Electrical Conduit in Post Tensioned Slab

[DesignIntegrity]

Regarding electrical conduit in Post-Tensioned Slabs. ACI 318 section 6.3.5 addresses conduit embeded in concrete. However, I am looking for guidelines/provisions for clearance and/or tolerance between electrical conduit and PT anchor zones or PT cables in general.

I am familiar with the common detail for vertical penetrations near tendon anchorage zones (shedule 40 pipe when vertical pentration is in the anchorage zone, etc.).

However, this condition is when an eletrical conduit runs perpendicular or parallel to the anchorage zone or to the tendons in the plane of the slab.

I have a condition where a contractor has installed a number of electrical conduits running through our PT anchor zones in both primary directions. We have requested that the contractor avoid the anchor zone, but we have no reference/provision for clearance between the anchor zones and conduits or between parallel tendons and conduits in general.

I certianly would appreciate any assistance regarding the matter.

DesignIntegrity

 

[VoyageofDiscovery]

Hi DesignIntegrity,

I don't know of any references, however have you designed for these voids or transverse loss of section?

A Contractor must not be placing conduits haphazardly throughout your structure. At any rate, any voids are better placed as close to the neutral axis as possible and be accounted for in the design. Transverse section loss which I presume are close to both top and bottom surfaces reduce section properties at those locations and must be accounted for.

HTH

VOD

 

[MikeT14]

DesignIntegrity,

From the PTI Desighn, Construction, and Maintenance of CIP Parking Structures:

"Sleeves in the Anchorage Zone: The area behind the anchorage devices (18 in. behind the achorage device at 45 degree angles on each side . . .) is a region of high stress. Sleeves, block outs, LARGE CONDUITS or any other voids create areas of weakness which can allow the concrete behind the anchorage to crush during thestressing operation. If penetrations within this region are necessary, the Engineer will usually require steel pipe inserts."

The accompaning figure dictates the schedule 40 pipe you mentioned. I haven't run across any details for conduit specifically, but generally anything in the above anchorage zone could lead to blowouts or anchorage depressions during stressing. I'd imagine that the conduits could crush during stressing if they are close enough to the anchors, especially those running perpendicular to the anchorage in the plane of the slab. So far, everything I've read leads me to believe that any penetrations in the 18" cone require pipe.

I'd be wary of any conctractor that would continue to install conduit after you expressed concern as to it's location relative to the anchorages. Even though it's not a "penetration" in the same vein as above, you're effectively reducing the area over which the stressing force can spread out (especially for conduit running parallel to the tendons) and could get significant local stress increases.