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RE: Is there a better way (2x4 SW Anchor Bolts to comply with ACI 318-05 Appendix D)?

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Hi, Freeman.

 

I’m not yet familiar with the new code; maybe you can clear something up for me. The breakout strength you’re talking about, is that a shear failure mode? If so, is the shear failure parallel with the edge? If it’s shear perpendicular to the edge, that’s not a critical condition for a shear wall, is it?

 

In the “good old days”, all you had to check was the shear capacity of the anchor in wood section (parallel to grain), in concrete, again no edge distance issues for loads parallel to the edge and then shear through the bolt section itself. Using Table 19-D, for a 5/8” diameter bolt the minimum edge distance for full capacity is 3-3/4” and can be reduced to half for half capacity. Half of 3-3/4” is almost 1-3/4” (within 1/8” anyway) and I’m thinking this edge distance is for load perpendicular to the edge anyway.

 

If you could clarify this area for me, I would be most appreciative.

 

I would caution about moving the bolt away from the edge towards the inside face of stud. I believe plate washers are still required in the 2007 CBC, right?

 

I think we are seeing just the tip of the iceberg…

 

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

ALLEN DESIGNS

Consulting Structural Engineers
 
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: Freeman C. Tang [mailto:ftang06(--nospam--at)yahoo.com]
Sent: Wednesday, December 12, 2007 11:32 AM
To:
seaint(--nospam--at)seaint.org
Cc: Saeed Bekam
Subject: Is there a better way (2x4 SW Anchor Bolts to comply with ACI 318-05 Appendix D)?

 

Hello all,

 

Please bear with me; I know it is a little tedious! 

 

I am revising the anchor bolt spacings of my typical wood framed SW schedule and it seems to me that 5/8" dia. ABs won't work anymore for exterior wood SW with working stress load higher than 437 plf.

 

Per CBC 2007 1908.1.16 D3.3.5, which is basically ACI 318-05 Appendix D requirements with amendments, anchors in concrete shall be designed to be governed by:

1) Ductile (tensile or shear) yielding of steel element, or

2) Ductile yielding of the attachment that the anchor is connecting to the concrete structure, or

3) The minimum design strength of the anchors shall be at least 2.5 times the factored forces transmitted by the attachment.

 

For a 2x4 exterior SW with 5/8" dia. anchor bolts, 7" min. embedded into concrete, at 1.75" (at center of sill) from the concrete foundation edge for a typical tract house, the anchor bolt concrete breakout strength (with f'c = 2500 psi) that I have calculated is 1019 lbs (accounting for all the reduction factors, edge distance, no supplementary reinforcement, no edge bars, strength reduction factor, and seismic design category D reduction factor).

The anchor bolt concrete pryout strength is still high (8994 lbs) and won't govern.

The anchor bolt steel strength is 3173 lbs. (also accounting for all the reduction factors)

The nominal strength an anchor bolt in 2x4 sill plate based on NDS 2005 Table 11.E and Tables in Appendix N for LRFD conversion is 930x2.16 = 2009 lbs.

 

It indicates that the anchor bolt concrete breakout strength of 1019 lbs will govern, ductile yielding of either the steel anchor bolt or the 2x sill attachment can not be achieved, and therefore I have to design the strength of the anchor bolts to 2.5 times the factored forces transmitted. For a lightly loaded wood SW with only 350 plf at working stress level, the anchor bolts need to be designed for 350x1.4x2.5= 1225 plf, which means at 8" o.c. (1019x12/8 = 1529 plf.). Any wood SW with a working stress load of 437 plf., then 5/8" dia anchor bolts won't work!

 

Adding edge rebars will increase the anchor bolt concrete breakout strength about 20% but there is no room to put even a #4 horizontal to maintain the minimum 1.5" concrete cover. I don't want to use 2x6 walls to increase the edge distance to 2.75" for a higher anchor bolt breakout strength if I don't really have to. Increasing the anchor bolt size will not help too much either because the 1.75" edge distance basically limits the anchor bolt concrete breakout strength. I can use (MAS) mud sill anchors even at 8" o.c.with 1080 plf. allowable load, which is actually spaced too close already, but the MAS won't be good for double-sided SWs with the load of as high as 1740 plf.

 

I wonder if anyone has come across a similar situation.   Is there any better way to comply with the current ACI 318-05 Appendix D requirements for wood framed SW anchor bolts?  Am I overlooking anything?  Any comments and suggestions will greatly be appreciated.

Thanks

 

Freeman Tang, S.E.

Gouvis Engineering, CA

 


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