 408, i think i may find something in there.

http://www.google.com/url?sa=t&rct=...YRstugDIMbFu7xh1MFFJHAw&bvm=bv.42080656,d.eWU

 

[pmblair]

I got it! It's 2.56" for #8 in 4000 psi .

Thanks

Again

 

[BAretired]

And how did you come to that conclusion?


BA

 

[a7x1984]

BA: Magic, obviously.

If I got paid for every hour I worked, I'd be a wealthy man.

 

[pmblair]

The method i used may be way to conservitive, but its at least a number i can calc, so i feel as a minimum i can live with it.

I used the formula for maximum bond stress in Aci 408, which came from Aci 318-63.

Then set that equal to the average bond stress at failure equation by Orangun, Jirsa, and Breen in ACI 408.

Rearanged the queation to solve for Cmin, which is "smaller of minimum concrete cover or 1/2 of
the clear spacing between bars". When rearanged there is a ld in the equation that only reduces Cmin as it goes up, so i took it as 0.

I know this isnt exact or meant to be used in this manner. But it was the best i could come up with on short notice.

If we had 6" slab with #5s in each direction in the center, i dont think we would even question if we had enough concrete for bond.