SED 3e (ePDF) VIBRATIONS IN STRUCTURES

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Vibrations in Structures

Induced by Man and Machines

«Vibrations in Structures» concentrates on vibrations in structures as excited by human motion or machine operation. Man-induced vibrations may arise from walking, running, skipping, dancing, etc. They occur mostly in pedestrian structures, office buildings, gym­nasia and sports halls, dancing and concert halls, stadia, etc. Existing publications treat by and large some isolated aspects of the problem; the present one attempts, for the first time, a systematic survey of man-induced vibrations. Machine-induced vibrations occur during the operation of all sorts of machinery and tools with rotating, oscillating or thrusting parts. The study concentrates rather on small and medium size machinery placed on floors of industrial buildings and creating a potential source of undesirable vibrations. The associ­ated questions have rarely been tackled to date; they entail problems similar to those of man-induced vibrations.

The book is consciously intended to serve the practising structural engineer and not primar­ily the dynamic specialist. It should be noted that its aim is not to provide directions on how to perform comprehensive dynamic computations. Instead, it attempts the following:

  1. to show where dynamic problems could occur and where a word of caution is good advice;
  2. to further the understanding of the phenomena encountered as well as of the underlying principles;
  3. to impart the basic knowledge for assessing the dynamic behaviour of the structures or structural elements;
  4. to describe suitable measures, both preventive to be applied in the design stage and remedial in the case of rehabilitation.

ISBN 3-85748-052-X


Description

Increasingly, the practising engineer sees himself confronted with the effects of loads on a structure which vary with time, that is to say, which are dynamic by nature. The following are some of the main reasons:

– Structures planned or constructed today are often more prone to vibrations than those of former years. Because of the use of high-quality material and the exploitation of their strength, larger and larger spans are chosen while the thicknesses of beams and slabs have kept decreasing. Hence, of the total load – mostly assumed to be non-dynamic – the dead or permanent load constitutes a smaller part. Slender structures with a small stiffness and a low mass are more prone to vibrations than more squat and stiffer structures with a larger mass. If such a modern-type structure is exposed to dynamic loads, considerable vibration problems can arise.
– Several dynamic actions have increased in intensity latterly. For example, rationali­zation has led to the installation of more efficient machinery in manufacturing plants. Efficiency is enhanced by, for instance, a higher production frequency which may raise the demands on the structure to sustain higher dynamic loads.
– Also, the demands on a structure for serviceability to the user have been increased. The environmental stress level being generally higher, people have become more sensitive to vibrations. Therefore, normative values for admissible vibrations are becoming stricter, and vibration amplitudes must be reduced. There is also a higher demand from the point of the precision of certain manufacturing processes (e.g. milling, weaving) requiring a lower vibration level of their machines.

Apart from this, new engineering tasks have called for a more exact consideration of loading types and of the dynamic behaviour of certain structures. This has been the case, for example, with offshore structures subject to high dynamic wave loads and with nuclear power plants required to remain safe under exceptional loading such as earthquakes. These kinds of problems, however, have evolved into comprehensive subjects of their own and are not treated here.
This book is more or less limited to vibrations of structures as excited by human motion or machine operation. Man-induced vibrations may arise from walking, running, skipping, dancing, etc. They occur mostly in pedestrian structures and particular buildings, such as office buildings, gymnasia and sports halls, dancing and concert halls, spectator galleries, etc. Existing publications treat by and large some isolated aspects of the problem; the present one attempts a systematic survey. Machine-induced vibrations occur during the operation of all sorts of machinery and tools with rotating, oscillating or thrusting parts. A substantial literature can be found on machine foundations, concerned with the support of mostly heavy machines on purpose-built substructures. The present treatise concentrates rather on small and medium size machinery placed on ordinary floors of industrial buildings and creating a source of undesirable vibrations. The associated questions have rarely been tackled to date. They entail problems similar to those of man-induced vibra­tions. Questions beyond this range, such as, for instance, machine-induced ground vibrations interacting with adjacent buildings or affecting other parts of the structure, are excluded.