In his email of 3/24/200, regarding the lateral design force requirements for
suspended equipment items, several inaccurate statements were presented which
need to be clarified.
Although footnote 14 of UBC Table 16-O specifies special a-sub-p and R-sub-p
values where vibration isolators are used, the upper force limits of Equation
32-3 would still apply. That is, Fp = 4.0 Ca Ip Wp is the maximum, or for the
example, 4.0 x 0.44 x 1.0 x 0.8 = 1.4 kips.
As Mr. Searer correctly noted, Footnote 14 also requires that if the equipment
is supported by shallow expansion anchors, the required design force (for the
anchors only) would need to be multiplied by 2.0. Mr. Searer also stated that
the allowable strength for the anchors have a factor of safety of 3.0. This
"allowable strength" is an ASD value, therefore the required design force
against the anchors could be further reduced by a factor of 1.4 into an
equivalent ASD value, when comparing against the published "allowable loads".
Impact forces caused by vibration-supported equipment impacting against
restraints is a real phenomenon, to which expansion anchors are especially
vulnerable. Also, cyclic-load test data for expansion anchors did not exist at
the time these code provisions were written - only now are we beginning to see
this data. However, there will likely always be some added requirements against
expansion anchors for impact-type load conditions.