Design of Shear Key Using ACI-318 Anchorage

[shesanEIT]

Hi all,

My problem and question stated below:

I have applied loads of 500kip uplift and 100 kip horizontal onto a 3' x 3' x 3.5' tall pedestal hooked into a 5.5 foot mat foundation. I have 12 - 1.5" diameter bolts arranged in a 4x4 bolt pattern with 8 inch spacing between bolts. The anchors are set down into the foundation in order to increase tension breakout and side-faced blowout capacity with the "infinity" edge distance. After performing the tension/shear utilization calculations from ACI-318 on the anchorage, I am still over the 120% utilization limit. Concrete Breakout in Shear was governing, so I added as many hairpins as I could with the spacing requirements, but still cannot get enough capacity (sitting at 136% at this point). I can't increase any bar sizes, nor can I increase the pier size.

I'm now looking at shear key options. I was trying to figure out how to use the concrete breakout equations in section 17.5.2 and use a 6" HSS member considered as "one anchor" to determine the breakout resistance. I took my critical edge distance from the edge of concrete to the center of the shear key (18").

Is this a valid way of attacking this problem? I'm not getting much capacity going at it this way.

Thanks for any comments!

 

[mike20793]

Those kind of forces in a pedestal that size are definitely manageable. I think you've got too many anchors. With that kind of uplift I'd be looking at an embedded plate for uplift with just enough anchors to get by for shear. That will help your shear breakout quite a bit and give you some more room to restrain that sucker.

 

[shesanEIT]

Do you mean an embedded plate at the bottom of the anchor group? I understand that would help pullout, but how does that help the concrete breakout in tension condition (governing)?

 

[mike20793]

I'm assuming you have quite a bit of reinforcement in the pedestal for bending and uplift forces so you can easily develop that into the breakout cone with hooks or mechanical devices.

 

[KootK]

This is probably the method for you: Link. I'd try not to embed the anchor bolts in the footing. Firstly, it can lead to tolerance issues at the bolt at the top of the pier. Secondly, you want the tension transferred to your pier rebar, not resisted with brittle concrete tension breakout.

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.