Drilling through existing precast double-tees to support new ducts

[GalileoG]

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.

 

[frv]

Yes, strands are sometimes depressed mid-span. However, straight strands may run up a significant portion of the tee stem. You need to be absolutely sure you will not accidentally core through one of them.

 

[ron9876]

Don't think that is a safe assumption. Attaching to the slab portion of the double-tee is probably the best place to make a connection unless you can find the shop drawings or do some type of non-destructive testing.

 

[GalileoG]

Now I'm having second thoughts.

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?

 

[frv]

Can't you hang them off of the flanges of the Double Tees and have the ducts below the bottom of the stem? There is a lot less danger there.

 

[GalileoG]

frv,

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.

 

[smb4050]

As much as we engineers don't want to admit it, a lot of stems get drilled without knowing exactly where the strands are located, but that shouldn't be your risk. Give your client a proposal for x-raying the stems recommending they approve it, and include written caution of hitting a strand if they don't.

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.

 

[frv]

The flanges don't have strands. And yes- you could use expansion anchors or epoxy anchors (although you must ensure that the epoxy is designed for sustained tensile load). Depending on how you detail it, you may even get some PAF's to work (Hilti XU, for example).

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.

 

[UcfSE]

I've had success by requiring the contractor to verify strand locations prior to drilling, either by x-ray or gpr.

 

[MrHershey]

Not double tees, but we've had contractor scan for strands in hollowcore + topping before they could core openings for piping. Do try to remember, however, that it's not an exact showing of what is there. Usually tried to stay at least a couple inches away from where scan indicated strands might be in case the xrays scattered or technician misdrew the line (our technician would draw with chalk where the strands were).

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.

 

[JAE]

Hilti has an anchor that only requires 3/4" or 1" depth of drilling - I think it is called HDI-P. Not a whole lot of capacity but might work for ducts.

Here's a link: Hilti HDP-I