Embedded Conduits in Shear Walls and Columns

[BHHH]

Hi all,

Do any of you have experience with embedded conduits (electrical, 2" or 3" in diameter) in concrete shear walls or columns?

The contractor insists he has always built this way but I have never seen such installations. It's also not the first time I've heard of it.

Does anyone have a code or textbook reference?

 

[BHHH]

 

[SlideRuleEra]

Conduits 2" to 3" diameter... what is the wall thickness?

One thing for sure, there is clearly at least one problem:

http://www.SlideRuleEra.net

 

[BHHH]

16" thick. Yes I would recommend 4" spacing minimum.

 

[SlideRuleEra]

Vertical, properly spaced nominal 3" diameter (probably 3.5" outside diameter, actually), located (with appropriate clearance) between two rebar mats is not going to be a problem... how to prove that to you may be. Many heavily loaded electrical duct banks are not constructed with that much cover. If you want some "insurance", you could require steel conduit instead of PVC, but not really necessary.

What would be of more interest to me, is detail of how the sweep conduit elbows at the bottom of the wall make the transition into the horizontal floor runs.

http://www.SlideRuleEra.net

 

[BridgeSmith]

As SRE said, if the conduits are between the mats of reinforcing they shouldn't affect the capacity of the wall. It would have to be a rare case of very high vertical axial load and/or a heavily reinforced section to have the concrete compression zone extend deeper than the reinforcing steel. You can check it if you're concerned.

 

[BHHH]

@SlideRuleEra & @HotRod10, my concern was primarily in the shear resistance of the wall. How do the conduits affect the contribution of the concrete? Would removing the diameters of the conduits from the shear depth be a realistic approximation? Does the concrete between the conduits fully contribute to the section's ability to resist shear?

 

[slickdeals]

Refer ACI 318-08 (Section 6.3.4) for columns. It has a limit of 4% of concrete area being displaced by conduits. For walls, similar to slab, they shall not be bigger than 1/3 the thickness of the wall and not spaced closer than 3 diameters.

 

[BHHH]

@slickdeals they are considered to replace concrete in compression if they are in steel and 2" in diameter (6.3.6.2). How are they accounted for otherwise?

 

[bones206]

At a given cross section, I visualize the shear stress flowing around the conduits into the solid concrete between conduits. So I would check the solid strips for the additional tributary shear flowing around each adjacent conduit. So if there is 6" spacing between 3" conduits, I would check 6" concrete capacity against 6" + 2*(3"/2) = 9" of tributary shear force. The rebar probably helps the tie the wall together in a manner analogous to a castellated beam, but in my mind the wall is basically perforated by those voids and it should be accounted for with analysis.

 

[bones206]

You could also look at it from a net section point of view as with masonry design.

 

[slickdeals]

You calculate your capacity based on whatever area of concrete your are displacing with a maximum of 4%. Your Ag would be 96%.