VSL - The Fricvtion Damper Presentation

 

 

VSL STAY CABLE SYSTEM THE FRICTION DAMPER

Yves Bournand 29 April 2002

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Contents

I.

Introduction

II.

Description of the friction damper

III.

Assembly and installation

IV.

Maintenance

V.

Design specification

VI.

References, publications

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I. INTRODUCTION

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Cable vibrations on cable-stayed bridges are known since several years, but it is only recently that this problem is becoming more and more critical, perhaps due to the new design of the bridges. Many different phenomena can generate cable vibrations. They are mostly non-linear and their analysis is delicate. The new PTI Recommendations [1] give some indications how to evaluate and reduce the risk of cable vibrations. But the PTI proposes tentative criteria based on limited data. Presently, the re installation of dampers to thedamping cable onofthe bridge deck are most the common countermeasure countermeasu to increase the structural cables and thus, tothe mitigate vibration. The most important criteria to be considered for a cable damping system are the following :  Adjustability . Easy access to the damper . Easy to assemble . Possibility of installation on existing bridges .  Aesthetics . Maintenance cost . Reliability Reliabili ty . Damping characteristics characteristics insensitive to temperature and frequency of vibrations . ! ! ! ! ! ! ! !

Until recently the most classical solution consisted in the installation of hydraulic or viscous dampers connecting the stay cable to the deck, near the anchorage. This installation can be as simple as for Brotonne Bridge, Elorn Bridge or Erasmus Bridge , or it can be more complicated as for Normandy Bridge. These solutions have the advantage to have easy accessibility for the maintenance operations, but the aesthetical appearance could be criticized in some cases. According to recent experience, its seems that hydraulic dampers have relatively high maintenance costs and complex adjustment. Some other systems are designed to be installed without connection to the deck, as a ring around the cable. The damper can be installed in the anchorage steel guide pipe embedded in the concrete deck or within a steel support pipe extending the anchorage guide pipe. Viscous dampers consist of freely moving plates (or rings) in a viscous, silicon-like material, which assures the dissipation of energy. Several bridges in Japan are equipped with viscous dampers because of estimated lower maintenance costs. An important disadvantage is that the damper characteristic is strongly depending on temperature and frequency of cable vibration. Some types of damper can be almost permanently solicited to small, non-critical vibrations and very quickly will have to support a high level of cycles and consequently may experience rapid deterioration and need frequent maintenance. To answer to these main problems of fatigue and maintenance, maintenance, VSL proposes the friction damper. The idea to use friction systems for vibration absorbers has been developed several years ago and such friction absorbers have been installed on different types of construction such as chimneys, buildings and bridges. In Section VI (References) a list of some structures equipped with friction vibration absorbers is provided.

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The main advantages of the friction damper developed by VSL are the following : !

!

!

!

!

! !

!

The damper is not activated for small and non-critical vibration amplitudes. Thus, we have reduced wearing and low maintenance costs. For UDDEVALLA Bridge, the friction force is adjusted to have an action of the damper only when the amplitude of vibration of the longest cable is exceeding 70mm. For each cable, the friction force damper is adjusted according to the allowable amplitude of vibration defined by of thethe designer. The friction damper is designed to be easily installed on existing bridges , where cables are subjected to unexpected vibrations.  All components components of the damper are accessible accessible and can can be easily inspected inspected and replaced, if necessary, during the maintenance operations. The characteristics of the damper can be easily adjusted during the maintenance operations. This adjustment consists of only turning the four screws supporting the friction pads. The friction forces are practically constant and independent independent of the speed of the point to be dampened. The damping characteristics are insensitive to the frequency of the vibrations. The friction damper is designed so that the damping of the stay cable is not affected by longitudinall movement of the cable due to load variations. longitudina For better aesthetic, the damper can be placed at a reduced distance from the anchorage.

The design and implementation of the friction damper on a particular project are defined in collaboration with experts in bridge and cable dynamics. Detailing and installation are achieved by the VSL Technical Centres and experienced VSL site teams.

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II. DESCRIPTION OF THE FRICTION DAMPER

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21.

Functional description !

The damper connects a specified point of the cable with the bridge structure. The damper consists of two parts, see Fig. 1. ASSEMBLY 1 is rigidly fixed to the cable by means of a steel collar (A) and moves together with the cable. The major element of Assembly 1 is the two wings (B)

projectingtotransversely transversel y toand thethe cable plane, with friction  (C) being attached the top side bottom side of hard the wings. Thepartners plane surface of the hard friction partners lies at a right angle to the cable axis. ASSEMBLY 2 is rigidly fixed to the t he bridge structure. It consists of two spring blade halfring pairs (D), both of them together surrounding the cable and Assembly 1. The two superposed half rings are clamped against against each other at the ends and fixed by bolts (E) to the substructure. Soft friction partners (F), which are pressed from the top and from the bottom against the hard friction partners (C) of Assembly 1, are held by the spring blade rings through an inwardly projecting plate (G). !

The damper is activated during transverse vibration of the cable. This results in a periodic relative motion between Assembly 1 and Assembly 2. Thereby, friction force f orce and damping reactions, acting against the movement, are produced between the soft and the hard friction partners. The friction force of the soft friction partners against the hard friction partners is adjusted by deflection of the spring blade rings .

!

!

!

!

!

The damper is friction-locked in any state and in any relative position between  Assemblies 1 and and 2. Due to the variations of the tension in the cable, the Assembly 1 (fixed to the cable) is moving along the longitudinal cable axis. The flexibility of the spring blade rings allows the soft friction partners to follow this movement and to be all the time in contact with the hard friction partners. To keep the friction force of the damper at a constant value, the spring blade rings are deflected at installation with a greater value than the calculated longitudinal longitudinal movement of the Assembly 1 (see Fig. 4) The damper is equipped with a mechanical safety stop, to limit the amplitude of the cable deformation at the damper in case of complete loss of the friction force.  All the component component assemblies of of the friction damper are are without play. For aesthetics, the damper is generally placed near the anchorage. So the vibration amplitude at the damper point of the cable is not more than 2 or 3mm. All play or flexibility in the assemblies would lead to loss of the efficiency. Some damper designs are not compatible with these small cable amplitudes and have to be placed at a greater distance from the anchorage. The damper is placed after installation of the cable. It is composed of two symmetrical parts which are fixed to the side of the completed cable and to the substructure. The damper design meets the following further basic conditions: a) The friction forces are simply and stepless adjustable. b) The dampers can be easily inspected ; the friction partners can be replaced,on site, if necessary. Page 6

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22. Location of the friction damper The damping system is placed on the cable so that the transition length “"L” (see Fig. 2) is about 0.015 (L) to 0.025 (L), where (L) is the length of the stay cable. For one cable we have generally one damping system which can be placed near the deck or the pylon. it is placed at the level of the deck to havetoan easy access. The design of theGenerally stay pipe has to be adapted to facilitate the access the damping system. Respecting the above value of “ "L” : For concrete decks, the damping system can be located at the end of the deck guide pipe . (see Fig. 2a). For steel or composite decks the damping system will be placed on a steel support. (see Fig. 2b). 23. Dimensions The dimensions of the friction damper will vary according to the dimensions of the cable and its length :

Cable unit (No. of strands)

12

to

127

Diameter A (mm)

430

to

850

B (mm)

140

to

300

C (mm)

200

to

240

See Fig 1 for definition of A , B and C

 

Note : these dimensions can be adapted according to the project .

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Fig. 2 – Installation of the friction f riction damper

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III. ASSEMBLY and INSTALLATION INSTALLATION

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31.

General aspects  Assemblies 1 and and 2, (see Fig. 1), are preassembled preassembled in the workshop , where the deflection and prestressing force of the spring blade rings are adjusted. The completed dampers delivered to the erection site are marked according to the cable numbers.

32.

Workshop assembly  After fixation the spring spring blade rings a are re deflected by screwing screwing of the soft friction partners partners (see Fig. 3) and their flexibility is measured. And each ring pair (two opposite blade rings) is assembled and prestressed to adjust the deflection of the two opposite blade rings to their final value " + 2 (z). See Fig. 4. " 

(z) 33.

thickness of the hard friction partner value of the calculated longitudinal deformation of the cable at at the damper location.

Assembly on site The friction damper is installed after the final tuning of the force f orce in the stay cable. First, the damper support is pre-installed and adjusted according to the cable geometry (see Fig. 5). The support will be connected to the deck structure with temporary bolts. Then, the pre-assembled elements of the friction f riction damper will be placed, with a special tool, to the end of the support and adjusted. The steel collar collar of the Assembly 1 (see Fig. 1) will be rigidly fixed to the cable at the damper location and all the damper compone components nts with the damper support will be permanently fixed.

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  n   o    i    t   a    l    l   a    t   s   n    I   e    t    i    S      5  .   g    i    F

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IV. MAINTENANCE

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One of the main advantages of the friction damper, compared to the other damping systems, is the fact that there is no movement of the damper for small amplitudes of stay cable vibrations. The number of fatigue cyles is reduced and consequently the fatigue life of the damper is increased. The access to the damper is by sliding-up the HDPE stay pipe, along the cable.  All the components components of the damper are then then accessible to be be easily inspected, inspected, adjusted or replaced. If necessary, the friction force can be adjusted on site. Fig. 6 shows the access to the friction damper components.

Fig. 6 : Access to the friction damper for maintenance .

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V. DESIGN SPECIFICATION

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51-

The rain-wind induced galloping The possibility of rain-wind induced vibrations is considered as the most onerous scenario. It is assumed that if an adequate solution to this type of vibration is found, it would also reduce the response due to other types of vibrations, e.g. vortex shedding and parametric resonance. The problem of rain-wind induced cable vibrations has been considered in a number of papers over the last years. But to our knowledge no complete and commonly accepted theory for analysis exists. The PTI Recommendations, 4th edition [1], contain a stability criterion. A shortcoming of this criterion appears to be that important parameters such as cable tension and natural cable eigenfrequency are not considered. These parameters are however, considered in the analysis of the friction damper which is based on the classical galloping theory, [2-3-4].  Across - wind galloping galloping is a dynamic dynamic aerolastic instability. instability. It is caused by aerodynamic aerodynamic excitation forces induced by the motion of the cable itself. These forces can act in phase with the cable velocity and under certain conditions (profile shape, incidence angle of the wind) the so-called aerodynamic damping of the cable can be negative, and due to the small internal damping of the cable, galloping instabili instability ty will occur. This gives rise to a strong growth of vibrations in the across-wind direction. Circular cables cannot gallop because of their cross-sectional geometry. But small deviations from a perfectly circular shape may imply galloping instability. This critical situation can be observed with a combination of rain and wind. Rain-wind induced galloping can be explained as follows: During certain wind velocities and wind directions, the rainwater flowing on the surface of the smooth cable is retained by the dynamic pressure in an upper position of the cross-section. A rivulet develops here, which together with the cable cross-section and the permanently existing lower rivulet form an oval-like cross-section cross-section susceptible to galloping. galloping. This situation becomes critical at the point when the wind velocity necessary for retaining the rivulet is at the same time higher than the critical wind velocity that initiates the galloping of the composite cross section formed by the pipe and the rivulets. On cable-stayed bridges, rain-wind induced vibrations have been observed with a wind speed between 10 m/s and 15 m/s. During these vibrations, the oscillation amplitudes may increase exponentially if the net damping ratio of the cable is negative. The net damping ratio is the sum of the internal damping of the cable and its aerodynamic damping ratio. ! net = ! int + ! ae  The internal damping is always positive, but the aerodynamic damping may be negative. Within the classical galloping theory, the critical wind velocity that initiates the galloping oscillation is given by : Ucr,i = (1/CL1)(8. )(8." ".!i.m / #.D ) f i

(1)

!i

net damping ratio

m

cable mass per unit length (kg/ml) Page 17

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#  air density (1,25 kg/m3) D Cable diameter (m) f i  eigenfrequency eigenfrequency of the cable ( at mode i ) CL1 load coefficient CL1 is a lift (galloping) coefficient, as function of : # The speed and angle of incidence of the wind # # #

The distribution and relative motion of the rain water along the cable The cable inclination The magnitude of the motion

 According to some experiments in wind wind tunnels [3] , and studies achieved achieved for Erasmus Bridge [5], we can consider CL1 % 1 at wind speed U o=15m/s.  According to the equation equation (1), the ae aerodynamic rodynamic (neg (negative) ative) damping ratio, at the critical wind speed Uo=15 m/s , is: ! i,ae = - ( CL1.#.D.U0 ) / ( 8." 8.".m.f i ) If we consider the logarithmic galloping : $ i,ae = 2 ".! i,ae We will have : $ i,ae = - ( CL1.#.D.Uo ) / ( 4.m. f i ) 52-

The VSL criterion The VSL criterion to evaluate the characteristics of an additional damping system is the following: $ net = $ o + $ d + $ 0 $ d  $ ae  &    

&   .

$ ae ' 0

internal (or structural) eigendamping of the cable ($ ( $=2 =2"! "!). ). additional damper. aerodynamic aerodynamic (negative) damping effect according to the galloping theory . safety coefficient covering uncertainties in load and damping estimates.

 As $ d varies with the midspan amplitude of the cable motion , we consider a safety coefficient &   =2 at the largest value of $ d. 53-

Main parameters of the damper design. The design of the damper takes account of the following parameters: !

The main cable data: The cable diameter, D The cable force, F The cable length, L The cable mass (per unit length), m The cable damping The stay pipe surface (for example, the use of helical ribs).

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! !

!

!

54-

The position of the damper on the cable, ( "L) The static deformations of the damped cable point. These are determined by considering : - The variation of cable cable sag due to variations variations of tension. tension. - The thermal expansion of the cable - The cable stretching due to variation variation of longitudinal force. The damper friction force. The friction coefficient between the soft and the hard friction partners lies between 0.17 and 0.20 The flexibility of the damper support. For aesthetic consideration, the damper is generally placed near the anchorage. and the vibration amplitude at the damper point of the cable is not more than 2 or 3 mm. Consequently Consequen tly it is important to fix the damper on a rigid support to not reduce significantly the damper efficiency.

Example of a typical concept design. The main design specifications of the friction damper are described within the following example , which consists to define the main damper characteristics according to the cable parameters . !

Main characteristics of the cable: Length Stay pipe diameter Permanent cable tension Cable mass Frequency Internal eigendamping eigendamping of the cable

!

L = 207m D = 0.2m S = 4.300 kN m = 58.5 kg/m f 1 = 0.653 Hz $ (+)eigen = 0.006

 Aerodynamic  Aerodynami c damping of the cable. cable.

 At the critical wind velocity velocity of 15m/s, the ap application plication of the galloping galloping theory leads leads to a negative damping decrement of: $ (-)galloping = - CL1 . #. D. Uo / 4.m.f with

CL1 = 1 ( at U0 = 15m/s ) # = 1,25 D = 0,2m M = 58,5 kg/ml f 1 = 0,653 Hz

$ (-)galloping = 0,024 With a safety factor &   . $ (-) = - 0,048 !

&   =

2:

In conclusion : $ (+) + &   . $ (-) < 0 The cable is dynamically unstable under galloping excitation.

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consent of VSL (International) (International) Ltd.

!2002.

 

 

!

Installation of a friction damper. To remove the instability we will install a friction damper near the deck anchorage, at a distance of 3.6m from the anchorage. The friction force (F) will be adjusted so that the vibration amplitudes must not exceed : max S ( L / 3000 max S % 70mm Generally the friction force is between 3 to 4 kN. Figure 7 represents the cable stability diagram (damping versus amplitude), amplitude), after the installation of the friction damper.

Fig. 7: Cable stability diagram

The friction force (F) has to be lower than the maximum dynamic force in the connection point during a vibration with an amplitude of 70mm; on the other side, it must not be as low as to allow an early instability at high amplitudes. With a reasonable safety we take F=3,6kN, which represents a stationary amplitude of about 50mm (point S) and a high amplitude instability of about 300mm (point U). The above diagram shows that the friction damper characteristic is non-linear. We follow up the principal stabilizing effect: with low vibration amplitudes prevails $ (+) < $ (-), i.e., the vibration remains unstable and the amplitudes increase up to the stationary point S. In turn, with amplitudes higher than S but lower than U prevails $ (+) < $ (-), i.e., the vibration is stable and the amplitudes fall down to the same stationary point. Point U represents the start of the high-amplitude high-amplitude instability, from where the damping is (i.e., would be if such an amplitude occured) definitely non-sufficient non-sufficient during rain-wind affectation. The damper has to neutralize the galloping ,with the safety ffactor actor defined above ,and has the following value : Necessary $ (+) damper =

&    $ (-) 

gall -

$ (+) eigen = 0.048 - 0.006 = 0.042

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!2002.

 

 

55-

Efficiency of the friction damper 551 Efficiency affected affected by support support and cable stiffness. The effective damping varies with the vibration amplitude. Under ideal conditions conditions,, the maximum decrement is: max $ = $ eigen+ "(%L/L) L "L

cable length distance between the friction damper and the fixed point point of the cable. cable.

Location of the external Damper "L

/ L = 0.015 to 0.025 

Fig. 8: Location of friction damper.

In reality, max $ is lower, due to : - The bending stiffness of the cable . - The flexibility of the support of the damper. Both reduction effects are assessed and considered in the design, as illustrated in Fig.9

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consent of VSL (International) (International) Ltd.

!2002.

 

 

Fig.9 : Computational representation of the friction damper with its supports. 552-

Efficiency related to the friction force The main parameter parameter of the friction dam damper per is the friction force F. If this fri friction ction force is not correctly realized, i.e. if F is too low or too high, the maximum value of the cable damping (max $ eff  ) will still be achieved but at a different amplitude value, see Fig. 10. Both cases can be resolved by adjustment of the damping force during simple maintenance . In an extreme case of the damper being stuck / blocked , the t he damper will act as a fixed point, and no damping will be provided to the cable. In the opposite extreme case, with zero friction, the cable will freely move into an extreme position where it will be stopped by a mechanical stop.

553-

Efficiency related to higher modes (see Fig. 11) For higher modes, the efficiency of the damper is the same as for the first mode but with a different starting amplitude. The damper starts to act when the transverse force on the cable , at the fixation of the damper , reaches the friction force. The transverse force is the cable tension multiplied with the sine of the cable deviation angle at the damper. Higher modes produce cable deviation deviation angles similar to the first mode at half, one third, etc. of the amplitude of the first mode, for the 2 nd, 3rd etc. mode, respectively. The efficiency of the damper will only become lower than given above for and beyond mode N, with N ' 0.5 ( L / "L)

N = number of eigenmode

We will note that the damping of the higher modes is less relevant. They are usually not excited or they are only excited with lower intensity. 554-

Efficiency with higher amplitudes (see Fig. 12). The efficiency of the damper begins to decrease first at the amplitude A & 2A0 , where A0 is the starting amplitude. At the amplitude A & (5 to 6)A0 , the efficiency falls to about 50%. The reduction of efficiency with higher amplitudes has no practical relevance. The damper is correctly dimensioned with a safety factor of 2 applied to the excitation.

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!2002.

 

 

Max $ = with

&   =

&   .

$ (-)galloping 

2

Hence, the amplitudes are stopped to grow at the stationary point S, i.e., usually at A0=L/3000. However, the damper has reserve capacity ( &   = 2 ) until it reaches its maximum damping. To produce instability, the amplitude would have to snap-through from this amplitude level A0 , up to a value of about (5.A0 ). This is a hurdle invicible in the practice. As an example with a cable of L=215 m (the longest cable of Uddevalla Bridge), the snap-through hurdle is about 350mm.

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consent of VSL (International) (International) Ltd.

!2002.

 

 

     0    1  .   g    i    F

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consent of VSL (International) (International) Ltd.

!2002.

 

 

     1    1  .   g    i    F

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consent of VSL (International) (International) Ltd.

!2002.

 

 

     2    1  .   g    i    F

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consent of VSL (International) (International) Ltd.

!2002.

 

 

VI. REFERENCES, PUBLICATIONS

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consent of VSL (International) (International) Ltd.

!2002.

 

 

60.

Use of friction systems in structures Friction vibration absorbers are installed since some years in different types of structures, as for examples: !

Elevated types of structures (chimneys) Equipped with multiple-plate friction absorbers developed in 1973 - Series of power plant chimneys in Dhekelia (Cyprus) - Clinicum Grosshadern - Munich (Germany) - Binding brewery - Frankfurt (Germany)

! ! ! !

61.

Friction damper on cable of the Koehlbrand Bridge - Hamburg (Germany) Pedestrian Bridge Bridge - Kiel (Germany) 1997 Vertical damping damping of the deck . Pedestrian Bridge Bridge - Duisburg (Germany) 1998 Horizontal-vertical Horizontal-vertical damping of the deck Pedestrian Bridge Bridge for Expo 2000 2000 Hannover 2001

Recent references in cable-stayed bridges The present design of the friction damper has been installed on the Uddevalla Bridge in Sweden and, on the longest cables of the Gdansk Bridge in Poland. !

Uddevalla Bridge

The 120 stay cables of the bridge have been equipped with friction dampers, in summer 2000. Since this date the bridge has been exposed to variable (up to strong) winds but no cable vibration has been reported since the installation of the friction dampers up to the end of 2001.  A report from JOHS HOLT, HOLT, the bridge designer, designer, about the performance performance of the stay cables cables equipped with the friction dampers is attached in the following page . The figures 13,14 and15 show the stay cables of Uddevalla Bridge ( Sweden ) during and after the installation of the friction dampers. Dampers for cable sizes 6-22 to 6-77 were installed . !

Gdansk Bridge

This bridge has been opened to the traffic in November 2001. A dynamic analysis of the stay cables has been achieved by the CSTB (Centre Scientifique et Technique du Bâtiment) to evaluate the risk of cable vibrations. One conclusion of this analysis has been to equip the four longest cables (6-42) of the bridge with the friction f riction damper. These dampers were installed in December 2001. !

Badajoz Bridge (Spain)

In 1996, the cables of the bridge have been excited mainly by rain-wind induced galloping. Cable vibrations have been removed by the installation of friction dampers (first generation) on all the cables (cable sizes 6-42 to 6-80).

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consent of VSL (International) (International) Ltd.

!2002.

 

 

62.

Publications on the friction damper !

!

!

Development of new stay cable dampers Yves Bournand FIB Symposium - Prague 1999 Damping devices against cable oscillations on Sunningesund Bridge I. Kovacs, E. Strommen, E. Hjorth-Hansen Cable dynamics Symposium - Trondheim 1999 Performance of a friction damping device for the cable on Uddevalla cable-stayed Bridge. E. Hjorth-Hansen, C. Hansvold, R. Ronnebrannt Ronnebrannt Cable dynamics Symposium - Montreal 2001

#

Le pont du troisième millénaire , à Gdansk . J. Mossot – Y. Bournand Travaux – Décembre 2001

#

Controlling vibration of stay cable Y.Bournand Fib congress – Osaka 2002

63.

Experience with dampers on Uddevalla Bridge  Attached is a copy of of a letter provided by the designer of the b bridge. ridge.

64.

Photos of friction damper  Attached are photos photos of the friction dampers dampers installed on the Uddevalla Bridge, Fig. 13, 14, and 15.

65.

References [1]

PTI (Post-Tensionin (Post-Tensioning g Institute) Recommendations Recommenda tions for stay cable design, testing and installation - February 2001

[2]

I. Kovacs, E. Strommen, E. Hjorth-Hansen Hjorth-Hansen - Damping devices against cable oscillations on Sunningesund Bridge - Cable Dynamics Symposium Trondheim 1999

[3]

G. Hirsch, H. Bachmann - Dynamic effects from wind - Vibration problems in structures - C.E.B. 1991

[4]

Den Hartog - Mechanical Vibrations, Mc Graw-Hill, 1956

[5]

J. Reusink, C. Geurts - Numerical Modelling of Rain-Wind-Induced Rain-Wind-Induced Vibration: Erasmus Bridge, Rotterdam - Sturctural Engineering International - May 1998

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consent of VSL (International) (International) Ltd.

!2002.

 

 

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consent of VSL (International) (International) Ltd.

!2002.

 

 

Fig. 13: Friction damper without protection sleeve

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consent of VSL (International) (International) Ltd.

!2002.

 

 

Fig. 14: Friction damper with protection sleeve

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consent of VSL (International) (International) Ltd.

!2002.

 

 

Fig. 15: Friction damper with protection sleeve.

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