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Re: Anchorage to concrete

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John,

Thanks for your responce,  that really helps my understanding that we are again trying to insure ductility in the connection and avoid brittle failure.  Is there an article on the CCD method you refered to ?

Robert Shaffer,  P.E.
Santa Cruz, CA

"Silva John (sj)" wrote:

Robert,

Let me try this one.  The idea of requiring an l/d of 8 was to force some
measure of ductility, rightly or wrongly, into the anchorage.  It was
assumed that for many cases, l/d = 8 would ensure steel rupture, and this
should be encouraged by punishing shallower embedments.

In many cases, (stem wall anchors, for instance), the concrete capacity is
severely limited by the geometry of the anchorage, and l/d = 8 cannot ensure
steel failure at ultimate.  It is important to note, however, that if the
ultimate capacity is limited by the concrete, increasing the bolt diameter
will not increase the anchorage capacity, although it may increase the
initial stiffness.

e.g., assuming you are not limited by near edges or adjacent anchors, and
looking simply at tension, your 3/4" anchor (let's assume a standard hex
A307 bolt) with 6" of embedment would develop a concrete capacity (concrete
cone failure mode) in nominal 2000 psi concrete of approximately 26 kips.
The tensile strength of the bolt at the threads (assuming an overstrength
factor of 1.25) is about 25 kips.  The tensile strength of a 7/8" A307 bolt
is 33 kips.  Increasing the bolt diameter without increasing the embedment
doesn't do much for you in this case.

With respect to your recommendation for designing embedments in stem walls
or narrow foundations, I would agree that the use of equations based on a
breakout cone failure for this case is inaccurate (this was the subject of a
previous thread, as I recall).  Splitting will be the controlling failure
mode for the concrete in many cases, and the equations for development
length are based (at least in part) on this failure mode.

(The original post indicated reducing the embedment of the anchor bolt to
2.75" to avoid an edge distance reduction.  I would strongly recommend
against this line of reasoning...)

Apropos strength design of embedments, I would recommend becoming familiar
with the method adopted for the IBC 2000 (the so-called CCD method).  It is
reasonably comprehensive and provides guidance for a wide variety of loading
conditions.

Regards,

John Silva