Wood Shearwall anchorage and Appendix D

[JRESE]

I’m looking at a typical light wood framed single family residential design with wood shearwalls on raised concrete pier and grade beam foundation. Location is the East Bay area of California, code is 2007 Ca. Building Code (2006 IBC with amendments).

Section 1911 requires that I use Section 1912 for anchorage design because of seismic load combos. Section 1912 means ACI 318-05 Appendix D.

So I run through Appendix D for a 5/8" anchor bolt in the top of a concrete grade beam using a 3x6 mudsill, 2.75" edge distance. Concrete breakout strength governs with a capacity of 2.622 kips (strength). However, Appendix D 3.3.4 requires that shear strength be governed by strength of a steel element, so with concrete breakout strength governing, I have a problem. The CBC gives me an out by using a 2.5 over-strength factor, 1908.1.16. Dividing by 2.5, and multiplying by .75 for the additional strength reduction for seismic (D3.3.3), and by 0.7 for ASD, gives me 562 pounds per anchor bolt (ASD). Compare that to the NDS which gives me 1570 pounds per bolt (ASD) in 3x Douglas Fir mudsills (includes 1.33 duration of load).

A wood shearwall with 3/8" CD plywood, 8d @ 6", with a capacity of 260 PLF (ASD) would require 5/8" anchor bolts @ 24" o.c. If the loading was only wind, that same shearwall would have 5/8" anchor bolts @ 60" o.c. For a wall with 8d @ 3", 490 plf capacity, bolts would have to be at 12" o.c. versus 32" o.c. for wind only. It gets even worse when using a nominal 4" wide mudsill and 1.75" edge distance, the per bolt capacity drops to 400 pounds each.

Based my 32 years of structural engineering experience, I have a real hard time believing that the bolt to wood connection is 3 to 4 times stronger than the bolt to concrete connection. Something is very wrong here.

How about if I cut smooth grooves in the anchor bolt at the concrete embedment line, forcing a steel shear ductile failure. That would eliminate the need for the 2.5 over-strength factor, resulting in a per bolt capacity of 1405 pounds. Too bad I would have to actually weaken the anchor bolts in order to get the higher capacity out of them.

What do you think?

 

[OldPaperMaker]

JRESE,
I have done the same calcs and arrive at the same concrete breakout strength (after the phi=0.65 and the additional 0.75) of 2690 lbs. For the shear on the steel, using f=60,000 psi & a root area of the anchor bolt of 0.226 sq inches (times the 0.65 & 0.75) I get 6610 lbs.

I wonder if the IBC code mavens intensions in modifying section 1908.1.16, D.3.3.4 was to overide the concrete breakout value with the value for the shear in the steel in the anchor bolt value.

I looked at the example on page 145 & 146 of the "2006 IBC Structural/Seismic Design manual, vol.2" & notice that they just used the NDS wood value for picking the size & spacing of the anchor bolts.

I hope that someone else will join in with their thoughts on this subject.

 

[skier1578]

The topic seems to have many opinions, but one opinion (that I agree with) is that the wood sill plate will act as a fuse. The anchor will "slip" within the plate before the concrete breaks, creating a ductile failure (CBC1908.1.16, D3.3.5).

So far, some building officials accept this argument, and some don't.

Good luck.

 

[msquared48]

In thinking a little further here, this seems tanamount to the "shear friction" concept here in that the plate would have to lift up from the concrete to slide or "fail". This local uplift is resisted primarily by the anchor bolts, some by the shearwall holddowns. The more dead load the shearwall sees, the more resistance to local uplift.

Comments?

Mike McCann
MMC Engineering