ACI 318-08 Appendix D

[smwa]

I'm curious if anyone has read the new ACI318-08 Appendix D, Section D3.3.6? Am I understanding correctly that if the anchor material is brittle, then the anchor design strength is limited to 0.4 x concrete failure strength per D3.3.3?

Why is the brittle anchor design strength a function of the concrete failure strength? If I have a very weak brittle anchor (say and old existing anchor) and very strong concrete, how is this properly calculating the steel failure mode in the anchor?

Thanks,

Stan

 

[JStephen]

If you have a very weak anchor, brittle or not, then the strength should be limited by steel strength, which is a separate check.

I think the idea is that an anchor that can deform plastically prior to failure can reduce the loading transferred to the concrete, whcn compared to one with the same ultimate strength that doesn't yield. Partly through reduced motion transferred, partly though increased damping when things start stretching plastically.

 

[smwa]

JStephen - Thanks for your explanation. It seems to me that the ability for the anchor to deform should include the length of the anchor that is allowed to stretch, not just a function of the material property. For example, if the bottom of a holdown is installed right at the concrete surface, then the anchor rod is not allowed to stretch, but if the holdown is installed a few inches above the concrete surface, then the anchor rod is allowed to stretch. D3.3.6 doesn't take the length of the material into account. If this is the reason for this provision, it seems like a clumsy way of dealing with this issue by indiscriminately reducing the concrete capacity by 60%.

 

[Lion06]

I don't know if I have a bum copy of ACI 318-08, but I don't have a D3.3.6 in mine.

That being said, 318-05 has a similar provision, but it only applies to seismic loads in SDC C, D, or E. It doesn't say anything about using the concrete capacity instead of the steel capacity, it just says the "minimum design strength of the anchors shall be at least 2.5 times the factored forces transmitted by the attachment". This reference is in IBC 06 on page 379 - this modifies a portion of App. D in ACI 318-05.

The anchor capacity is the lowest of the different concrete failure modes and the steel.