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- [Bachmann-1987] Vibrations in structures Induced by man and machines.pdf
- Flow-Induced Vibrations
- Vibrations in structures induced by man and machines
- Vibrations in structures
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[Bachmann-1987] Vibrations in structures Induced by man and machines.pdf
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Reinforced and Prestressed Concrete 3rd-Edition. Human-induced vibration of steel structures Hivoss. Jump to Page. Search inside document. No part of this book may be reproducedin any formor by any means, electronicor mechanical, including photocopying, recording, or by any information storage andretricvalsysicm, without permission in writing ftom the publisher. ISBN Skipping a General characterization b Load-time function c Tnfluence of the number of people d Deliberate excitation 2.
Gymnasia and sports halls a Frequency tuning b Calculation of a forced vibration c Special measures 2. In practice, however, slowly varying loads can be treated as quasi-static since the inertia and damping forces are negligible. The presence of inertia and damping forces is in fact the important distinction between dynamic and static loading.
These forces arise from the accelerations and velocities induced in the structural member, and they have to be included in the calculation of stress resultants and support reactions.
The magnitude of these forces and their funetion against time depend both on the kind of excitation from outside and on the intrinsic dynamic behaviour of the structural member, i. According to their time function, dynamic loads can be categorized Figs. Additional aspects and criteria, respectively, are the number of load cycles, the ensuing loading or strain rate in the affected member, the probability of occurrence of the load or the peak values it may attain Tab.
They affect the structure for a sufficiently long time to permit a steady-state vibration response Fig. They may be caused by — machines with synchronously rotating masses which are slightly out of balance e. Again, the duration of the load is long enough for steady-state response to develop. Periodic loads can also be caused by wind, such as by vortex shedding, even though its general nature is transient.
Also the duration of the load is arbitrary Fig, 1. Their duration, however, is so short that the structural member affected reacts in quite a different manner Fig. Characterization of Dynamic Loads 5 1. Effecis on Structures A structure can be affected in both its load-bearing capacity and its serviceability. Design against fatigue, as it is called, does not just depend on the typeof loading, but also on the planned or expected life time of the structure.
Distress due to large stress excursions, of which a few are sufficient to cause collapse, is termed low- cycle fatigue Tab. For steel and concrete, the usual construction materials in civil engineering, the strain-rate effect is mostly beneficial to the load- bearing capacity of the structure.
Impairment of the serviceability entails damage mostly to nonstructural elements of a building, such as cracks in partitions, loss of cladding, etc. Effects on People The serviceability is impaired when the vibration of the building under dynamic load causes disturbance and discomfort to the occupants. Secondary vibration of the installations may hinder the production process, affect the proper functioning of machines, and cause associated material-tech- nological problems.
They can be periodic in nature e. The resulting vibrations can lead to the following forms of distress: overstressing of the structure, in extreme cases to the loss of structural integrity damage to nonstructural elements e.
Again, avoidance of resonance is the primary precaution, with provision of additional damping to suppress remaining vibrations. The resulting structural vibrations interfere with the wind flow leading to motion-induced wind loads, which may enhance the vibrations.
Over a longer period of time clastic strain energy accumulates in a fault zone, often of historic origin, until the rock strength in the critical direction isreached. During the rupture a substantial part of the energy released isradiated as kinetic energy in the form of seismic waves.
They induce in buildings both horizontal and vertical motions. To contzol such damaging effects, the precautions have to include — the choice of a suitable structural system with emphasis on symmetry — appropriate seismic design loads for the structure — careful detailing of structural members and nonstructural elements.
They occur in a directly loaded structure e. The vibrations can be kept within bounds by using a smaller pile hammer adjusting the frequency of the vibrator choice of an alternative method of construction e. While the local behaviour refers to influences in the zone of impact, the global behaviour describes the loading and vibrations spreading through individual structural members or the structure as a whole, Depending on the deformability of both the impacting object and the part of the structure directly affected, the impact may be characterized as hard or soft 12].
Impact loads may result from — vehicles crashing into piers, parapets, ete. The effect can also be one of a sudden load removal. Under these circumstances — often a direct consequence of an external impact -, the subsequent collapse of neighbouring structural members or the progressive collapse of the structure as a whole can only be prevented if these members have the ability to redistribute the forces quickly and to sustain the resulting stresses. Apart from protective measures against impact e.
Structures intended for moderate live loads of an assumed static nature are often designed with rather slender dimensions, the possibility that dynamic loading might govern the design being overlooked or underestimated. Another factor is the advent of new kinds of user activities such as fitness classes to the accompaniment of strongly rhythmic music in gymnasia, which may bring about a considerable dynamic loading.
Although overloading of the structure itself is not the primary concern, secondary building elements e. They may fee! It then shows their effects. The countermeasures are classified according to categories of structures in which man-induced vibrations are to be expected. The loads may be of periodic or transient nature Fig. Of course, this is just a crude categorization. Other forms of motion such as rhythmic skipping during fitness classes, jazz dance sessions, foot stamping, hand clapping and body rocking at a concert, etc.
Transient Joads result primarily from a jolting motion imparting a single impulse to a structural member e. A closer look at the time function of the periodic loading reveals that considerable forces are not just transmitted in the actual frequency of the walking, skipping or dancing rhythm, but also in the frequencies of upper and lower harmonics.
A systematic categorization of man-induced loading is difficult. Apart from the aforementioned factors, the number, for example, of the people involved in the excitation plays a role as well. The following section deals first with periodic loads due to walking and running, skipping and dancing.
Then the transient loads caused by a jolling motion are briefly discussed Section 2. Pacing Rate The pacing rate f, dominates the resulting dynamic load. It is sometimes given as footfalls per second FFs , but its nature as loading frequency is more adequately expres- sed in Hz. For normal walk on horizontal surface, both Matsumoto [2.
Assuming a Gaussian normal distribution around the mean of 2. Kramer [2. For sprinting it may be as high as 5. On public pedestrian structures, however, pacing rates above 3. Forward Speed The speed or velocity of pedestrian propagation v, is coupled with the pacing rate f, through the stride length 1,. Naturally, different people may possess quite different stride lengths and paces for the same forward speed.
Figure 2. The main parameters affecting the load-time function are the following see also [2. As Fig. The weight of the test person was N. This feature disappears with increasing pacing rate and degenerates to a single maximum of sharp rise and descent when the person is running.
From low to high pacing rates, the width of the signal decreases, and the load maximum increases. While for strolling with a frequency below 1 Hz the maximum load hardly exceeds the weight of the person, it increases by a quarter or a third for 2 Hz and by a half around 2. For relatively high pacing rates above ca. For fast running the maximum load can increase to three times the weight Fig. The time function given in [2. In contrast to the behaviour during walking, ground contact during running is interrup- ted, the ratio of contact duration to pace period becoming smaller with increasing pacing rate.
If the most unfavourable combination of the different harmonics is to be captured, the phase angles need to be varied. In most cases, however, a forced vibration induced by walking is governed by just one harmonic, so that phase angles become immaterial.
Man-induced vibrations may arise from walking, running, skipping, dancing, 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 book is consciously intended to serve the practising structural engineer and not primarily the dynamic specialist.
Outlines some basic forms of dynamic loading which cause vibrational problems in structures. Presents information on the possibility of structural damage occurring from vibration. Discusses the human response in terms of its often being the limiting factor in terms of amplitude which can be tolerated within a structure. Details industrial vibrational problems, covering areas of traffic, piling, forced vibration and industrial plant. Williams, C. Report bugs here. Please share your general feedback.
Book written by Hugo Bachmann and Walter Ammann and published by International Association for Bridge and Structural Engineering (IABSE) in in the.
Vibrations in structures induced by man and machines
Realistic simulation of the dynamic effects of walking pedestrians on structures is still a considerable challenge. This is mainly due to the inter- and intrasubject variability of humans and their bodies and difficult-to-predict loading scenarios, including multipedestrian walking traffic and unknown human-structure interaction HSI mechanisms. Over the past three decades, several attempts have been made to simulate walking HSI in the lateral direction.
This paper presents an analytical approach to analyze the vertical vibration of a simply supported beam subjected to pedestrian-induced loads. The loading time history of an individual footstep is simplified as a rectangular force pulse, and each identical footstep load acts at different locations along the beam depending on a step length. Although the loading model is very simple, it enables us to find analytical relations between the pacing parameters and the beam response. The results showed that the dependence of the pacing period, footstep contact duration, time delay of traveling between two pedestrians on the natural period of the beam as well as the step length can influence the dynamic response of the beam significantly. Yin S.
In many plants, vibration and noise problems occur due to fluid flow, which can greatly disrupt smooth plant operations. These flow-related phenomena are called flow-induced vibration. This book explains how and why such vibrations happen and provides hints and tips on how to avoid them in future plant design.
Vibrations in structures
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