Concrete Break Out

[CANPRO]

I'm wondering how to address the following setup:

I have multiple anchors installed in concrete. The anchors are close enough that the break-out cones overlap. However, the anchors will not be loaded at the same time.

I don't have any set dimensions I'm applying this to, I just need to know how to deal with the general concept.

3 scenarios to consider. First, the unloaded anchor falls completely within the cone of the loaded anchor. Second, the cone of the loaded anchor pass through the unloaded anchor. Third, the unloaded anchor falls outside of the cone of the loaded anchor.

Conservatively I could just apply all spacing reductions to my anchors. But I'm thinking for the first and third scenarios I could just ignore the effect of the unloaded anchor and in the second scenario the unloaded anchor would reinforce the concrete if anything (not that I would count on that). I'm leaning towards just ignoring the effect of the unloaded anchor.

Since only 1 of the 2 anchors are loaded at a time, this is a cylic load. So maybe I should consider the history of the loading and apply all spacing reductions.

Any opinions are appreciated.

 

[wadavis]

CANEIT
Sounds entirely reasonable to exclude the unloaded anchors entirely from the design, those anchors would be loaded under separate load cases with no interacting stresses.

 

[OHIOMatt]

In scenatios 1 and 2, the cone of the loaded anchor is compromised to some degree by the hole created by the adjacent anchor. I would want to research the impact that this would have on the loaded anchor cone. Not sure what type of load you are expecting, but if as you not it is truly cyclic, then I would be very concerned with stress connecntrations around the un-loaded anchor.

 

[wadavis]

If the anchor is set when the concrete is poured or is an adhesive anchor would you treat it as typical shear reinforcing (conservatively disregard) and not worry about stress concentrations?

 

[Ron]

If the influence cone gets interrupted by anything, take the reduction. In your case, the loaded cone is interrupted by the unloaded anchor....the result is an unpredictable load distribution around the interruption.

Set up a test section and test your load condition and sequencing...then you'll know.

 

[CANPRO]

My gut feel is that I can ignore the unloaded anchor. But unless I have something solid to back it up I have to take the reduction.

I would love to set up a test section and see for myself, but I don't think its going to happen this time around. Thanks for the input.

 

[JedClampett]

The same thing happens when a group of anchors are subject to a pure moment. Some are in tension and some see compression (basically unloaded to ACI). I ignore the overlap effects of the compression anchors. It's not discussed in ACI, but I think a reasonable reviewer would agree.

 

[southard2]

I concur with Clampett. If I'm designing sign post anchor bolt base plate connection with say only four bolts. With there being only moment and shear at the base you can use all four bolts for the shear part of the interaction equation. Then for the tension portion you would use a two bolt group for your pull out cone analysis. Totally reasonable. You have match the failure modes with the equations.


Just make sure that whatever you do is done conservatively.

John Southard, M.S., P.E.

http://www.pdhlibrary.com/

 

[amec2004]

>>3 scenarios to consider. First, the unloaded anchor falls completely within the cone of the loaded anchor. Second, the >>cone of the loaded anchor pass through the unloaded anchor. Third, the unloaded anchor falls outside of the cone of the >>loaded anchor.

I would say if the anchors have overlaped failure cone they are close enough to work as group. All the anchors in the group shall have the tensile load at the same time.

anchor bolt design crane beam design

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