December_1970 — Page 35

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Vibrations in buildings

This second and final part in a feature by Britain's Building Re- search Station discusses some of the ways in which nuisance from vibration can be avoided or reduced to a tolerable level - by treat- ment at source, by reducing the transmission of vibration and by providing protection against vibration from an external source

VIBRATION can usually be isolated or reduced to a tolerable level by fairly simple and relatively inexpensive means if the precautions are incor- porated in the initial design. It is likely to be more difficult and expensive to provide a remedy in existing buildings and installations.

The ways in which nuisance from vibrations can be treated include: a, re- duction of the vibration at its source; b. reduction of the vibration transmit- ted to the immediate surroundings of the source; c. placing machines that cause vibration as far as possible from people and equipment likely to be affected; d. protecting sensitive ap- paratus against external vibration; e. protecting a whole building or a part of it against external vibration.

It may be necessary to apply two or more of these measures to any one problem. For example, if a alone is unlikely to give the required improve- ment, then a and b, a, b and c, or other combinations may prove to pro- vide the answer.

Reducing vibration at source can be done by the maker of the machine by dynamic balancing of the moving parts so far as this is possible. An alteration in the speed of the machine may be effective in some cases, parti- cularly if a state of resonance would otherwise exist between the machine and the structure.

From machines

The transmission of vibration from machines may be reduced by various

means:

By mounting the machine on a heavy base, of weight comparable to that of the machine, resting directly on the ground; the vibrations are then reduced by the increase in mass.

By placing an insulating mounting between the machine and a slab of concrete resting on the ground; this is particularly suitable for low or medium power machines that are not

Far East BUILDER, December 1970

likely to generate very low frequency vibrations.

By the combined use of a heavy base and an insulating mounting.

By the combined use of a heavy base and an insulating mounting sunk into a pit. The insulation may be either above or below the machine base. An air space should be left all round the block but the gaps may be sealed at the top by a strip of flexible material to prevent ingress of debris.

The insulating material may be of cork, felt, rubber or composite mate- rials; steel springs are sometimes used. Each material has its advantages and disadvantages for any given conditions and the choice is a matter for specialist advice. Some firms deal only with a particular material or type of unit but others will advise on the type most suitable for a given installation.

In many cases a compromise solu- tion may have to be accepted. For example, bolting a machine tightly to a rigid foundation may prevent vibra- tion of the machine but may transmit harmful vibrations elsewhere. On the other hand, a very flexible mounting, while it may greatly reduce the trans- mitted vibration, may result in a large increase in amplitude of movement of the machine which may be incon- venient to the operator, or even harm. ful to the machine.

A satisfactory result is likely when the natural frequency of the spring- supported mass is no higher than about one-third of the forcing frequen- cy, the latter depending on the rate of operation of the machine. If the re- quired natural frequency can be deter- mined the required static deflection can be found and, hence, the stiffness of the spring or mounting. The sug- gested frequency ratio of 3 to 1 would, theoretically, reduce the trans- mitted vibration amplitude by more than 80 per cent; a higher value of fre- quency ratio would achieve a still greater reduction.

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PART II

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