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Re: Foundation Vibration Design

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> Wouldn't it be better to keep the foundation block soil system frequency
> GREATER than the equipment frequency at all times? I look at it this way:
> When the equipment is not running, it's frequency is zero. Turn it
> on, and the frequency will vary from 0 to its maximum natural
> frequency. If the Foundatiuon system frequnecy is less than the
> Equipment frequency, at some point the two will be equal. Then I
> would think we would have some amplification (a la the Tacoma
> Narrows bridge).

At first glance this would appear to be the case.  However, there 
are good reasons why high tuning is not always the best approach.

1) For a single-degree-of-freedom system, anytime that the forcing 
frequency is less than the natural frequency the dynamic 
amplification factor (DAF) is greater than 1.0.  But when the forcing 
frequency is at least twice the natural frequency, the DAF is only 
1/3; that is, the dynamic response is considerably LESS THAN the 
static response.

2) For equipment with high operating frequencies (or on soft soil 
conditions) it is not practical (and is, in fact, sometimes 
impossible) to provide a foundation system with a natural frequency 
higher than the forcing frequency.

If the equipment has a high steady-state operating frequency and 
fairly quick startup, resonance of a low-tuned system is usually not 
a problem.  Conditions to watch out for with low-tuned foundation (or 
structural) systems are:

1) Equipment that can have steady-state operation at lower 
frequencies.

2) Equipment with very slow startup (from zero to operating 
frequency).  Several good dynamics texts address how the number 
of cycles at resonance plays a part in the problem.

3) Multiple-degree-of-freedom systems might experience resonance at 
the higher operating frequency due to higher mode participation.  [So 
be sure to check modal separation at higher modes.]

Time-history analysis can be used to investigate startup behavior, 
higher mode effects, multiple excitations, etc.

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201