Drilling through existing precast double-tees to support new ducts
Drilling through existing precast double-tees to support new ducts
(OP)
I am trying to support new ducts on existing precast double-tees. I am able to show that the additional stress on the tees from the new ducts is less than 5%, and so I do not need to provide any strengthening measures to the tees to support new ducts. However, my concern is with drilling through the tee leg to hang these new ducts.
First, correct me if I am wrong, but most tees are pretentioned and therefore the strands should have a straight profile close to the bottom of the leg. Is that a fair assumption? Do strands in precast tees ever have a tapered profile? I am thinking of specifying that drilling is to be made mid-height of leg (likely to avoid the strands) but that the contractor should confirm location of strand prior to drilling and to avoid drilling through the strands.
I am sure this is done all the time but just wanted to hear it from others. Thanks.
First, correct me if I am wrong, but most tees are pretentioned and therefore the strands should have a straight profile close to the bottom of the leg. Is that a fair assumption? Do strands in precast tees ever have a tapered profile? I am thinking of specifying that drilling is to be made mid-height of leg (likely to avoid the strands) but that the contractor should confirm location of strand prior to drilling and to avoid drilling through the strands.
I am sure this is done all the time but just wanted to hear it from others. Thanks.






RE: Drilling through existing precast double-tees to support new ducts
RE: Drilling through existing precast double-tees to support new ducts
RE: Drilling through existing precast double-tees to support new ducts
There is risk of drilling through the strands - but how else can these ducts be supported? This is in a large industrial facility where drilling would take place on at least 50 double tees.
Ultimately the contractor has to be dilligent about where to drill by performing NDT tests to determine strand locations. But I'd rather another way now.
Ron, how would you attach it to the slab?
RE: Drilling through existing precast double-tees to support new ducts
RE: Drilling through existing precast double-tees to support new ducts
How would I connect it to the flanges? I suppose mechanical expansion anchors? I'm not sure as to how thick the flanges are. But that's certainly something I can look into.
I was thinking perhaps right at the top of the stem, just below the tee flange? There's no way strands will be that far up. But drilling through the stem, anywhere, even that far up - it provides the contractor with an opportunity to screw up with disastrous results. Hmm.
RE: Drilling through existing precast double-tees to support new ducts
Don't drill in the middle of the stem and stay away from the ends! Drill as close as possible to the top of the stem for various reasons with missing strands being one of them. Strand patterns vary depend on the depth of stem, number of strands, and they're not always depressed. Due to cost of shipping, pre-cast usually is supplied by the nearest fabricator(s), so I suggest you start calling pre-cast fabricators in the area and ask if they supplied the project. If you locate the supplier, they may provide insight on strand location, and if you're really lucky will have shop drawings. I advise using through bolts with sloped washers on each side of the stem.
RE: Drilling through existing precast double-tees to support new ducts
Double Tee flanges are typically either 2" with a 2" topping or 4", although I guess a 2" flange with no topping is possible.
I don't think drilling at the stem to flange intersection is a viable option- not enough clearance.
RE: Drilling through existing precast double-tees to support new ducts
RE: Drilling through existing precast double-tees to support new ducts
To be completely safe, would look at expansion anchors in the flanges if possible. Would double check with Hilti's (or Simpson's or whoever's) engineering department about whether expansion anchors still work okay across the interface you'll have between DT and topping.
RE: Drilling through existing precast double-tees to support new ducts
Here's a link: Hilti HDP-I