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fixing a steel console with only 2 anchors? 3

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n3jc

n3jc

Civil/Environmental
Nov 7, 2016
187
I have to fix a steel console into existing RC beam. Console is supporting a steel canopy beam.
Since RC beam is 250/300 mm I m very limited about how to destribute anchors. I have to consider minimum edge spacing from the edge of concrete and between anchors. So I came up with this:

Normally Id use more than single line of anchors, but in this case forces are small so I think it is OK. Calculations shows that 2 anchors are more than sufficient bu I still feel somewhat uneasy because I have only a single line of anchors. Id add another line but since RC beam is only 300 mm high, I have a problem with steel beam since it shouldnt be too high above the RC beam.


SIDRANJE_gwtnoo.png
 
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What Im asking is do you feel comfortable with one line of anchors for this type of problem?

Anchors are M12 (HILTI HST3)
Do you see any mistake in my calculation?

m12_pxygcg.png
 
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Can you flip the angle bearing?... better anchorage... and use a thicker angle if necessary and avoid the stiffener (it only forces the point of load application further from the concrete)? I don't know how big the gap is, but, I would treat the load as coming down near the end of the beam, not at the 'bolt holes?'.

The beam is so much stiffer than the supporting angle.

I missed the question; a single line of anchors would be fine... and your e might be 15mm. If you can imagine the force in the bolt (not the anchors)... if you think the bolt will be in tension from the loading...

Dik
 
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Instead of using an angle shape, you could use a rectangular plate, maybe 250 mm high, with a tee shape welded to it. That would allow for an additional upper row of anchors. I would definitely want redundancy in this connection, even if the numbers work out with just one line of anchors.

Another possible configuration is to anchor a bent plate to the face of the concrete beam, where the top horizontal leg of the bent plate bears on top of the beam and is anchored through the top of the beam, in addition to the side anchors.
 
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Your calculations don't account for prying on the anchors, which is a real load in this situation that doesn't appear in the Hilti engineering guides or anywhere else.. this would make me very nervous with a load that high.
 
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jgKRI,

n3jc is accounting for prying. That's where the tension comes from.
 
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No, prying is a specific force related to levering the nut/head of a bolt due to deformation of the anchored bracket. It's detailed in the start of chapter 9 in the AISC steel manual.

I also still haven't seen anything from OP that they checked combined shear breakout of the two bolts, this close to a free edge and combined with tension load I can almost guarantee that this is being overlooked.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
 
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N3jc:
Use a WT shape (maybe a fabricated shape) in place of your angle bracket, as bones206 suggests. This way you can get two lines of anchors into the RC beam in more favorable locations.
 
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I have no problem with the single line of bolts if the numbers work (~2k isn't all that much). I'd keep the stiffener with this connection design as, in my opinion, it alleviates the prying action which is difficult to assess when your steel is bearing against concrete.

I would strongly urge you to heed TME's comments regarding concrete shear breakout however. That's likely the governing failure mode and is really the numerical means of determining whether or not one line of fasteners is sufficient.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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With less than 3" clearance from the edge, I'd be willing to bet that this doesn't meet the minimum edge distance requirements of the anchor, even if you can technically get the concrete breakout calculation to work.

Edit: It looks like you've overestimated your anchor's shear capacity.

hiltianchor_ahggui.png


Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.
 
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you have just lost a bet sir

hst3_vs5pbz.png
 
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I ran a few numbers on this and noted a few more things.

I wouldn't take the moment arm e as 60 mm as was noted above. Closer to the end of the bracket is conservative and likely more realistic. I also wouldn't take the moment arm of the compression/tension forces in the anchorage as the full depth. Rather, I'd consider a small rectangular compression block which gives a tension component in the two bolts of around 10.5 kN.

I don't believe you're going to get additional prying loads based on the center stiffener but without knowing your plate thickness I can't be sure. Definitely should be checked regardless.

You noted you're using a wedge anchor but using 1/2" wedge anchors that close to the edge is usually not recommended as the wedge force creates a fairly large bursting force and thus you need something like 80 mm of edge distance or whatever Hilti recommends.

Edit: You addressed this above. I would have also lost a bet. Hilti knows their stuff though so if they say you can do it without bursting the concrete then I'd believe them. Still worth pushing the holes back away from the edge if you can. Just because an expert Hilti installer can avoid cracking the concrete does not mean Joe "Hammer Drill" McMuscles can.

I confirmed that the interaction of concrete breakout in shear and tension definitely controls and I highly suspect your anchoring system will be undersized for the loads you have.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
 
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briancpotter... that's why I suggested 'flipping' the angle...

Dik
 
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Seems I have, color me surprised. Like TME, I'd still be pretty skeptical of this as it's frighteningly close to the edge, and an expansion anchor is going to push outward against it's hole. And you've still dramatically overestimated your shear capacity.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.
 
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Why don't you check the bolts and concrete with Hilti Profis Anchor? With only two bolts close to edge I would certainly check it.
 
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Are you sure you will not have problems drilling in rods that close to the bottom? I would expect it is likely there is rebar close where you want to drill, and you are bound to hit it. I like dik's idea if you do not want to use a four bolt connection.
 
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TehMightyEngineer, typical prying doesn't apply here since there are no tension loads normal to the anchored surface. Prying as defined by AISC and what jgKRI was discussing only occurs when you have tension normal to the anchored surface with eccentricity between the load and the anchor along with enough lever arm beyond the anchor to generate additional tension.

The only "prying" that is in this situation is based on the eccentricity of the vertical load. The tension generated by this eccentric load has been accounted for. I have not verified that their design is correct, but the tension from "prying" is there.
 
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Yes, the primary force in the bolt is a "prying" reaction but if you have a thin enough bracket plate then the tension stress in the bracket can still cause an additional, traditional prying force in the fasteners. See attached.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
 
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