Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: International Conference on Vibrations

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Sherman


 I hope that these comments are of use.


<<Are there any rules of thumb on when a structural vibration analysis
needs to be done or can be waived, say for a steel support frame, based on
readily available equipment data?  (equipment weight, RPM, centrifugal vs
reciprocating, etc)>>

I would expect the rules of thumb to be highly related to the types of
construction that you do.

. << Typically I don't know the "disturbing force" magnitude, amplitude,
etc.>>

The magnitude of the disturbing force is often not as critical as the
location of the disturbing force.  If you locate a disturbing force at the
right location and if the disturbing force matches the natural frequency of
the structure you will find that the magnitude is not a issue.  Engineers
have excited the primary natural frequency of a high rise by running back
and forth on the roof.

<<  And it is usually quite difficult to achieve
required stiffness to provide a high natural frequency of a support for
rotating equipment.>>

An common option to stiffening the structure is to place vibration
isolators under the equipment.

 <<When can design be based on an increased load (such as
impact percentages per AISC for supports for machinery) in lieu of a
frequency analysis? >>

Unless you understand the natural frequencies just increasing the stiffness
will not necessarily solve the problem.

<<2. Does vibration excite all vibration modes/directions or only
directions
associated with the direction of the disturbing force?  For centrifugal
equipment (pumps, fans, etc), must vibration be considered on all principal
axes, i.e. vertical and two lateral directions for a support frame?>>

Each mode has a unique shape and when the mode is excited every point on
the structure will vibrate about the undeflected shape proportional to the
modal ordinates.  Thus if you excite a continious structure structure in a
horizontal direction you will  typically see movement in directions normal
to the exciting force.  

You can only excite modes that have modal ordinates parallel to the
exciting motion and that also have a non-zero modal participation factor
associated with the direction of excitation.

<< A vibration analysis appears reasonable for this case as the pumps are
fairly
heavy, but I haven't come across any good published rules of thumb for the
above questions.>>  

You might find the services of a vibration consultant to be usefull.  They
may be able to suggest an alternate approach that will save time designing
to tight criteria or help to reduce cost of construction.  Often an
Acoustical / vibration consultant is hired to work with the engineers
designing the equipment.

The question of what is an acceptable magnitude of vibration that is
acceptable needs to be addressed.  The problem is that there are few clean
answers since you are dealing with individuals perception.  There is a lot
of literature on this subject but it probably takes somebody who has worked
with these problems before to interpret the data and make recommencations.

A good review of a text on dynamics might answer some of these questions
and provide some insight into what the problems are.

Mark Gilligan