GB2346667A

GB2346667A - Device for absorbing vibrations in a structure

Info

Publication number: GB2346667A
Application number: GB0002624A
Authority: GB
Inventors: Stuart Cameron Holdsworth
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-02-13
Filing date: 2000-02-07
Publication date: 2000-08-16
Anticipated expiration: 2020-02-07
Also published as: GB0002624D0; GB9903184D0; GB2346667B

Abstract

A device for absorbing vibration of a structure having a particular fundamental frequency. The device (20) comprises a mass (50) arranged for pivoting movement on an arm (30) about a pivot point (26) and means (45, 46) for limiting the angular range of such pivoting movement and for damping such movement. The embodiments include, means (52, 56, 58) for adjusting the position of the mass (50) on the arm (30) and various arrangements for transferring the pivoting movement of the mass to the movement limiting and damping means. An application of the device is the absorption of induced vibrations in a building, in particular a grandstand.

Description

Title-Device for Absorbing Vibrations This invention relates to a device for absorbing vibrations, in particular to a vibration absorbing device suitable for fitting to civil engineering structures, though the device may be applicable to any structure which requires damping.

All structures can be made to vibrate when excited by an oscillating force. The amplitude of the induced vibration is a function of the applied force and its frequency. An exciting force has the greatest effect when applied at the fundamental frequency of the structure. The structure is then excited at resonance, and in the case of a lightly damped structure the induced movement can be many times greater than the deflection caused by the equivalent static force. The ratio between the two effects is called the magnification factor.

Vibrations in a structure have two effects. First, the very high peak accelerations can mean that the effective weight of the vibrating structure increases several-fold, and this may cause its destruction. Secondly, people on or inside the structure feel these accelerations, which can be uncomfortable or even dangerous.

In civil engineering structures, the greatest problems of vibration are encountered with structures whose fundamental frequency is a small multiple of some human activity, eg walking, running or jumping. It has been found that such activities can excite structures with fundamental frequencies of up to 8.4Hz. This includes a wide range of bridges, grandstands, etc. In order to avoid such problems, structures have hitherto been made stiffer, in order to increase the fundamental frequency, or the mass of the structure has been increased, to keep accelerations low. However, both these approaches suffer from disadvantages, not least that they lead to increased cost.

There has now been devised a device which can be incorporated in, or attached to, a civil engineering or other structure, which overcomes or substantially mitigates problems of vibration.

According to the invention, there is provided a device for absorbing vibrations suitable for incorporation into, or attachment to, a structure having a particular fundamental frequency, said device comprising a mass arranged for pivoting movement on an arm about a pivot point, and means for limiting the angular range of such pivoting movement and for damping such movement.

The device according to the invention is advantageous primarily in that it reduces the amplitude of vibrations induced in the structure by, for example, people walking over or within the structure. Vibrations of the structure are transmitted to the pivoting arm and it oscillates in opposition to the structure at its fundamental frequency so that no peak in the amplitude of the vibrations of the structure is reached. Instead, a first peak is reached at a frequency slightly below the fundamental frequency of the structure and a second peak at a frequency slightly above the fundamental frequency. However, the magnification factors of these peaks are much lower than that at the fundamental frequency, depending on the ratio of the equivalent masses of the device and the structure, and so the amplitude of the vibrations induced in the structure is much lower. The use of the device obviates the need for structures to have increased stiffness or mass, thereby reducing the cost of construction and allowing lighter weight structures to be used. The device itself is relatively light in weight, compact and easy to incorporate into, or attach to, a structure. In general, the stresses to which the device is exposed are low, which results in low requirements for maintenance and long life expectancy, in many cases as long as the structure on or in which the device is used. The device can be produced in any of a number of configurations, to suit the structure to which it is to be applied.

It is particularly preferred that the position of the mass on the pivoting arm should be adjustable. This renders the device of the invention in effect tuneable, the fundamental frequency being adjustable simply by altering the position of the mass on the pivoting arm.

The means for limiting the range of angular movement of the pivoting arm and for damping its movement most preferably comprises resilient means which act against the movement of the arm. Since the arm pivots in two directions, suitable resilient means are preferably provided to limit and damp movement in both those directions. The pivoting arm is conveniently operably linked to a piston which reciprocates within a cylinder in which a pair of suitable compression springs are captivated. As an alternative to mechanical springs, other types of springs, such as air, gas, oil or electromechanical springs, may be used.

The device may incorporate more than one type of spring. For example, two pairs of compression springs may be used, a first pair of compression springs damping vibrations of amplitude below a certain magnitude and the second pair of springs acting only when the amplitude exceeds that magnitude.

The pivoting arm may be operably linked to the piston by simple mechanical linkages.

Alternatively, the linkage may be via a gear or cam-type mechanism.

In general, it is found that satisfactory results are achieved using a mass corresponding to from about 2% to 8% of the equivalent mass of the structure with which the device of the invention is to be used, and more preferably from about 2% to 4%.

A single mass mounted on a single pivoting arm may be convenient for most applications, but more than one pivoting arm (eg two arms) may be provided, each carrying a part of the mass.

The device of the invention may be mounted in any suitable orientation. To absorb vertical vibrations, the device is preferably configured such that the mass pivots about a horizontal axis, whilst to absorb horizontal vibrations the mass preferably pivots about a vertical axis.

The device of the invention may be used on any of a wide range of civil engineering structures. Examples include bridges (including footbridges and road or rail bridges) and grandstands, eg at sports stadia.

The invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which Figure 1 is a side view in cross-section of a grandstand, being a structure to which vibration absorbers according to the present invention can advantageously be fitted; Figure 2 is a perspective view of a vibration absorber according to the invention; Figure 3 is a plan view of the vibration absorber of Figure 2 ; Figure 4 is a cross-sectional side view of the vibration absorber of Figure 2; Figure 5 is a schematic diagram of a second embodiment of a vibration absorber according to the invention; Figure 6 is a schematic view of a vibration absorber according to the invention, mounted vertically; Figure 7 is a schematic diagram of part of a third embodiment of a vibration absorber according to the invention; Figure 8 is a schematic view of a gear type linkage which may be used in a vibration absorber according to the invention; and Figure 9 is a schematic view of a cam type linkage which may be used in a vibration absorber according to the invention.

Referring first to Figure 1, a grandstand (generally designated 10) is a typical civil engineering structure to which vibration absorbers according to the invention can advantageously be fitted. The grandstand 10 comprises a lower tier of seating 12 and an upper tier 14. The upper tier 14 in particular is susceptible to vibration induced by human activity, eg people walking to or from their seats or rhythmical lapping or stamping of feet.

The upper tier 14 consists of a number of sections arranged side by side. In order to reduce the vibrations to acceptable levels, vibration absorbers according to the invention are fitted to each such section, and function in the manner described below.

The vibration absorber according to the invention is generally designated 20 and is shown in Figures 2 to 4. The vibration absorber 20 comprises a support plate 22 which is provided with six fixing holes 21 by which the vibration absorber 20 can be secured to a structure (in this case, to the grandstand 10), eg by means of bolts. At one end of the support plate 22 a pair of flanges 24 support a shaft 26 on which a pivoting block 28 is supported. An arm 30 of box-section is mounted at one end in the block 28.

A further pair of flanges 32 are formed integrally with the block 28 and support a second shaft 34 which forms a linkage between the arm 30 and a piston rod 36. The piston rod 36 is connected to a piston 38 (see Figure 4) housed within a cylinder 40. The cylinder 40 is pivotally mounted on a pair of stub shafts 42 which engage in openings in upstanding (as viewed in the drawings) flanges 44 at the end of the support plate 22 which is remote from the arm 30. A pair of compression springs 45,46 (see Figure 4) are captivated within the cylinder 40 and act between the piston 38 and the respective end plates 47,48 of the cylinder 40.

The arrangement is such that the arm 30 is able to pivot over a restricted angular range about the central position illustrated in the drawings. The range of movement of the arm 30 is limited by the throw of the piston 38 in either direction, against the action of the springs 45, 46 and movement of the arm 30 is damped by the action of the springs 45,46. The range of movement of the piston 38 is limited by inwardly projecting stops 47a, 48a which are formed integrally with the respective end plates 47,48 and one of which 48a has an axial bore through which the piston rod 36 passes.

A mass 50 is slidably mounted on the arm 30. The mass 50 is of generally cuboid form and is provided with a bore corresponding in shape to the cross-section of the arm 30 and through which the arm 30 passes. The position of the mass 50 on the arm 30 is adjusted by means of an adjustment rod 52 which is fixedly mounted on the arm 30 by means of a bracket 54 and which extends through a bore in an extension piece 56 fixed to the upper (as viewed in the drawings) surface of the mass 50. The adjustment rod 52 is disposed parallel to the arm 30.

The mass 50 may be fixed in position relative to the adjustment rod 52 by any suitable means.

For instance, locking screws may be provided to clamp the extension piece 56 to the adjustment rod 52. Alternatively, as illustrated, the adjustment rod 52 and the bore in the extension piece 56 may be threaded so that the position of the mass 50 relative to the adjustment rod 52 can be altered by rotating the rod 52. A pair of locking nuts 58 can be provided to fix the adjustment rod 52 to the bracket 54, the locking nuts 58 being loosened to permit adjustment and then tightened.

Altering the position of the mass 50 on the arm 30 alters the effective stiffness of the vibration absorber 20, and hence its fundamental frequency.

If the vibration absorber 20 is to be fitted to an existing structure, tests are first carried out to determine characteristics of the structure such as the fundamental frequency, its equivalent mass, stiffness and damping ratio. The magnitude of the mass 50 is selected to provide optimum acceleration reduction and the springs 45,46 are constructed in such a manner that they resist the motion of the arm 30 and constrain that motion to a range which is less than the extreme limits of motion of the piston 36 defined by the stops 47a, 48a. The size of the mass 50 will generally be between 2% and 4% of the equivalent mass of the structure.

The vibration absorber 20 is then mounted on the structure, and tests carried out to check its effectiveness. Adjustments of the position of the mass 50 on the arm 30 are made until the optimum performance is achieved. The vibration absorber 20 can be fixed at any convenient point in the structure, provided that the plane of oscillation of the arm 30 is the same as (or at least has a substantial component in) the plane of the vibration of the structure which it is intended to absorb. In the case of the grandstand 10, the vibration absorber 20 may, for instance, be fixed to the upper tier of seating in the area marked X.

If the vibration absorber 20 is intended for a projected structure, then a similar procedure is used, but with the size of the mass 50 and the design of the springs 45,46 based on theoretical calculations. The vibration absorber 20 is fitted to the structure once the structure has been constructed and, as for an existing structure, adjustment of the position of the mass 50 is carried out until the optimum result is achieved.

Referring now to Figure 5, a second embodiment of vibration absorber according to the invention is shown schematically and generally designated 60. This embodiment differs from that described above in that the arm 62 on which mass 64 is mounted pivots about a point 66 which is offset from the shaft 62. As before, vertical vibrations of the mass 64 are damped by the action of springs 68,70 on a piston 72.

Figure 6 is a schematic view of a further embodiment, generally denoted 80, which is similar to the first embodiment described above save that it is mounted vertically. In this configuration, the vibration absorber 80 can be used to absorb horizontal vibrations. A mass 82 is mounted on a shaft 84 which is formed integrally with a transverse extension 86. The shaft 84 and extension 86 pivot about a point 88 such that horizontal vibrations of the mass 82 are transmitted to a piston 90. The vibrations of the piston 90 are damped by springs 92,94.

Figure 7 shows a modified vibration absorbing mechanism. In this embodiment, a piston 100 again vibrates on the end of a piston rod 102 within a cylinder 104. In this case, however, two pairs of compression springs are captivated within the chamber 104. A first pair of springs 106,108 are of lesser stiffness and act upon the piston as soon as it is displaced from its rest position. A second pair of springs 110,112 are of greater stiffness but have inner ends which are spaced from the rest position of the piston 100. These springs 110, 112 thus act upon the piston only when the displacement of the piston 100 from its rest position exceeds a certain threshold. The first pair of springs 106,108 therefore damp small amplitude vibrations, the springs 110,112 of greater stiffness coming into play only when larger amplitude vibrations occur.

Finally, Figures 8 and 9 show alternative mechanisms by which oscillations of the mass may be transmitted to the damped piston. In the embodiment of Figure 8, the mass 120 is mounted as before on a shaft 122 which in this case is fixedly mounted on a sectored plate 124 which in turn pivots about a pivot point 126. The arcuate edge of the sectored plate 124 is toothed and engages a toothed rack 128 formed on the terminal portion of piston rod 130.

Oscillations of the mass 120 are thus converted to reciprocating motion of the piston rod 130.

In the embodiment of Figure 9, the mass 140 is again mounted on a shaft 142 which pivots about a point 144. The shaft 142 is mounted on the pivot 144 by means of a bracket 146 which is provided with an elongate slot 148 aligned coaxially with the shaft 142. The piston rod 150 is provided near its right hand (as viewed in the drawing) end with a lug 152 which engages in the slot 148.

Oscillations of the mass 140 cause the sides of the slot 148 to act upon the lug 152 with a camming action, again converting the oscillatory movement of the mass 140 and shaft 142 into reciprocating motion of the piston rod 150.

Claims

Claims 1. A device for absorbing vibrations suitable for incorporation into, or attachment to, a structure having a particular fundamental frequency, said device comprising a mass arranged for pivoting movement on an arm about a pivot point, and means for limiting the angular range of such pivoting movement and for damping such movement.
2. A device as claimed in Claim 1, wherein the position of the mass on the pivoting arm is adjustable.
3. A device as claimed in Claim 1 or Claim 2, wherein the means for limiting the range of angular movement of the pivoting arm and for damping its movement comprises resilient means which act against the movement of the arm.
4. A device as claimed in Claim 3, wherein resilient means are provided to limit and damp movement of the arm in both directions.
5. A device as claimed in Claim 4, wherein the pivoting arm is operably linked to a piston which reciprocates within a cylinder in which a pair of compression springs are captivated.
6. A device as claimed in Claim 5, wherein two pairs of compression springs are provided, a first pair of compression springs damping vibrations of amplitude below a certain magnitude and the second pair of springs acting only when the amplitude exceeds that magnitude.
7. A device as claimed in any preceding claim, wherein the mass corresponds to from 2% to 8% of the equivalent mass of the structure with which the device is used.
8. A device as claimed in any preceding claim, wherein the mass pivots about a horizontal axis.
9. A device as claimed in any one of Claims 1 to 7, wherein the mass pivots about a vertical axis.
10. A structure fitted with one or more devices as claimed in any preceding claim.
11. A structure as claimed in Claim 10, which is a grandstand.
12. A method of damping vibrations in a structure, which method comprises fitting to the structure one or more devices as claimed in any one of Claims 1 to 9.
13. A device for absorbing vibrations, substantially as hereinbefore described and as illustrated in Figures 2 to 4 hereof.