Please bear with me, I know it is a little tedious!
I am revising the AB's spacing
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,
minimun design strength of the anchors shall be at least 2.5 times the factored
forces transmitted by the attachment. (this is one of the
For a 2x4 exterior SW with
5/8" dia. anchor bolt at 1.75" (at center of sill) from the concrete
foundation edge for a typical tract house, the anchor bolts 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 very high (8994 lbs) and won't govern.
The steel strength of
the anchor bolt is 3173 lbs. (also accounting for all the
The nominal strength of the anchor
bolt in 2x4 sill plate based on NDS 2005 table 11.E and tables
in Appendix N for LRFD conversion is 930x2.16 =
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, therefore I have to design
the strength of the anchor bolt 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 bolt
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 minimun 1.5" concrete
cover. I don't want to use 2x6 wall 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 anchor 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.
wonder if anyone has a similar situation? am I overlooking anything?
Is there any better way to deal with this ACI 318-05 Appendix D
requirements for wood framed SW anchor bolts? Any comments and suggestions
will greatly be appreciated.
Freeman Tang, S.E.