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RE: base plates subject to uplift

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Sherman Williams wrote in response to Charlie Carter:

<Charlie, I am curious as to why AISC feels that the "usual application" of
a column base plate is to transfer only compression. In addition to uplift
or moment, many base plates transfer shear forces. Thus, an oversized hole
creates problems for load transfer under many conditions. It seems that
AISC should address these situations as well as the "simple compression"
case.>

This comment raises the question as to how shear forces actually get from the
column or brace into the foundation.  There are several possible mechanisms: 1)
friction between base plate and supporting grout or concrete, 2) bearing between
base plates holes and anchor rods, 3) shear keys, 4) bearing between far edge of
base plate and supporting grout or concrete.

Mechanism 1) is probably the initial load path, especially if the anchor bolts
have been pretensioned.  Unless the shear force is accompanied by enough tension
and or overturning moment to completely "uplift" the base plate, this mechanism
will probably resist the entire shear force.  However friction cannot be
considered when resisting code earthquake loads, and another design calculation
method must be utilized.

Mechanism 2) is usually considered in design and is probably sufficient
consideration for light shear loads.  It represents the shear limit state if the
base plate has overcome friction and has displaced relative to the anchor rods.
The anchor rods are usually checked for combined shear and tension.  you could
also check the anchor rods for bearing, but usually the base plates are so thick
that this is not a problem.

Mechanism 3) should be considered for heavy shear loads, although welding and
construction issues are raised.  If a shear key is used, it is probably both the
initial load path and the shear limit state.  If tension and/or overturning
loads are present, anchor rods need to also be provided to resist tension
forces.

Mechanism 4) requires base plate bending and/or resisting to mobilize and should
not be relied on.

In summary, I don't think that the oversize holes cause a problem in shear
transfer.  Most of the shear force will probably transfer in friction, even if
the codes do not acknowledge it.  If all of the anchor rods were perfectly
centered in the base plate holes (not very likely), the column base plate would
have to laterally displace half the annular space to mobilize the bearing limit
state.  Even with the AISC oversize holes, this distance is not very much.  Of
course in the real world, where all of the anchor rods are not centered, the
travel distance will be much less before the first rod engages.

Rick Drake, SE
Fluor Daniel, Aliso Viejo