ACI Appendix D - high seismic - 2.5 factor

[ATSE]

I've seen similar threads, but to be painfully clear:

Consider a single headed stud in concrete using Appendix D
1. Moderate-high seismic zone (say, San Diego)
2. f'c = 3,000 psi
3. 5/8" diameter, A193, B6 (stainless steel, futa=110ksi), 7" embed in 12" thick concrete
4. No supplemental reinf (Condition B)
5. Tension only
6. Edge distance > 24"

phi*Nsa = design strength, steel tension = 18.6 kips (not including seismic reduction)

phi*Ncb = design strength, conc breakout = 17.0 kips (not including seismic reduction)

However, the punishing comes when the seismic reduction factors are applied via D.3.3.3 and D.3.3.5.

It appears that I need to penalize phi*Ncb by 0.75 / 2.5 = 0.30. That is, reduce the capacity by 70%. Phi*Nn =0.3 * 17.0 = 5.1 kips.

I've done this calc a few different times now, but I always wonder if I'm missing something. Can I get a confirmation on this?
I think I can get better anchorage values out of bubble gum.

Hilti has fairly nice design aids and design examples, but does not use omega (high seismic areas) in their example calc (page 56-57 of latest design manual).
Simpson’s software has a warning regarding the 2.5 factor, but does not explicitly apply it to the reported values on the results page.

As an aside, I can use a little more embedment to get steel to govern. However, epoxy by any manufacturer is nearly impossible to get a ductile (steel) failure to govern over bond for reasonable embedments (and available hammer bits), and resulting tension capacities are super low. It seems to encourage the bad practice of using larger diameter anchors, since you're going to have to use 2.5 no matter what you do. Am I missing something?

 

[Lion06]

Nope, you're not missing anything. You have to multiply both capacities by 0.75 for the moderate to high seismic zone. Always try to get the steel to control so that you don't have the additional reduction factor of 2.5. If that means using a smaller diameter stud or providing deeper embedment, then just do it. I would also be hesitant to use an epoxy anchor for seismic loading, but if you must, use smaller diameter anchors with the deepest embedment you can to get away from the additional reduction factor of 2.5. All of this only applies if the load is actually a seismic load. If you're in the moderate to high seismic area, but the load under consideration is gravity or wind, you don't have to comply with the additional requirements.

 

[OCI]

Where is the 2.5 factor shown in the ACI app.D?

 

[OldPaperMaker]

OCI,
The 2.5 number applies when designing structures in Seismic Design Category C, D, E or F.

The specific reference is found in the 2006 IBC in Section 1908.1.16 as a modification to ACI 318-5 Section D.3.3.5

It is also found in ACI 318-08 in Section D.3.6 but it is worded as "0.4 or 0.5 (for stud bearing walls)times the design strenght determined in accordance with D.3.3.3."

There is a report, that is found on

http://www.awc.org,

that describes a series of tests on anchor bolts in wood framed shearwalls. The report confirms that using appendix D to design anchor bolts for wood shearwalls is very conservative. But, until the code Czars make a change I suspect that we are stuck with it.

 

[OCI]

Thanks!, I was aware of and have been using the .75 factor but missed the 2.5. Ouch! I thought these anchors were hard enough to get to work. It's crazy to have to reference 3 code books to design an anchor! ACI, IBC, CBC.

 

[OCI]

Is this in addition to Omega for the design of LFRS anchors?
If so, when you combine (2.5*Omega)/.75 using omega = 3 this is 10X design strength for normal factored loads!! Is this correct?

 

[OldPaperMaker]

OCI,
ACI318-Appendix D is used to calculate the capacity of the anchor bolt in concrete.

I am pretty sure that the Overstrength Factor (Omega sub zero in ASCE 7-05)is only used to calculate the Seismic Load Effect (E sub mh & E sum).

I would recommend that you keep the capacity and the loads seperate.

I am not sure what you are referring to with "LFRS". Do you mean LFRD as used in steel design?

 

[OCI]

Thanks,

I only combined the loads and capacity for the example. By "LRFS" I meant the lateral system. I think it's "Lateral Force Resisting System" or something like that. I'm just pointing out that it seems overly conservative and want to be sure I'm understanding the code correctly.