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RE: Is there a better way (2x4 SW Anchor Bolts to comply with ACI 318-05 Appendix D)?
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Is there a better way (2x4 SW Anchor Bolts to comply with ACI 318-05 Appendix D)?
- From: "Gaines, David" <David.Gaines(--nospam--at)hdrinc.com>
- Date: Wed, 12 Dec 2007 14:13:36 -0600
- Cc: "Saeed Bekam" <sbekam(--nospam--at)gouvisgroup.com>
For shear walls over 350 plf you need to install 3x sill
plates. Try putting the anchor bolt inside of center on the sill plate when
you're at the edge of slab. You'll need 1&1/2 bolt diameters minimum to the
edge of the wood. By providing 1&1/4" of edge distance to the bolt in wood
you achieve 2&1/4" edge distance at the edge of slab. The rest of this
write up as it pertains to anchors is interesting. Thanks for
COMPANY | Many
251 S. Lake Ave, Suite
Pasadena, CA 91101
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
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
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
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.
Freeman Tang, S.E.
Gouvis Engineering, CA
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