azcats
Structural
- Oct 17, 1999
- 693
I'm looking for guidance (code supported or otherwise) on the placement of conduit for drilled pier foundations or pedestals atop mat foundations.
Structures are telecom towers typically with large diameter anchor bolts embedded into the pier footing (sometimes concrete pedestal atop a mat foundation) with enough depth to develop the vertical reinforcing in the concrete above the bolt anchorage. The towers usually need (2) to (6) 4" PVC conduits that enter the pier from the side below grade then bend upward and exit thru the top center of the concrete element at grade and into the bottom of the tower.
The current pier I'm reviewing is 78" diameter with (26) #9 verts and ties at 6" centers. 2 1/4" anchor bolts are embedded about 66" on a 55.5" bolt circle. The conduits usually enter the side at about 3' below grade - so right in the middle of the bar development area.
I've reviewed a lot of threads on similar subjects and am aware of the 4% number for columns in older versions of ACI 318 (6.3.4), but I don't see that in the 318-14 and this really isn't a column. Even with (6) conduit on this pier, I'm well below 4%. Do any limits like this exist in the current code?
In addition, how does a conduit in proximity to a bar being developed impact that development? If I maintain at least one bar diameter clear from bar to conduit, does that satisfy the development of 25.4.4.2 for the top line formulas in the table?
My instinct says to let them stack (2) 4" conduits vertically, maintaining at least 1 bar diameter cover from all reinforcing and anchor bolts. Additional pairs can be inserted at least two bar spaces away around the pier. I'm really not sure on a maximum number yet.
Thoughts welcome.
Structures are telecom towers typically with large diameter anchor bolts embedded into the pier footing (sometimes concrete pedestal atop a mat foundation) with enough depth to develop the vertical reinforcing in the concrete above the bolt anchorage. The towers usually need (2) to (6) 4" PVC conduits that enter the pier from the side below grade then bend upward and exit thru the top center of the concrete element at grade and into the bottom of the tower.
The current pier I'm reviewing is 78" diameter with (26) #9 verts and ties at 6" centers. 2 1/4" anchor bolts are embedded about 66" on a 55.5" bolt circle. The conduits usually enter the side at about 3' below grade - so right in the middle of the bar development area.
I've reviewed a lot of threads on similar subjects and am aware of the 4% number for columns in older versions of ACI 318 (6.3.4), but I don't see that in the 318-14 and this really isn't a column. Even with (6) conduit on this pier, I'm well below 4%. Do any limits like this exist in the current code?
In addition, how does a conduit in proximity to a bar being developed impact that development? If I maintain at least one bar diameter clear from bar to conduit, does that satisfy the development of 25.4.4.2 for the top line formulas in the table?
My instinct says to let them stack (2) 4" conduits vertically, maintaining at least 1 bar diameter cover from all reinforcing and anchor bolts. Additional pairs can be inserted at least two bar spaces away around the pier. I'm really not sure on a maximum number yet.
Thoughts welcome.
