Adhesive for overhead anchors

[ajk1]

Is there any adhesive that can be safely used for overhead anchors, and won't run out when being installed. Please see the attached sketch? Preferably a Hilti product (I would caall them, but I don't expect that they are open today). I would use a mechanical anchor except that they are not supposed to be used where closely spaced bars may prevent the mechanism from opening properly to created its wedging action.

 

[ajk1]

Sorry -- forgot the attachment, so here it is now.

 

[thaidavid40]

Getting the adhesive into an overhead hole, and keeping it there, is not all that hard. All it takes is a piece of duct tape covering the hole, and inserting the anchor through that until it sets. The bigger issue is the adhesive itself, when used overhead. Be very careful, and wary, of using an adhesive anchor where there is a long-term, sustained, tension (pull-out) load. They tend to fail after a time (refer to the Boston "Big Dig" tunnel's overhead liner failures a few years ago). Mechanical anchors are better suited for such applications, and therefore preferred. Use adhesives is such an application only after considering, and accounting for, the long-term creep effects of the adhesive system selected.
Dave

Thaidavid

 

[SAIL3]

I personally would not use adhesive anchors in an overhead situation for the reasons mentioned unless it was a minor load and I could live with the consequences of pull-out....

 

[ajk1]

To thaidavid40 and SAIL3: Yes I am well aware of the causes of the Boston Big Dig failure and I am very reluctant to use even adhesives that have passed the creep test (which the Boston anchors did not). I think I will either use a mechanical anchor into the rib bottom but make it long enough that the expansion mechanism is past the rebar, or use and adhesive anchor into the side of the rib. I think the anchor into the side of the rib, to resist the vertical force in shear, is the better choice, and hope that they can get it in without hitting the rebar. Another option might be a thru rod extending right thru the 6" wide rib and out the other side. Then I suppose no adhesive wuld be required if the bolt goes thru the steel plate each side of the rib.

Thanks for the comments. Much appreciated.

 

[thaidavid40]

@ajk1,
You are most welcome. And I like your idea of through-bolting the connection through the web.
Dave

Thaidavid

 

[hokie66]

Just make sure you have enough room for the drill. Don't know what your rib spacing is...

 

[ajk1]

to thaidavid40 (Structural)- thanks.
to Hokie66 - the 5" wide ribs are at 24" centres so should be ok.

I will sketch up the thru-bolt scheme. I have been working on different schemes all day and each one though seemingly promising at the start turns out to have problems.

What is making it difficult is that the top flange of the steel beam is 2½" below the soffit of the concrete rib, so there is not only horizontal force to resist, but also vertical force as a result of this difference in elevation between where the force is applied and where it is resisted. And the rib is so narrow and congested with rebar that only relatively small diameter fasteners can be used, and even then it is dicey whether they can get them in.

I hope the thru bolt scheme works. I am not sure how much shear capacity the thru bolt will have and how tight it can be in the hole...for a half inch diameter threaded rod, how tight a hole can be drilled for it to fit into? What shear capacity would it have from bearing on the concrete?

This is a good example how easy it would have been to do it right in the beginning (i.e. use a beam with an Lu corresponding to the actual conditions), than to try to fix it later.