vsl - the fricvtion damper presentation

7/29/2019 VSL - The Fricvtion Damper Presentation

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This document is the property of VSL (International) Ltd, and must not be copied, reproduced, duplicatednor disclosed totally or partially to any Third Party, nor used in any manner whatsoever without prior written

consent of VSL (International) Ltd. 2002.

VSL STAY CABLE SYSTEM

THE FRICTION DAMPER

Yves Bournand29 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 onlyrecently that this problem is becoming more and more critical, perhaps due to the newdesign of the bridges. Many different phenomena can generate cable vibrations. They aremostly non-linear and their analysis is delicate.The new PTI Recommendations [1] give some indications how to evaluate and reduce therisk of cable vibrations. But the PTI proposes tentative criteria based on limited data.

Presently, the installation of dampers to the cable on the bridge deck are the most commoncountermeasure to increase the structural damping of cables and thus, to mitigate thevibration.

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 .! Damping characteristics insensitive to temperature and frequency of vibrations .

Until recently the most classical solution consisted in the installation of hydraulic or viscousdampers connecting the stay cable to the deck, near the anchorage. This installation canbe as simple as for Brotonne Bridge, Elorn Bridge or Erasmus Bridge , or it can be morecomplicated as for Normandy Bridge. These solutions have the advantage to have easyaccessibility for the maintenance operations, but the aesthetical appearance could becriticized in some cases. According to recent experience, its seems that hydraulic dampershave relatively high maintenance costs and complex adjustment.

Some other systems are designed to be installed without connection to the deck, as a ringaround the cable. The damper can be installed in the anchorage steel guide pipeembedded in the concrete deck or within a steel support pipe extending the anchorageguide pipe.

Viscous dampers consist of freely moving plates (or rings) in a viscous, silicon-likematerial, which assures the dissipation of energy. Several bridges in Japan are equippedwith viscous dampers because of estimated lower maintenance costs. An importantdisadvantage is that the damper characteristic is strongly depending on temperature andfrequency of cable vibration.

Some types of damper can be almost permanently solicited to small, non-critical vibrationsand very quickly will have to support a high level of cycles and consequently mayexperience rapid deterioration and need frequent maintenance.

To answer to these main problems of fatigue and maintenance, VSL proposes the frictiondamper.

The idea to use friction systems for vibration absorbers has been developed several yearsago and such friction absorbers have been installed on different types of construction suchas chimneys, buildings and bridges. In Section VI (References) a list of some structuresequipped 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, wehave reduced wearing and low maintenance costs.For UDDEVALLA Bridge, the friction force is adjusted to have an action of the damperonly when the amplitude of vibration of the longest cable is exceeding 70mm.For each cable, the friction force of the damper is adjusted according to the allowableamplitude of vibration defined by the designer.

! The friction damper is designed to be easily installed on existing bridges , where cablesare subjected to unexpected vibrations.

! All components of the damper are accessible and can be easily inspected andreplaced, if necessary, during the maintenance operations.

! The characteristics of the damper can be easily adjusted during the maintenanceoperations. This adjustment consists of only turning the four screws supporting thefriction pads.

!

The friction forces are practically constant and independent of the speed of the point tobe 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

longitudinal movement of the cable due to load variations.! 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 incollaboration with experts in bridge and cable dynamics. Detailing and installation areachieved 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. Thedamper consists of two parts, see Fig. 1.

ASSEMBLY 1 is rigidly fixed to the cable by means of a steel collar (A) and movestogether with the cable. The major element of Assembly 1 is the two wings (B)projecting transversely to the cable plane, with hard friction partners (C) beingattached to the top side and the bottom side of the wings. The plane surface of the hardfriction partners lies at a right angle to the cable axis.

ASSEMBLY 2 is rigidly fixed to the bridge structure. It consists of two spring blade half-ring pairs (D), both of them together surrounding the cable and Assembly 1. The twosuperposed half rings are clamped against each other at the ends and fixed by bolts(E) to the substructure. Soft friction partners (F), which are pressed from the top andfrom 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 aperiodic relative motion between Assembly 1 and Assembly 2. Thereby, friction forceand damping reactions, acting against the movement, are produced between the softand the hard friction partners.

The friction force of the soft friction partners against the hard friction partners isadjusted by deflection of the spring blade rings .

! The damper is friction-locked in any state and in any relative position betweenAssemblies 1 and 2.

! Due to the variations of the tension in the cable, the Assembly 1 (fixed to the cable) ismoving along the longitudinal cable axis. The flexibility of the spring blade rings allowsthe soft friction partners to follow this movement and to be all the time in contact withthe 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 thecalculated longitudinal movement of the Assembly 1 (see Fig. 4)

! The damper is equipped with a mechanical safety stop, to limit the amplitude of thecable deformation at the damper in case of complete loss of the friction force.

! All the component assemblies of the friction damper 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 orflexibility in the assemblies would lead to loss of the efficiency. Some damper designsare not compatible with these small cable amplitudes and have to be placed at agreater distance from the anchorage.

! The damper is placed after installation of the cable. It is composed of two symmetricalparts 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.

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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 deckor the pylon. Generally it is placed at the level of the deck to have an easy access. Thedesign of the stay pipe has to be adapted to facilitate the access to 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 cableand its length :

Cable unit

(No. of strands)

Diameter A (mm) 430 to 850

B (mm) 140 to 300

C (mm) 200 to 240

12 to 127

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 damper

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III. ASSEMBLY and INSTALLATION

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31. General aspects

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

32. Workshop assembly

After fixation the spring blade rings are deflected by screwing of the soft friction 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.

thickness of the hard friction partner

(z) value of the calculated longitudinal deformation of the cable at the damper location.

33. Assembly on site

The friction damper is installed after the final tuning of the force 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 damper will be placed, with a special tool,to the end of the support and adjusted. The steel collar of the Assembly 1 (see Fig. 1) willbe rigidly fixed to the cable at the damper location and all the damper components with thedamper support will be permanently fixed.

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Page 13

Fig.

5

-Site

Installation


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IV. MAINTENANCE

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One of the main advantages of the friction damper, compared to the other dampingsystems, is the fact that there is no movement of the damper for small amplitudes of staycable vibrations. The number of fatigue cyles is reduced and consequently the fatigue lifeof the damper is increased.

The access to the damper is by sliding-up the HDPE stay pipe, along the cable.

All the components of the damper are then accessible to be easily inspected, adjusted orreplaced. 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 alsoreduce the response due to other types of vibrations, e.g. vortex shedding and parametricresonance.

The problem of rain-wind induced cable vibrations has been considered in a number ofpapers over the last years. But to our knowledge no complete and commonly acceptedtheory for analysis exists. The PTI Recommendations, 4th edition [1], contain a stabilitycriterion. A shortcoming of this criterion appears to be that important parameters such ascable tension and natural cable eigenfrequency are not considered.

These parameters are however, considered in the analysis of the friction damper which isbased on the classical galloping theory, [2-3-4].

Across - wind galloping is a dynamic aerolastic instability. It is caused by aerodynamicexcitation forces induced by the motion of the cable itself. These forces can act in phasewith the cable velocity and under certain conditions (profile shape, incidence angle of thewind) the so-called aerodynamic damping of the cable can be negative, and due to thesmall internal damping of the cable, galloping instability will occur. This gives rise to astrong growth of vibrations in the across-wind direction.Circular cables cannot gallop because of their cross-sectional geometry. But smalldeviations from a perfectly circular shape may imply galloping instability. This criticalsituation can be observed with a combination of rain and wind.

Rain-wind induced galloping can be explained as follows: During certain wind velocitiesand 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 forman oval-like cross-section susceptible to galloping. This situation becomes critical at thepoint when the wind velocity necessary for retaining the rivulet is at the same time higherthan the critical wind velocity that initiates the galloping of the composite cross sectionformed by the pipe and the rivulets.

On cable-stayed bridges, rain-wind induced vibrations have been observed with a windspeed between 10 m/s and 15 m/s.During these vibrations, the oscillation amplitudes may increase exponentially if the netdamping 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 gallopingoscillation is given by :

Ucr,i = (1/CL1)(8..i.m / .D ) fi (1)

i net damping ratiom cable mass per unit length (kg/ml)

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air density (1,25 kg/m3)D Cable diameter (m)fi 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 tunnels [3] , and studies achieved for Erasmus

Bridge [5], we can consider CL1 1 at wind speed Uo=15m/s.

According to the equation (1), the aerodynamic (negative) damping ratio, at the criticalwind speed Uo=15 m/s , is:

i,ae = - ( CL1..D.U0 ) / ( 8..m.fi )

If we consider the logarithmic galloping :i,ae = 2 .i,ae

We will have :i,ae = - ( CL1..D.Uo ) / ( 4.m. fi )

52- The VSL criterion

The VSL criterion to evaluate the characteristics of an additional damping system is thefollowing:

net = o + d + . ae 0

0 internal (or structural) eigendamping of the cable (=2). d additional damper. ae 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, DThe cable force, FThe cable length, LThe cable mass (per unit length), mThe cable dampingThe stay pipe surface (for example, the use of helical ribs).

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! 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 sag due to variations of tension.

- The thermal expansion of the cable- The cable stretching due to variation of longitudinal force.! The damper friction force.

The friction coefficient between the soft and the hard friction partners lies between 0.17and 0.20

! The flexibility of the damper support.For aesthetic consideration, the damper is generally placed near the anchorage. andthe vibration amplitude at the damper point of the cable is not more than 2 or 3 mm.Consequently it is important to fix the damper on a rigid support to not reducesignificantly the damper efficiency.

54- Example of a typical concept design.

The main design specifications of the friction damper are described within the followingexample , which consists to define the main damper characteristics according to the cableparameters .

! Main characteristics of the cable:

Length L = 207mStay pipe diameter D = 0.2mPermanent cable tension S = 4.300 kNCable mass m = 58.5 kg/mFrequency f1 = 0.653 Hz

Internal eigendamping of the cable (+)eigen = 0.006

! Aerodynamic damping of the cable.

At the critical wind velocity of 15m/s, the application of the galloping theory leads to anegative damping decrement of:

(-)galloping = - CL1 . . D. Uo / 4.m.f

with CL1 = 1 ( at U0 = 15m/s ) = 1,25D = 0,2mM = 58,5 kg/mlf1 = 0,653 Hz

(-)galloping = 0,024

With a safety factor= 2 :. (-) = - 0,048

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

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! Installation of a friction damper.To remove the instability we will install a friction damper near the deck anchorage, at adistance 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 stabilitydiagram (damping versus 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 connectionpoint during a vibration with an amplitude of 70mm; on the other side, it must not be as lowas to allow an early instability at high amplitudes. With a reasonable safety we takeF=3,6kN, which represents a stationary amplitude of about 50mm (point S) and a highamplitude 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 thestationary 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 stationarypoint. Point U represents the start of the high-amplitude instability, from where the dampingis (i.e., would be if such an amplitude occured) definitely non-sufficient during rain-windaffectation.The damper has to neutralize the galloping ,with the safety factor defined above ,and hasthe following value :

Necessary (+) damper = (-)gall - (+)

eigen = 0.048 - 0.006 = 0.042

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55- Efficiency of the friction damper

551 Efficiency affected by support and cable stiffness.

The effective damping varies with the vibration amplitude. Under ideal conditions, themaximum decrement is:

max = eigen+ (L/L)

L cable length

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

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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Location of the external Damper

L / L= 0.015 to 0.025


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

552- Efficiency related to the friction force

The main parameter of the friction damper is the friction force F. If this friction force is notcorrectly 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 casescan be resolved by adjustment of the damping force during simple maintenance . In anextreme case of the damper being stuck / blocked , the damper will act as a fixed point,and no damping will be provided to the cable. In the opposite extreme case, with zerofriction, 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 adifferent starting amplitude. The damper starts to act when the transverse force on thecable , at the fixation of the damper , reaches the friction force. The transverse force is thecable tension multiplied with the sine of the cable deviation angle at the damper. Highermodes produce cable deviation angles similar to the first mode at half, one third, etc. of theamplitude of the first mode, for the 2nd, 3rd etc. mode, respectively. The efficiency of thedamper 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 notexcited 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 isthe 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 damperis correctly dimensioned with a safety factor of 2 applied to the excitation.

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Max = . (-)galloping

with = 2

Hence, the amplitudes are stopped to grow at the stationary point S, i.e., usually atA0=L/3000. However, the damper has reserve capacity (= 2 ) until it reaches itsmaximum 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 examplewith a cable of L=215 m (the longest cable of Uddevalla Bridge), the snap-through hurdle isabout 350mm.

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Fig.

10

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F

ig.

11

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Fig.

12

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VI. REFERENCES, PUBLICATIONS

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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)

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

61. Recent references in cable-stayed bridges

The present design of the friction damper has been installed on the Uddevalla Bridge inSweden 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 summer2000. Since this date the bridge has been exposed to variable (up to strong) winds but nocable vibration has been reported since the installation of the friction dampers up to theend of 2001.

A report from JOHS HOLT, the bridge designer, about the performance of the stay cablesequipped with the friction dampers is attached in the following page . The figures 13,14and15 show the stay cables of Uddevalla Bridge ( Sweden ) during and after theinstallation 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 thestay cables has been achieved by the CSTB (Centre Scientifique et Technique duBtiment) to evaluate the risk of cable vibrations. One conclusion of this analysis has beento equip the four longest cables (6-42) of the bridge with the friction 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 (firstgeneration) on all the cables (cable sizes 6-42 to 6-80).

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62. Publications on the friction damper

! Development of new stay cable dampersYves BournandFIB Symposium - Prague 1999

! Damping devices against cable oscillations on Sunningesund BridgeI. Kovacs, E. Strommen, E. Hjorth-HansenCable dynamics Symposium - Trondheim 1999

! Performance of a friction damping device for the cable on Uddevalla cable-stayedBridge.E. Hjorth-Hansen, C. Hansvold, R. RonnebranntCable dynamics Symposium - Montreal 2001

Le pont du troisime millnaire , Gdansk .J. Mossot Y. BournandTravaux Dcembre 2001

Controlling vibration of stay cableY.BournandFib congress Osaka 2002

63. Experience with dampers on Uddevalla BridgeAttached is a copy of a letter provided by the designer of the bridge.

64. Photos of friction damper

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

65. References

[1] PTI (Post-Tensioning Institute)Recommendations for stay cable design, testing and installation - February 2001

[2] I. Kovacs, E. Strommen, E. Hjorth-Hansen - Damping devices against cableoscillations on Sunningesund Bridge - Cable Dynamics SymposiumTrondheim 1999

[3] G. Hirsch, H. Bachmann - Dynamic effects from wind - Vibration problems instructures - C.E.B. 1991

[4] Den Hartog - Mechanical Vibrations, Mc Graw-Hill, 1956

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

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Fig. 13: Friction damper without protection sleeve

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Fig. 14: Friction damper with protection sleeve

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Fig. 15: Friction damper with protection sleeve.

vsl - the fricvtion damper presentation

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