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RE: Grade @ Anchor Bolts

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ACI 349 Appendix B, which started most of the ductile headed bolt design,
gives specific details for size of anchor head to initiate the shear cone.
Section B.4.5.2 requires the "bearing area of the anchor head (excluding
the area of the tensile stress component) is at least 1.5 times the area of
the tensile stress component".  If you work out the math, most heavy hex
heads and nuts meet this requirement.  ACI 349 Appendix B commentary goes
on to state "Failure is initiated at the outside periphery of the
mechanical anchor (anchor head) and therefore the area of the anchor head
does not contribute to the pullout strength of the concrete and should be
subtracted from the projected tensile stress area in computing pullout
strength.  Thus, it is desirable to keep the effective size of anchor head
as small as possible to reduce embedment requirements.  Tests have shown
that the head of a standard anchor bolt or stud, without a plate or washer,
is sufficient bearing area to fully develop the tensile strength of

Thomas Hunt, S.E.
Duke/Fluor Daniel

---------------------- Forwarded by Tom Hunt/DFD on 08/21/2000 07:26 AM

Charlie Carter <carter(--nospam--at)> on 08/18/2000 02:33:07 PM

Please respond to seaint(--nospam--at)

To:   "'seaint(--nospam--at)'" <seaint(--nospam--at)>

Subject:  RE: Grade @ Anchor Bolts

>Does the "chunk" behavior apply just to multiple anchor bolts connected to
>single plate or would it also apply to anchor bolts each having its own
>individual plate (such as a plate washer) acting in group behavior?

It applies to both. The single plate spanning all anchor rods would
probably represent the worst-case situation. For the case of rods with
individual washers, it's similar but less drastic. Here's what I mean.

Start with a group of rods (without washers) placed so that the cones
representing the pull-out failure for each rod in the concrete intersect.
Now, add plate washers and gradually increase their size. As the size
increases, the failure cones intersect more quickly (closer to the bottom
of the embedments). The larger the plate, the less surface area you get on
the intersecting cones. Eventually, the individual washers approach the
case of a continuous single plate as their size is increased.

So really, the maximum cone pull-out strength is attained with the minimum
plate washer size, which is no washer (i.e., just the head or nut). So then
the question is, can the head or nut alone provide the necessary bearing
area. Research (see Marsh and Burdette "Anchorage of Steel Building
Components to Concrete" in the 1st Quarter 1985 AISC Engineering Journal)
has shown that a head or nut is sufficient to develop the necessary


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