Seismic Anchors and Appendix D

[fredPE]

I am retrofitting an existing braced frame connection by installing new epoxy anchors. Basically, I was running in to compounding load factors in using ACI Appendix D . . . Seismic anchorage forces are designed for Omega (Ordinary braced frame). For bond strength, you get a regular phi of 0.55 for the epoxy anchors in the ICC-ES Report, seismic forces get an additional phi factor of 0.75 in ACI 318 Section D3.3.3, and epoxy anchors get an additional 0.65 alpha factor based on Hilti's ICC-ES 2322 report table 9. Profis applies all three of these phi factors in checking bond strength of an epoxy anchor. Even still, for breakout forces, you get a regular phi factor of 0.7 combined with the additional seismic factor of 0.75.

Somewhere, I thought I seen a reference where you were not intended to use Omega forces with all of these other phi factors, so was looking for somebody who may know about this. Obviously, with large forces, there isn't much you can do short of supporting your column, chipping out the concrete in the footing and installing new anchors. The existing anchors are only 10" deep and were actually strong enough based on the 1997 UBC Anchor methodology. I am going in with epoxy, doubling the number of anchors and going 20" deep, and I am overstressed by a factor of 2! I don't know what I am missing here.

 

[ishvaaag]

I transcribe a note I made when doing worksheets for anchors according to ACI 349-2R-89 / 1994

"Derived from Klingner in ACI SJ 93 1 p.132) (to get the usual confidence of 95%) you will need an additional safety factor for anchorages of 2, that is, if you usually for forces amplify by 1.7, for anchors extra amplify multiplying again by 2 to get 3.4. You need this load factor to get the proper level of 95% confidence using 0.65 as pullout strength reduction factor and 0.9 steel strength reduction factor."

ACI SJ refers to ACI Structural Journal magazine.

What above doesn't include any factor for a seismic application so no wonder one safety factor or another needs be even further amplified (or another introduced) to keep reliability consistent all along the structure for seismic action. This as always, since connections use not to be ductile, to ensure the failure happens outside connections.

 

[ishvaaag]

And when making another sheet for anchors according to PCI Handbook 4th edition I noted:

"Additional safety factors might be needed to get the proper 95% statistical reliability.
Without a pull-out strength reduction factor the formulas maybe would overestimate extraction capacity and would be not conservative
See ACI SJ Vol. 94 Nº 5 p. 594.
Per PCI Hbk. p. 6-4 an additional security factor from 1 to 1.33 has been conventional in the industry. Say, from 1.6 to 2.99."

In this case it seems the safety factor was taken additional, as a sum, instead as a factor.

The same consideration referring to seismic in my previous post applies.

 

[Lion06]

Have you tried using LESS embedment? I know it doesn't make sense, but based on the equations, there comes a point where deeper embedment means less capacity (depending on spacings and edge distances). I would also say that I've found PROFIS to be very conservative in 2 cases. I haven't looked super hard at it, but for what I've looked at, they do take the ultra-conservative route.

Which failure mechanism is controlling the capacity?