From: Gerard Madden, SE
Sent: Wednesday, December 12, 2007
Subject: Re: Is there a
better way (2x4 SW Anchor Bolts to comply with ACI 318-05 Appendix
On Dec 12, 2007 11:32 AM, Freeman C. Tang <ftang06(--nospam--at)yahoo.com>
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
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
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
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.