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RE: Foundation A.B. Capacity

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When you are dealing with an embedment in a narrow member, the controlling
failure mode is generally splitting, not the formation of a failure cone.
The code doesn't recognize this failure mode (yet) but is probably
conservative enough to cover you in most instances.  In your case, a 1-1/4"
anchor bolt in an 8-inch stem wall seems a little out of proportion, and I
would agree with the suggestion that you check just how much foundation mass
you can mobilize to resist the uplift.  The flexural reinforcement that you
put in the foundation to accommodate the negative moment will also help for
the splitting.  Think a little about how much stiffness you expect from this
connection.  If the connection and foundation are out of proportion to the
load you expect them to resist, you are unlikely to have the required
stiffness to attract that load in the first place.  This implies that the
load will go elsewhere (or, if you're lucky, may not be generated in the
first place...).
I haven't tried to re-create your calc that gave you diminishing returns for
increasing embedment, but it's not hard to see how this comes about.  The
increased "cone" area that you calculate cannot keep up with the reduction
for near edges, which increases as you increase the embedment.
Unfortunately, a method which works pretty well for headed studs embedded in
unreinforced concrete where edges and spacing are not dominant falls apart
when the failure mode anticipated is no longer at issue.
If you design a rebar embedment, you must be able to assure yourself that
you are transferring the load to other rebar directly, i.e., as a lap
splice.  Again, you need transverse reinforcement to carry the attendant
splitting (hoop) stresses.  If you design the rebar as a dowel, then the
rules you would apply to a discrete anchor bolt apply.


J. Silva, SE
Hilti, Inc.

> ----------
> From: 	Jake Watson[SMTP:jwatson(--nospam--at)]
> Reply To: 	seaint(--nospam--at)
> Sent: 	Tuesday, July 20, 1999 5:25 PM
> To: 	seaint(--nospam--at)
> Subject: 	Foundation A.B. Capacity
> I am trying to find the capacity of an anchor bolt in a residential
> foundation wall.  The case has a large uplift do to a "Simpson" holdown
> (about 15k service loading). It is a traditional 8" foundation wall with
> a simpson HD15 anchor that requires a 1.25" dia anchor bolt.  Simpson
> doesn't make a corresponding SSTB.  I tried to design an anchor bolt
> using the provisions of ch19, div III but am very confused.  According
> to the provisions, I first calculate the projected area area at the top
> of the wall (Ap) then reduce according to edge distance.  The reduction
> amounts to edge distance divided by embedment length.  I have two close
> edges (4") so I squared the reduction.  After doing all this, the UBC is
> trying to tell me that a 30" long A.B. is weaker than a 12" long A.B. 
> If someone could clarify these provisions, I would be very grateful.
> P.S. Here's the fun part, if I design it at as a rebar, then I have more
> than 10x the capacity of an A.B.
> Suggestions?
> Thanks in advance, 
>         Jake Watson, E.I.T
>         Salt Lake City, UT