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Anchors in shear perpendicular to the edge per ACI 318 Appendix D

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I’m trying to put a hold down in an existing raised floor foundation and I’m having difficulty, particularly due to the cracked concrete requirement. It’s not the cracked concrete so much (1.0 vs 1.4 factors, etc.) but the methodology.

 

I first tried an epoxy (Simpson SET) with a threaded rod attached to a Simpson PHD. Due to the limited edge distances (c1=2 in; c2=4 in because the stem width is 6 in), that approach didn’t give me enough capacity.

 

I then looked at installing a MST strap on the face of the stem wall and installing a Simpson Strong Bolt and loading it in shear. This is where I need some help. It appears that no matter how far I am from the edge, I can only consider one anchor in shear. I have 318-02 and the picture is in section d.6.2.2. In the picture above the caption “Note: One assumption…” shows Vn/2 loading an area AV=2(1.5c1)h. If I thought that this geometry would yield a value for Vn/2, I believe I would be wrong, at least according to the Simpson program. If my stem wall height is 22.5 inches and the bolt spacing is 5.25 inches, the capacity for either one bolt or two bolts is the same (2,970 lbs ultimate).

 

This really doesn’t make sense to me. I’m thinking that, If the distance from face of concrete on the loaded edge to the first anchor is 1.5hef, then I should get added capacity when I put anchors behind it. Based on what I’ve explained above, this is not the case.

 

What am I missing?

 

TIA,

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
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