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fixing a steel console with only 2 anchors?

fixing a steel console with only 2 anchors?

(OP)
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.





RE: fixing a steel console with only 2 anchors?

I wouldn't exactly say that's a small load. Have you truly verified the anchors can take that combined tension and shear load?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: fixing a steel console with only 2 anchors?

(OP)
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?

RE: fixing a steel console with only 2 anchors?

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

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

jgKRI,

n3jc is accounting for prying. That's where the tension comes from.

RE: fixing a steel console with only 2 anchors?

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
https://www.facebook.com/AmericanConcrete/

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

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.



Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

RE: fixing a steel console with only 2 anchors?

(OP)
you have just lost a bet sir



RE: fixing a steel console with only 2 anchors?

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
https://www.facebook.com/AmericanConcrete/

RE: fixing a steel console with only 2 anchors?

briancpotter... that's why I suggested 'flipping' the angle...

Dik

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

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.

RE: fixing a steel console with only 2 anchors?

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
https://www.facebook.com/AmericanConcrete/

RE: fixing a steel console with only 2 anchors?

TMH, I don't believe there is any prying action here.

Your sketch implies there is a tension load away from the anchored surface originating between the bolts, but there is none. The tension load is directly at the bolts as a result of the eccentric load.

The deformation shown in your mini-sketch (on the bottom right) cannot occur. The only deformation that can occur is the leg of the angle bending just below the joint between the two legs. If anything the bending in the vertical leg of the angle (especially if the stiffener was not there) would force the steel between the two bolts into the concrete surface, not away from it.

See below. I've removed the stiffener to make it more clear.



RE: fixing a steel console with only 2 anchors?

I would argue that your sketch is the perfect example of prying action.

RE: fixing a steel console with only 2 anchors?

jayrod,

There is a difference between the tension reaction caused by this scenario and how AISC defines "prying action". While the sketched scenario is "prying" on the bolts, it's not "prying action" as defined by AISC.

"Prying action" is a secondary tension effect cause by plate bending due to a tension load applied directly to the plate element eccentric from a line parallel to and at the centerline of a bolt. See Chapter 9 in the AISC Manual. The scenario we are discussing does not have this tension load and can't have "prying action".

RE: fixing a steel console with only 2 anchors?

Fair enough. However I assume the people here indicating prying are thinking of your sketch.

RE: fixing a steel console with only 2 anchors?

DETstru, I would argue that because the stem is there that it provides a stiff path for a tensile stress component from the flexure of the stem. This tensile stress can cause the typical prying. Obviously there should be some kind of "two-way" action with the compressive stress at the base of the stem but if the plate is thin enough I could definitely see prying action occurring.

I fully agree that the unstiffened angle that you show doesn't receive any prying action, unless the plate was so flexible that it's actually prying the nut itself (but this would have to be a very large deformation I would imagine).

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: fixing a steel console with only 2 anchors?

Quote (jayrod)

I would argue that your sketch is the perfect example of prying action...I assume the people here indicating prying are thinking of your sketch.

I agree. Couldn't have drawn it better myself. In my experience, this is the version of prying action that most folks are concerned with in these situations. The center of compression reaction migrates away from the toe of the angle in a manner that is difficult to predict. This is the version of prying for which I like the stiffener.

TME's version of prying is also legitimate when the stiffnener is in play although I doubt much would come of it for most conventional geometries.

So you've got two kinds of prying potentially. And neither is textbook AISC prying so they're rather difficult to assess. All good reasons to try to detail the issue away as has been suggested several times above.

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.

RE: fixing a steel console with only 2 anchors?

TMH,
The horizontal leg of the angle will prevent any meaningful bending that could create "prying action". This situation is similar to the "thick" plates defined in AISC design guide 4 that allow you to ignore "prying action". Once the connecting element is stiff enough, horizontal "prying action" can be ignored.
That's why WT's used to bolt members with tension loads are such a problem, no horizontal leg to stiffen the plate.

Koot,
For the same stiffness reason above, I don't think there is any meaningful two way action. Just the tension load generated from the T/C lever.

jayrod,
Yes they may be thinking that but my original comment was responding to someone saying OP's calc didn't account for prying. I was saying they did account for it. Then the rest of this conversation was based on TMH's next comment specifically about the AISC-defined "prying action"

RE: fixing a steel console with only 2 anchors?

As the leg of the angle deflects, the point load application will move to the left and minimise the e value... the bolt will go into tension...

Dik

RE: fixing a steel console with only 2 anchors?

dik, not sure what you mean. The vertical deflection is negligible (effectively nonexistent with the stiffener) and the bolts are always in tension anyway.
Regardless, that doesn't generate "prying action" since the ledger doesn't bend in the horizontal direction.

RE: fixing a steel console with only 2 anchors?

If there's any deflection... the bolt will go into tension... also deflection will be caused by the point load acting near the end of the beam... it will not occur at the fastener location.

Dik

RE: fixing a steel console with only 2 anchors?

Well, I think we beat that to death, probably best to either make a new topic or move on. For OP's consideration I believe it's safe to say any "prying action" is negligible.

Far more critical is the edge distance.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: fixing a steel console with only 2 anchors?

Dik, my apologies but I really don't understand what you're getting at.
The bolts are always in tension. I don't think anyone is saying they aren't.
What does the tip deflection have to do with prying action in the horizontal plane??

RE: fixing a steel console with only 2 anchors?

Not saying anything about prying action... (you will have a little of it at both the concrete as well as the steel connection; don't know if it's significant) and, if the bolts were only finger tight, they would tighten a bit due to the deflection of the angle... my thesis is that the point of load application should be closer to the end of the beam... not 60mm from the concrete face...

Dik

RE: fixing a steel console with only 2 anchors?

Ahh ok thanks.
FYI- the bolts can't just be finger tight. They're wedge anchors so without tightening they don't work.

I'm going to stick with my conclusion of no prying action in the horizontal plane because of the stiffness of the angle in that plane. The only forces on these bolts are shear and tension from the eccentric load (via T/C lever arm). No secondary "prying action."

RE: fixing a steel console with only 2 anchors?

sorry... should have clarified it... the bolts securing the steel...

Dik

RE: fixing a steel console with only 2 anchors?

(OP)
I have read through this thread several times now. TNX everyone for contribution/replies.

What I still dont understand is: why is there so much talking about value e - distance from beam action on angle to edge of the RC beam? As far as I know If I choose larger value of e, the larger eccentricity is which means larger moment and larger tension forces in bolts... So why so much talk about e being closer to the RC beam? e being larger means being more conservative - being on the safe side right?
I use e as a distance between half of a contact surface between a steel beam and steel angle and edge of RC beam

RE: fixing a steel console with only 2 anchors?

Oh, I was suggesting to use an e at the end of the bracket to be conservative but and e to the middle of the bearing surface would also be reasonable. You are correct that a smaller eccentricity is less conservative.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: fixing a steel console with only 2 anchors?

With the stiffener in there, I favour at least a triangular distribution increasing away from the heel. Point load at the end probably isn't too far off reality.

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.

RE: fixing a steel console with only 2 anchors?

(OP)
Thanks for replies. I agree.

I want to ask another question in a case of 2 lines of bolts as shown below.

What is your opinion about lever arm r1 = 80 mm and r2 =80 + 80 = 160 mm.
Would you consider rotation at the bottom of RC beam or higher? What if there was no stiffener but just an angle? would you consider the same? I think it all depends on plate thickness of steel angle.


RE: fixing a steel console with only 2 anchors?

2
I think that's fairly rational with one exception: you keep showing the point of rotation as being at the very bottom of the bracket. I feel that there needs to be some acknowledgement of the fact that the compressive stress block against the beam takes up some finite amount of space which will reduce your effective lever arm.

For small scale applications like this, I'll assume the point of rotation (neutral axis) to be right at the upper anchor with a triangular compression stress block projecting downwards. That's a physical impossibility, of course, but I feel that it's nicely conservative and it takes me five minutes. In that case, I'll either ignore the lower anchors or assume that they only participate in shear.

In all the hubbabaloo over the taxonomy of the particular kind of prying action we have here, I fear that we may have lost sight of what is some very wise and time honored wisdom: turn the angle leg upwards if you can, as others have recommended above. Do that because:

1) It greatly reduces the need for the stiffener.
2) It moves your anchors to a more favorable location for shear breakout, probably eliminating the need for two rows.
3) It keeps your center of concrete compression pretty reliably near the angle heel reducing that source of non-AISC "prying".
4) It eliminates the non-AISC prying associated with the heel of the angle drifting over laterally (P-delta effectively).

These are all good things when it comes to developing a simple, reliable connection.

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.

RE: fixing a steel console with only 2 anchors?

The AISC Steel Construction Manual assumes the beam reaction is applied at a distance of 80% of the bearing length (0.8W) from the face of the support for stiffened beam seats. I actually worked on a project with dozens of beam seat connections already installed in the field that were designed assuming an eccentricity of W/2. When someone pointed out the 0.8W requirement, all of the beam seats had to be re-evaluated and a few required reinforcement in the field.

That being said, I think assuming that it's acting at the edge is appropriate and conservative when you have a stiffener.

RE: fixing a steel console with only 2 anchors?

(OP)
This has become quite a thread :)

tnx for replies/help!

regards

RE: fixing a steel console with only 2 anchors?

KootK... the fourth posting...

Dik

RE: fixing a steel console with only 2 anchors?

What about the fourth posting dik? You mean your recommendation to flip the angle? I was backing you up on that with my latest.

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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