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ANCHORAGE TO CONCRETE

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In my original post, I was trying to justify the anchorage that so many
residential contractors in this area (Tucson, AZ)use for hold down anchor
bolts in a concrete stem wall for 6" stud walls. I was attempting to justify
it with a simple truncated cone on a j-bolt.  I now understand that the cone
failure may not be very applicable for a j-bolt.

In my simple analysis I was not proposing to reduce the emedment length from
12 inches to 2.75".  I was only basing the failure cone to be that equal to
a 2.75 inch embedment since the edge distance for a j-bolt embedded 12
inches is only 2.75 inches on each side for an 6 inch stem wall. This was
not adequate to develop the published load of a Simpson HD2A with a 5/8"
anchor bolt much less develop enough to ensure a ductile failure of the
bolt.  (All they are usually trying to develop is the 1800# uplift for the
prescriptive braced wall panel required in UBC '94 2326.11.4).  I was
looking for justification to use more surface area of the cone failure with
12 inches of embedment; however, it appears that I need to look at a
different failure mode for a j-bolt.

Does the CCD method that John Silva referenced in the IBC 2000 address a
smooth j-bolt (that is typically used) in a narrow stem wall.  Has anyone
calculated the allowable service load for a 5/8" diameter j-bolt with 12
inches of embedment in a 6 inch stem wall.



From: "Silva John (sj)" <Silva(--nospam--at)hilti.com>
To: seaint(--nospam--at)seaint.org
Subject: RE: Anchorage to concrete

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