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Re: In-Slab Ducts in P/T Slabs

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Our office has been asked to investigate or include these mechanical ducts on at least two recent projects. (within the last 6 months) On the first project, a condo, the system was studied and the engineer and contractor dismissed the concept because of difficulties in building this system. On the second project, a hotel, they were included in the original design. However, after the contractor bid the project they were value engineered out. One reason was construction difficulties. Alot of time was spent by me reviewing the mechanical drawings to ensure that the ducts were layed out per our directions. The ducts are placed at slab-mid depth and need to remain a certain distance away from the columns so that the punching shear capacity is not reduced too much. (this distance needs to be determined by the engineer and coordinated w/ the mechanical engineer) Also the ducts need to be located properly so that they clear tendons in the perpendicular direction. The tendons will be at mid depth of the slab approximately at the 1/3 points. Therefore, the tendons can cross between a certain distance from the column (determined by you for punching shear) and a certain distance from the 1/3 point to provide adequate clearance between the tendon and the duct. The other crossing point is at mid span where the tendon will have to be pushed all the way down to clear the duct at mid depth. Therefore, even if you have a short span that does not require full drape, in a thin slab you will likely have to drop the profile or the duct cannot cross at that location. My experience was that the structural engineer needs to spend allot of time reviewing the mechanical drawings as the mechanical engineer does not understand the requirements fully.

The manufacture of these ducts is a Canadian company. The system is called Eccoduct and has been used in Canada in both mild and p/t slabs. They have some documentation, although vague, that shows that it has been used in p/t slabs and mild slabs ranging from 6.5" to 8".

If you don't have their literature, you can get ahold of them at (604) 530-4151.


Jeff Creagan wrote:

 We have been asked on several recent projects about the feasibility of introducing in-slab mechanical ducts (i.e. 2" x 12") into our flat plate post-tensioned concrete slab design.  Apparently, this is a system that is very common in Canada, where they are utilized in conventional mildly reinforced slabs.  Any opinions?