Harold Sprague wrote:
> The only thing similar to a rule of thumb is contained in ACI 350 Section
> 2.9 - Impact, vibration, torque, and seismic loads (only 3 pages long).
> There is Table 2.9.2(a) "Natural frequencies of beams" and Table 2.9.2(b)
> "Recommended maximum structural deflection for given equipment operating
ACI 350 does provide some useful formulas for vibration design - however
these are basically just modifications of the natural frequency formula
using f=((g/delta)^1/2)/2*pi. These formulas would still require very stiff
support structures for even minor vibrating equipment.
The problem frequently arises when the frequency of a piece of equipment
is not less than 1.5x the structure natural frequency. But if it is not a
large or heavy piece of equipment, it becomes questionable as to whether
the extra cost of stiffening the structure is cost effective. The only
rule of thumb I have come across is that electrical motors are generally
a well balanced piece of equipment and generally should not require
design for an unbalanced condition.
Another design method commonly used is inclusion of an "impact factor" such
as specified in AISC ASD A4.2. (i.e., 20 percent impact "for supports of
light machinery, shaft or motor driven" and 50 percent impact "for supports
of reciprocating machinery or power driven units"). Again, no limits are
given as to what constitutes "light machinery" or when a full dynamic
analysis is needed. Ultimately it comes down to "engineering judgement" but
different engineers can come up with different answers.