Cantilevered Anchor Bolt - Effect of Shim? 3

[justhumm]

I'm looking at a contractor-proposed situation. They need to put a steel shelf angle up on an existing concrete wall. And they need to leave a gap between the wall and the angle. So they're proposing to post-install the anchor, put on a shim pack (steel plates), put up the angle, and tighten it with nuts.

If the shim pack isn't there, the anchor is obviously loaded eccentrically, and it's going to bend and apply a relatively significant tensile load on the anchorage.

Now, if you put the shim pack there, it's going to reduce the deflection, moment, and tension to some extent.

Is anyone aware of an accepted analytical approach (with reference) to showing how much of an effect the shims have?...increasing the effective section / Moment of Inertia of the bolt?...etc?

 

[Agent666]

The degree by which any design actions are effected really has to do with whether or not there is any slip between the plates. If they slip then its almost as bad as a cantilever. I don't know of any recognised way to quantify the effect, and I imagine nothing will exist for something that looks so unengineered.

Due to the unknowns in effectively assessing something like this, I'd just go back to them to use a larger angle or fabricated bracket. You'll sleep easier at night, and they will have a safer solution.

 

[WARose]

You'll need enough pinching force to let that transfer by friction (otherwise it is a cantilever). And since the failure cone of the anchor pullout doesn't overlap with the shim.....you are going to be most limited in the amount you can pre-load the bolt. (As per Appendix D of ACI 318.)

 

[JLNJ]

It seems me that the load is trying to pry the angle off the wall.

Regardless of slip between shim planes, moving the load away from the wall increases the moment arm significantly (maybe doubles it?), and with it the prying force. If you tack the shims so they don't slip, you still have this moment to deal with.

If the shims are smaller than the vertical leg of the plate, the resisting moment arm is reduced, driving the prying action up even higher.

 

[TehMightyEngineer]

This seems like a terrible idea...

Weld the shims and angles into one unit?

Edit: No, better idea: have a clamping nut on the far side of the shims against the concrete.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

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

 

[justhumm]

Just for the sake of argument...

Let's assume that it's not a shim pack and it's not a horseshoe...considering it a single solid plate with a standard hole in the center. And neglect any prying effects on the nut.

Once the anchor begins to deflect, at some point, won't it just be pressing against the bottom half of the washer?

And could you then argue that you're getting an upward friction force between the concrete and the washer?

 

[CANPRO]

In the scenario pictured in the OP it looks like you’d get prying between the angle and the shims and then again between the shims and the wall.

What is the application? Looks like a new shelf angle for exterior siding. Lightly loaded?

An angle or bent plate with a longer horizontal leg, bolted directly to the wall would be my first choice. If forced to provide the gap I’d prefer to have either one solid shim or maybe a small steel tube used instead of the shim pack. If the wall is reasonably straight this wouldn’t be difficult. Depending on the loads maybe have the shim/tube welded to the angle.

 

[KootK]

 

[justhumm]

@KootK, interesting idea, but I don't think it will fly. The 'tractor said it owner or designer won't allow the shelf to remain in place after installing the masonry block above. And any anchors need to be cut flush when the shelves are removed. That's why they're proposing an angle with the vertical leg down.

@CANPRO, yeah it's for a masonry block replacement on an existing building. I think the max factored vertical dead load on one of the anchors was about 900 lb (conservatively estimated). I'm not sure why the 'tractor didn't just propose the longer angle, like you mentioned, but I'm thinking it might be worth bringing up with them.

 

[KootK]

@KootK, interesting idea, but I don't think it will fly.

Maybe flip the HSS and run the bolt straight through the vertical angle leg and the tube.

I assumed the gap was born out of some kind of thermal requirements. If it serves no purpose then, yeah, L6x6 all the way.

 

[Celt83]

@KootK, interesting idea, but I don't think it will fly. The 'tractor said it owner or designer won't allow the shelf to remain in place after installing the masonry block above. And any anchors need to be cut flush when the shelves are removed. That's why they're proposing an angle with the vertical leg down.

So this may be dumb and I'm sure I don't understand the end game here, but what supports the masonry block after they've cut away the shelf? If this is just a temporary base for a lintel why not just do some post shores and a beam as a base and avoid all the hassle and short comings of the shimmed relief angle and epoxy'd anchors?

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[BridgeSmith]

I think KootK has it - mitigate the prying action by extending the 'spacer' down the wall.

 

[KootK]

mitigate the prying action by extending the 'spacer' down the wall.

Yeah, that. Well articulated. I've been at this so long that I'm often only marginally aware of the logic underpinning my own choices.

When I found out that my original detail wasn't going to work, I was tempted to give OP an e-flogging for not providing enough information in the problem statement for me to be able to work efficiently. It was really my own fault though. I did notice the down turned angle and I did catch the mention of "temporary" in the original detailing. I chose to turn a blind eye to those things because I'd already fallen in love with the detail that I wanted to propose. Classic SDB (Sexy Detail Bias). Rookie mistake.