Anchor rod in bearing

[BAGW]

Hi,

When we have series of anchors placed 4 x d distance from each other, does ACI code provides any guidelines in reduction in bearing strength based on group effect?

 

[lexpatrie]

Hard to say with such a broad question.

If you mean actual anchor rods, like for structural steel with base plates, usually the holes in the base plate are so large that it's unlikely you'll get all four to act together, I'm used to seeing engineer's only count the two that are in compression as working for shear.

If you mean what are more conventionally called anchor bolts, for attaching a wood sill plate to a concrete masonry stem wall or poured concrete slab, I'm not aware of a penalty (unless it's high seismic). (There was some research done on this topic circa 2000 by the Structural Engineer's association of California as I recall). The ACI code seems to be fairly punitive to anchor bolts due to a lack of research on them in particular, because you can't transfer all that much force through an anchor bolt, either due to limitations of the shear wall, the mud sill / sill plate, or the diameter and strength of the bolt itself.

Something from 2010 -

https://www.structuremag.org/wp-content/uploads/2014/08/C-GuestColumn-Seaoc-Aug101.pdf

Regards,
Brian

 

[BAGW]

Yeah I meant the actual anchor rods like column to base plate connection. I have like 6 anchor rods with washers welded to base plate. So all anchors share equal shear load. Anchors are spaced with 4xd dimension. In such scenarios how to consider the overlap of anchor rod bearing area? Is that even a real thing?

 

[Eng16080]

If I understand you correctly, you're concerned with failure of the concrete due to a shear force being transferred to the concrete through the anchor rods. If that's the case, then I'm not aware of a design check specifically for the rod bearing area. (Or at least, it's not called that.) What you do want to do is follow the requirements of ACI 318, Chapter 17. In particular, per Table 17.3.1.1, you'll need to check:
[ol 1]
[li]Steel strength in shear[/li]
[li]Concrete breakout strength in shear[/li]
[li]Concrete pryout strength in shear[/li]
[/ol]

The "concrete breakout strength in shear" is most likely what you're thinking of in your question. The calculations involved account for overlapping failure cones as well as the overall geometry of the concrete. I'd recommend checking your calculations with software, like Hilti Profis because the Chapter 17 requirements are (in my opinion) very complicated.

 

[BAGW]

@Eng16080

Yes I am doing all those checks. For concrete breakout in shear I have shear reinforcement and hence this failure mechanism does not happen.

Not sure, If I have to consider group effect for bearing of anchors. It is not specified anywhere in ACI nor AISC DG.

 

[ads0221]

What group effect are you referencing? ACI recommends the 4 x Da for untorqued anchor bolts and 6 x Da torqued anchor bolts.

If you are following this then as referenced above checking all of your failure modes the lesser design strength will control.

 

[dik]

If I need it for bearing, I weld plate washers to the column base plate to engage the anchor rods.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Eng16080]

BAGW,

Ok, I understand your question now. I agree with you that ACI doesn't seem to have a specific bearing strength check where the concrete breakout is resisted by reinforcement. As ads0221 mentioned, you should set the anchor locations in accordance with the edge distance and spacing requirements of Section 17.7.

The commentary to Section 17.5.2.9 does recommend that "the enclosing anchor reinforcement ... should be in contact with the anchor and placed as close as practicable to the concrete surface." So, I think, if you were to do this, concrete bearing would be less of a concern since the force transfer would be directly from the anchor rod to the reinforcement which it's in contact with.

To go further on this, if you were to analyze this connection using the strut and tie method (STM), based on the design guide "Anchorage Design for Petrochemical and Other Industrial Facilities," it seems that you would check concrete bearing based on a concrete stress of 0.85Fc', and you would use a load bearing length of the anchor, le, in accordance with Section 17.5.2.2, which is not to exceed 8 x Da. (I'll admit that I'm not an expert on the STM method, so you should do your own research on this.)

 

[haynewp]

There isn't a group bearing check I have seen before and I wouldn't worry about it if it meets the other ACI anchor checks. You can check the bearing of a single bolt using the load in the bolt applied to the area of bearing and use the standard ACI bearing equation in Chapter 14.