Hanging ductwork off rebar in slab? 2

[jk2017]

Hi all,

A contractor for a reno job I'm working on wants to hang ductwork by chipping away at an existing slab and hooking the ductwork off existing rebar. Has anyone encountered something like this before and have watch-it's/opinions? The ductwork is very light, to note. I told him I'd look into it but that we much preferred they use typical HILTI anchors, of which we have already approved. For reference, the existing structure is cindercrete with draped rebar from the 1920's (encased steel beams provide the main gravity support).

 

[phamENG]

I wouldn't do it. The rebar is draped, so you're either loading the rebar in shear/bending at its point of highest tensile stress while also breaking the bond between the concrete and the rebar where it's needed most, or you're digging through to the top of the slab at the point of highest shear stress. A Hilti anchor would be a lot less invasive, easier to install, and less likely to do significant damage. I've revised my response due to other contributions below bringing up good points about the soft concrete the was likely used. I still wouldn't go into the slab and hang on the rebar, but a solution other than a post installed anchor may also be needed.

 

[jk2017]

Agreed, and we haven't gotten any submittals about it. He seemed to insinuate that this is a common practice, but I've never heard that before.

 

[dhengr]

Jk2017:
One problem could be the load ratings of any anchors in that old, kinda soft/porous, cinder concrete which makes up the slab. I agree that it is a crazy idea to mess around with the rebar as a means of supporting hanging mechanicals.

 

[kingnero]

Not to mention the long term corrosion issues. If he's suggesting that's a common practice, I'd very very careful what other "common practices" he practises...

 

[r13]

There are many more simpler solutions. Here is one.

Link

 

[Blackstar123]

I remember reading about this in one of the structure magazine articles.

 

[jk2017]

I should have mentioned: we have load tested HILTI epoxy anchors to 500 lb, so knowing this we may just tell them to limit the load at each anchor to 500#/safety factor (4?).

 

[phamENG]

5 may be more appropriate - Hilti does a good job explaining their allowable loads and safety factors in the front matter of their catalogs.

 

[jayrod12]

And depending how you tested it, you potentially only checked it for bond strength pullout. You have not tested it for breakout.

The conversation has been had a few times on this board, if you ask Hilti to come test an anchor, they'll put a jack on it and pull, but the collar from the jack will press back against the concrete surrounding the anchor meaning you are only testing one failure mode of the anchor.

I agree with the safety factor of at least 5. As indicated though, I'd even be hesitant that the 500lb ultimate load is accurate for all possible failure modes.

 

[Ingenuity]

if you ask Hilti to come test an anchor

In the US, Hilti no longer do such testing...liability.

I agree with jayrod12 and the type of test undertaken to check failure modes.

I have done many anchor tests, and when you explain to the client (owner, contractor or EoR) what type of test do they require I typically refer them ASTM E3121:

This is the most common test, that only has failure modes of bond and tensile of anchor body: