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RE: International Conference on Vibrations[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "INTERNET:seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
- Subject: RE: International Conference on Vibrations
- From: Mark Gilligan <MarkKGilligan(--nospam--at)compuserve.com>
- Date: Wed, 10 Nov 1999 01:06:21 -0500
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
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