The use of anti-vibrating bearings in the Taiwan High ... - Alga S.p.A.

The use of anti-vibrating bearings in the Taiwan High Speed RailwayAgostino MARIONICivil EngineerALGA S.p.A.Milano, ITALYAgostino Marioni, born 1943, received his degreefrom Politecnico di Milano in 1966. He isChairman of ALGA, bridge bearings, antiseismicdevices and post-tensioning systems manufacturerand Chairman of CEN TC 167, the EuropeanTechnical Committee for the standardisation ofbridge bearingsSummaryThe paper describes the isolation system utilised in the Taiwan High Speed Rail to protect from thevibration a High-tech industrial area. In particular are described in details the conception of thebridges, the design of the bearing system applying the concept of the mass-spring system and thenumerous static and dynamic tests performed on material samples, prototype bearings and currentproduction in order to optimise the performances and assure the quality.Key words: structural bearing, elastomeric bearing, rubber bearing, vibration1. IntroductionThe Taiwan High Speed Railway is one of the largest Civil Engineering Projects currently underconstruction. It includes over 8000 bridge spans totalling over 250 km length. Most of the bridgesconsist of simply supported pre-stressed concrete box girders ranging from 20 to 35 meter span.Part of the railway, for a length of 5 km approximately, crosses an important Hi-tech industrial parkin which many industries very sensible to vibrations will be located. To reduce the vibrations in thesurrounding environment flexible structural bearing has been adopted, realising the so called massspring-system.The mass-spring-system is well known to be the most efficient solution to reducevibrations and in this case it is particularly performing due to the fact that the mass supported by theflexible bearings consists of the entire bridge superstructure and superimposed dead loads. Thevertical stiffness of the bearings has been selected in order to obtain a natural frequency of thesuperstructure for vertical vibrations of the order of 10 Hz, providing a sufficient gap with thenatural frequency of the vehicles from one hand and the frequencies of structure-borne sound on theother hand. The bearings with the required vertical stiffness has been developed utilising a very lowdamping natural rubber compound suitable to minimise the dynamic stiffening effect. They havebeen designed for the service conditions -vertical loads and flexural rotations - in accordance withthe European Standard Draft prEN 1337-3. The horizontal movements due to creep, shrinkage andthermal effects are allowed, at the movable end of each span, by PTFE and stainless steel slidingsurfaces in order to minimise the shear deformation of the rubber. All horizontal loads due toservice and earthquake loads are supported by mechanical shear keys totally independent from therubber bearings.

2. The mass-spring systemVibrations caused by a railway supported by a bridge are transmitted through the foundations to thesurrounding soil ant to the near buildings. The vibrations are caused by discontinuities andirregularities of both the running wheel and the rail/track construction and act in the frequencyrange between 1 and 60 Hz.The most efficient solution to avoid the propagation of the vibrations is the so called mass-springsystem. Its basic concept is to prevent vibrations from penetrating in the substructure and from herein the soil by inserting a linear harmonic oscillator with a very low natural frequency.The natural frequency should be as low as possible in order to let the attenuation of vibrations startat the low end of the excitation frequency spectrum. Due to practical limitations one cannot gobelow a fundamental frequency of the system of 5 Hz, but is also important not to exceed 14 Hz.Normally the calculated natural frequency of a mass-spring-system lies between 8 and 12 Hz andthis provides a sufficient gap from the natural frequency of the vehicles (1 to 4 Hz) on the one handand the frequencies of structure borne sound (mainly 20 to 60 Hz) on the other hand.The “spring” usually consists of elastomeric elements (bearings), which can be characterised by alinear behaviour over the whole loading range. From the load-deformation curve of the bearing, thespring stiffness (k = load/deflection, kN/mm) can be evaluated. If the damping of the spring isneglected, it can be demonstrated that a dead load deflection of the bridge equal to D (cm), leads toa natural frequency F of:F =5D(1)For example, a deflection of 3 mm under the weight of the superstructure results in a theoreticalfrequency of 9.1 Hz. The actual frequency usually turns out to be 10%-20% higher than thecalculated one due to non-linear effects, depending on the elastomeric material used in themanufacturing of the bearings.Elastomeric springs always have dampingproperties (that are essential to avoidresonance) and therefore the mass-springsystem may be schematised by a systemconsisting of a mass m attached by means of aspring k to an immovable support; the springacts in parallel to a viscous damper.Fig. 1: Single degree of freedom system withviscous damping, excited in forced vibrationThe differential equation of motion for the single degree of freedom system with viscous dampingshown in Figure 1, when the excitation is a force F = F 0 sinωt applied to the mass, is:m&&+ x cx&+ kx = F0sinωt(2)An important factor that may be deducted by the equation of motion and well represents theeffectiveness of the mass-spring system is R d , the dimensionless response factor giving the ratio ofthe amplitude of the vibratory displacement to the spring displacement that would occur if the forceF is applied statically. At very low frequencies R d is approximately equal to 1; it rises to a peak nearthe eigenfrequency value and approaches zero as the frequency becomes very large.

For the Taiwan High Speed Rail a fundamental period of the system of around 10 Hz has beenchoosen and the spring has been realised with elastomeric bearings. The dimensionless responsefactor is plotted in the following diagramFig. 2: The response factor of a mass-spring system with eigenfrequency of 10 HzFrom the diagram it may be seen that a certain level of damping, as given by the rubber bearings, isessential in order to avoid resonance effects.3. Static scheme of the bridgesThe mass-spring antivibrating system hasbeen applied for a bridge length of 5390 mincluding 184 spans ranging between 20 and35 m. The bridge structure consist of simplysupported prestressed concrete box beams.Each beam is supported by four antivibratingbearings carrying the vertical loads. Thehorizontal loads due to braking, earthquakeand other effect are supported by a fixed and amovable shear key placed in accordance withthe following scheme.Fig. 3: Static scheme of the bridgesThe fixed shear key is designed to transmit the horizontal loads in any direction, whilst the movableone is designed to transmit the horizontal load in the transversal direction allowing the movement inthe longitudinal one. Both shear keys are designed to allow rotation around three axis and thevertical displacement in order to accommodate the deflection of the antivibrating bearings without

interfering in their load bearing capacity, what would reduce the isolation from the vibrations. Thefeatures of the shear keys are shown in the following pictures.Fig. 4: The portion of Taiwan High Speed Railwith antivibrating bearings under constructionFig. 5: Detail of the upper part of a fixed shearkey before casting the concrete4. Features of the antivibrating bearingsThe antivibrating bearings mainly consist of reinforced elastomeric bearing designed in accordancewith the European Standard prEN1337-3. They are of circular shape with different diameter infunction of the required load bearing capacity as shown in the following table. In addition to theload bearing capacity the most important property of the antivibrating bearings is the verticalstiffness that was established by the designer of the bridge and is the main parameter defining theisolation property of the mass-spring system.Type Span Diameter Stiffness Max Vertical load SLS Max Vertical load ULSm mm kN/mm kN kNA 20 to 30 750 1600 7100 10090B 20 to 30 800 1600 8600 10190C 20 to 30 850 1600 9370 12520D 35 850 2200 9740 11880The antivibrating bearings include also external steel plates with anchors for fixing to the concretestructures and two lateral antilifting devices. The antilifting devices are required to withstand thenegative reaction that is developed by the lateral earthquake forces in the loading condition of onetrain only on one rail. The antivibrating bearings placed on the movable bearing axis are providedwith a sliding surface consisting of PTFE and stainless steel in accordance with EN 1337-2. Thescheme of the antivibrating bearing and a picture of an installed one are given in the followingfigures.The stiffness of the bearings was selected as 1600±200 kN/mm for the bridges up to 30 m span and2200±200 mm for the bridges of 35 m span corresponding to a deflection of the bearings underdead load of approximately 2,5 mm, corresponding to an eigenfrequency of 10 Hz according toequation (1).

Fig. 6: Scheme of an antivibrating bearingFig 7: An installed antivibrating bearingIt is very difficult to predict the deflection of the rubber springs because there is always aconsiderable discrepancy between the theoretical formulae and the real behaviour of the springs. Inaddition to that the rubber springs present different stiffness for quasi-static and dynamic loading:all rubber springs are stiffer if tested dynamically and the so called dynamic stiffening depends onmany factors but specially the rubber compound and the testing frequency. The dynamic stiffeningis put in evidence in the following diagram where the load-deflection plot for a quasi-staticcompression is superimposed to that obtained dynamically cycling the bearing with a load of2250±225 kN at a frequency of 8 Hz. The increase of stiffness may be evaluated from thedifference of inclination of the two superimposed plots and is equal to 60% approximately. Inaddition to that the stiffness of the elastomeric bearings is affected by temperature and ageing. Toget the appropriate dynamic stiffness a series of prototype test had to be performed modifying therubber compound and thickness and number of the rubber bearings until the required performancewas achieved.Fig. 8: Superimposition of the static and dynamic deflection plots of an antivibrating bearing. Thedynamic behaviour corresponds to the small loop that also puts in evidence the dynamic stiffeningand the damping

5. Test resultsSeveral tests has been performed to adjust the performance of the devices on prototypes and toassure the quality of the installed bearings.All the tests have been performed in ALGA’s laboratory that is equipped with static and dynamicequipment suitable to apply loads up to 50000 kN. The test performed were of three different types:• Prototype tests that aimed to adjust through a trial and error procedure the rubber compound andthe geometry of the devices until the required dynamic stiffness has been reached. Several rubbercompounds and several geometries with different number and thickness of rubber layers havebeen tested statically and dynamically until the optimum solution was reached.• Dynamic production tests, in order to verify the dynamic stiffness of the antivibrating bearingduring the current production. The dynamic tests have been performed pre-loading statically thebearings with a load corresponding to the permanent one for a period of one hour and thenapplying a sinusoidal load corresponding to the live load effect. This procedure has beenfollowed in order to avoid creep effects that would have been shown applying the sinusoidal loadwithout pre-loading. The dynamic tests have been performed on approximately 5% of theproduced bearings• Quick production tests. This kind of tests have been performed in accordance with EN 1337-3for 100% of the manufactured bearings. The aim of these test was to verify that the static verticalstiffness was in line with the expected one and to prevent any possible vulcanisation defect onthe bearings to be installed in the bridges.A detail of a bearing during the test and a typical output of the dynamic tests are shown in figures 9and 10.3200315031003050Load (kN)30002950290028502800275027002,3 2,35 2,4 2,45 2,5Deflection (mm)Fig. 9 Detail of an antivibrating bearingduring a dynamic testFig. 10: Typical output of a dynamic test5. ConclusionThe application of antivibrating bearings described in this paper is probably the most important oneever utilised for a High Speed Rail in the world and represent a very innovative solution to protectthe surrounding areas from the vibrations.The success of the application has been possible thanks to a very extensive program of preliminaryresearch and testing and to a very severe quality control during the production phase.