Engineering Failure Analysis 14 (2007) 1065 1082 Author's ... · M. Siegwart et al. / Engineering...

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Transcript of Engineering Failure Analysis 14 (2007) 1065 1082 Author's ... · M. Siegwart et al. / Engineering...

This article was originally published in a journal published byElsevier, and the attached copy is provided by Elsevier for the

author’s benefit and for the benefit of the author’s institution, fornon-commercial research and educational use including without

limitation use in instruction at your institution, sending it to specificcolleagues that you know, and providing a copy to your institution’s

administrator.

All other uses, reproduction and distribution, including withoutlimitation commercial reprints, selling or licensing copies or access,

or posting on open internet sites, your personal or institution’swebsite or repository, are prohibited. For exceptions, permission

may be sought for such use through Elsevier’s permissions site at:

http://www.elsevier.com/locate/permissionusematerial

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Obtaining evidence of structural safety using a fibreoptical monitoring system on the example of the ‘‘Wave of Bern’’

M. Siegwart a,*, M. Wanner b, P. Zwicky a

a Basler & Hofmann AG, Consulting Engineers, Structural Monitoring, Forchstrasse 395, 8032 Zurich, Switzerlandb Schweizerische Bundesbahnen, SBB, Olten, Switzerland

Received 8 August 2006; accepted 30 November 2006Available online 6 February 2007

Abstract

In 2004, the roof structure of the main train terminal in Bern was extended to cope with increasing passenger volume.For this reason, an innovative architectural structure was erected across the passenger platforms. Although the wave-shaped roof with its choice of light materials was designed with diligence and care, there were some remaining uncertaintiesto the full extent of the roof behaviour under the given wind exposure, which could not be resolved using the conventionaltheoretical design approach alone. To clear these minor uncertainties, the structural monitoring department of Basler &Hofmann was approached to install a fibre optical structural monitoring system on the most exposed steel/glass roof struc-ture (GD 5) of the train terminal, so that the influence of high wind loads on its behaviour could be monitored. The ridgepurlin of the main roof is subject to vertical and horizontal loads from various loads, which sometimes act on the structurewith eccentricity. Therefore, the purlin is subject not only to bending but also to torsion stresses. Therefore, it was impor-tant to carry out multidirectional vibration monitoring of this ridge purlin. A variety of different sensors was used to reg-ister the static structural and dynamic behaviour. The vibrations of the purlin induced by vertical loading were measuredusing optical strain sensors (System OSMOS); the torsion behaviour was monitored using inclinometers and accelerome-ters. The latter sensors were also used to register any wind induced wobbling of the outer edges of the steel/glass roof. Thecorrelation between wind speed and vibration behaviour was accomplished by using a wind anemometer which wasinstalled on the roof structure. The aim of the monitoring system was to obtain a clear understanding of the behaviourof the roof, especially when it was subject to strong wind loads. The results were needed to make reliable predictions ofthe long term aerodynamic stability of the roof. In consequence, structural monitoring can be used on innovative struc-tures to verify the theoretically expected behaviour and gain confidence in the stability of the structure. In the long term,monitoring can be used as effective asset management tool to optimise spending on maintenance and repair. In case ofunforeseen events such as very high snow loads, monitoring could act either as alarm system or allow for the structureto being operated safely.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Structural monitoring; Vibration analysis; Strain sensors; Wind speed; Aerodynamic stability

1350-6307/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2006.11.060

* Corresponding author. Tel.: +41 044 387 1371; fax: +41 044 387 1100.E-mail address: [email protected] (M. Siegwart).

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1. Introduction

The train terminal in Bern was extended due to a significant increase in passenger volume induced by thechanges caused by the substantial overhaul of the Swiss Train Schedule, due to the general expansion of theSwiss Railway infrastructure and stock, which is part of the strategy ‘‘Bahn 2000’’. As part of the extensionworks a new and innovative roof structure representative in design for the capitals’ main train station was con-structed across the passenger overpass. The overpass was covered by five wave-shaped roofs in glass–metalconstruction (see Fig. 1).

The form of the roof in combination with its wind exposed location required from a structural engineerspoint of view proof of safety against wind induced vibrations under extreme wind loads. However, due tothe complexity of the geometrical form and the resulting lack of knowledge about the fluid mechanics ofthe roof this structural proof of safety was not adequately possible using theoretical models. Therefore, onehad to either model the performance of the roof in a wind tunnel, with all the cost and remaining uncertaintiesor to equip the roof with a monitoring system (System OSMOS). The commercialisation of fibre optical strainsensors based on the measurement of light attenuation is inexpensive and robust and thus has facilitated tocarry out accurate long term strain measurements where this had previously not been possible due to commer-cial or technical constraints [1,2]. Therefore, the technique was used to monitor the vibration behaviour of the‘‘Wave of Bern’’, the afore mentioned roof structure.

Fibre optics in monitoring eliminates some of the problems that exist with conventional, i.e. electrical sen-sors, one of them is the relative robustness of the sensors. The second one is their inertness against other elec-trical sources. For example, resistance based sensors can not be installed nearby operating electrical fields asthis would give erroneous readings. Fibre optics in contrast, can be used in combination with electrochemicalrepair methods [3,4] to provide continuous monitoring results while the repair is being carried out. Further,with the prices of these systems falling, monitoring systems can be integrated from the beginning as tool forproactive asset management strategies [5].

One of the most important features of fibre optical sensors is the lack of the time drift phenomenon [6] and,therefore, the suitability for long term monitoring in cases when recalibration of the system is not possible.

Fig. 1. Computerised picture of the Wave of Bern, Switzerland.

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In general, monitoring systems are only applied when they are financially and technically advantageousover traditional surveying methods such as inspections [7]. In this particular case, the combination of the costadvantage of monitoring of the real structure compared to wind tunnel experiments, the quality of data gainedfrom monitoring, i.e. the real structural behaviour and also the already high level of confidence one had intothe structural design, lead to this monitoring system being used. The common believe is that monitoring sys-tems are installed when a structure is in critical condition. This is the reason why monitoring is often associ-ated with a safety issue. When it then comes to apply a monitoring system on a newly build structure there isalways some degree of reservation on side of the ownership and the designer as nobody likes being associatedwith owning or designing ‘‘unsafe’’ structures [8]. In this case, the monitoring system was installed to reinforcethe structural design statement.

In May 2005, the monitoring system comprising a monitoring cabinet, fibre optical strain sensors and anumber of conventional sensors was installed at the glass roof GD 5. The monitoring period was 12months and in July 2006, the system was de-installed. During the monitoring period, it was possible togain first hand information on the structural performance of the roof under strong (extreme) wind loads,such as during the winter storm period. A cross check with theoretically obtained parameters (natural fre-quency) was carried out after three months of operation to confirm the suitability for the measurementtask.

2. Monitoring system, sensors and installation

2.1. General description of the monitoring system

The measurement system installed at the Wave of Bern comprised a fibre optical interrogator and four lightattenuation based fibre optical sensors. Further, four conventional 3D accelerometers and one inclinometerwere installed. A wind anemometer was placed on the roof to measure wind speed and direction used to triggerdynamic measurements. A temperature sensor to distinguish between temperature inducted deformation anddeformation due other causes (e.g. wind) was installed nearby one of the strain sensors.

All of the sensors were connected to a data collection, pre-analysis (for alarms) and data-transmission unit.The data was transferred to a secure web server, from which it was viewed and downloaded in frequent inter-vals or after strong winds had occurred. The data was carefully examined and analysed at the end of the mon-itoring period.

The fibre optical strain sensors have a dynamic resolution of ±1 lm/m. The sensors length varied between1.5 m and 2.0 m. However, it is possible to use sensor with lengths of up to 10.0 m. This way, it is possible todetected distributed, phenomenon such as cracks across the length of a beam, which cannot be seen usingsmall patch strain sensors, such as conventional strain gages or optical fibre Bragg sensors [9].

During the calibration period the recording rate of the system was set equal to its sampling rate of 100 Hz(i.e. 100 values per second were stored). The range of ‘‘normal operation’’ was defined during an initial cal-ibration period, where data was recorded almost continuously and also using a cross check with available lit-erature. Dynamic data were not required for the condition of ‘‘normal operation’’ as the roof was well withinsafe limits of operation. The trigger for storing dynamic events was set at 100 km/h wind speed or when thedeviation of the current wind speed value compared to the last average value was greater than 40 km/h. Dur-ing normal operation data was stored as average value (i.e. 1 value per 100 s) in order to reduce the amount ofdata to a manageable size.

2.2. Sensors on the roof GD5 of the ‘‘Wave of Bern’’

The sensors were placed in such a way that the complicated structural behaviour of the roof as combinedtorsion and bending moment bearing structure could be measured simultaneously and be recorded dynami-cally, if required.

Two optical strands (fibre optical strain sensors) were installed at midspan of the ridge purlin (steel tube) atthe side and soffit. Further, two strain sensors were installed at the secondary purlins, which are formed bywelded steel H-profiles. The optical strain sensors measure the deformations directly at the outer perimeter

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of each structural element. Because of this, the maximum stress amplitudes due to wind induced dynamicbending action and due to static effects could be registered.

An inclinometer was installed on the secondary beam in very close proximity to the ridge purlin. It was usedto obtain any vibration induced torque of the ridge purlin (i.e. dynamic torsion behaviour of the roof).

An accelerometer was installed at each of the four corners of the glass roof to measure the maximum accel-eration amplitude at the outer rim of the roof, allowing thus a cross check with theoretical models through thecalculation of the natural frequency. To measure the wind speed and direction a wind anemometer wasinstalled on the top to the roof number 5 (GD 5). A temperature sensor was also installed to differentiatebetween the temperature induced deformations and deformation due to other causes such as wind or the pres-ence of snow. In general, the temperature induced deformations are greater than those caused by other effects.

The measurement components, i.e. computer and fibre optical interrogator was placed in a cabinet on topof an elevator shaft to hide it from the views of the general public. The location of each sensor on the structureis shown in Fig. 2 and a photograph of each sensor at the structure is given in Figs. 3–8. The following sensorswere used in the monitoring system:

1. Optical strand on purlin (secondary beam, western side, measurement length 2.0 m).2. Optical strand on purlin (secondary beam, eastern side, measurement length 2.0 m).3. Optical strand on side of ridge purlin (main beam, measurement length 1.5 m).4. Optical strand on underside of ridge purlin (main beam, measurement length 1.5 m).5. Wind anemometer/wind direction sensor.6. Accelerometer south west (ACC_SW).7. Accelerometer north east (ACC_NE).8. Inclinometer at ridge purlin (sensitivity ±1�).9. Accelerometer south east (ACC_SE).

10. Accelerometer north west (ACC_NW).11. Temperature sensor on ridge purlin.

Fig. 2. Location drawing of sensors on roof GD 5 at the ‘‘Wave of Bern’’.

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Fig. 3. Optical strands on ridge purlin, the wind anemometer is visible through the glass.

Fig. 4. Optical strand on purlin (secondary beam).

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pyFig. 5. Fixings (of the inclinometer on secondary beam nearby the ridge purlin).

Fig. 6. Accelerometer with levelling wedge located at the outer edge of the roof.

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Fig. 7. Wind anemometer for measurement of wind speed- and direction.

Fig. 8. Measurement cabinet with PC, modem and fibre optical interrogator located above the elevator shaft between the glass roofs GD 4and GD 5.

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3. Monitoring results and discussion

3.1. Comparison of measured data with expected data range of wind speeds

The monitoring system was installed to measure dynamic effects such as resonance vibrations or criticalstrains which could have been induced by exceptionally strong wind loads. The term ‘‘exceptionally strong’’had to be defined for this particular location. For the purpose of classification a comparison was made withofficial meteorological data collected by swissmeteo. As reference event for which data was cross referenced,the storm Lothar, which devastated Switzerland and Europe during the night 26th of December 1999, waschosen. During this night peak wind speeds of up to 133 km/h were registered at the meteorological observa-tion station at Bern Liebefeld.

However, wind speed alone is not critical for the structural behaviour, the pressure exceeded by a certainwind speed is more important for the effects on the structure [10]. The relationship between wind speed andwind pressure depends on various parameters, but a simplified relationships can be used to obtain a validapproximation of the wind pressure [11]. This simplified relationship is shown in Fig. 9. The calculated windpressure during the monitoring period was approximately 0.3 kN/m2. However, the wind pressure at the sta-tion Bern Liebefeld easily can reach double that value.

The official weather station at Bern Liebefeld, however, is not representative of the conditions found attrain terminal Bern due to different height above ground and also due to the surrounding built environment.Therefore, more representative data for cross reference purpose had to be found. It became necessary to usedata sourced by a private company (Meteotest). The company operates a monitoring station in theLanggassstrasse (street) nearby the train terminal of Bern.

Like the train terminal, the house in the Langgassstrasse is located in the city centre of Bern. Therefore, itbetter resembles the conditions at the train terminal than the weather conditions at the official site at BernLiebefeld. This is the reason why the wind speed data collected by a private company was chosen for crossreference rather than the officially available data. During the storm Lothar in 1999 the station in theLanggassstrasse registered average wind speeds of 27.0 km/h and peak wind speeds of 52.0 km/h.

The average wind speeds from May 2005 until June 2006 recorded at the train terminal are shown in Fig. 10and set in relation to the wind speeds that can be expected at this location. The highest average wind speed was54.9 km/h; it was registered on 18th of June 2006 at 13:55 local time. However, the accompanying peak wind

Fig. 9. Measured wind speeds (average and peak winds) compared to officially registered storm peak wind speeds against static pressure.

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pyspeed (dynamic event) was below the threshold for the registration of dynamic events (100 km/h). Therefore,dynamic data was not recorded during this time.

In order to calibrate the system the dynamic triggers were set to a very low level during the first 2 months ofoperation. Whenever the trigger value was exceeded data was recorded at 100 Hz. The peak wind speedrecorded shortly after the installation of the system on the 29th of June 2005 at 16:36 local time. The averagewind speed was approximately 40 km/h, the peak wind speed was approximately 80 km/h. The effects causedby this peak wind were carefully analysed (see next paragraph). Negative dynamic effects did not occur. Fur-ther, the normal stresses and strains were well below any critical design limit state. Therefore, the threshold forregistration of dynamic events was chosen to avoid being set of constantly. At the same time, it was importantto set the trigger so that one would still be able to record useful data. The trigger was set to 100 km/h to makesure data would be recorded well before the structure would be in a potentially interesting situation and per-haps start to show resonance effects.

With respect to the wind speeds observed at the reference station, it was safe to assume that the best part ofthe possible range of wind speeds had already been registered during the period from May 2005 until June2005. It would be unlikely to obtain greater wind speeds than the ones already measured by the continuationof the monitoring regime.

3.2. Average deformations through wind loads and annual cycle

The dynamic deformations of the ridge purlin and secondary purlin caused by the peak wind speeds of80 km/h on the 29th June 2005, shortly after 16:36 local time are shown in Fig. 11. The accompanying accel-erations at the outer edges are shown in Fig. 12. Both graphs show a good correlation between wind speed andstructural peak effect. The gust winds caused short peaks of deformation and peak accelerations, which fadedout after a few seconds without causing resonance effects or critical stresses. It can also be seen from thegraphs in Figs. 11 and 12 that the thermally induced deformations over the monitoring period of more than

Fig. 10. Measured average wind speeds from the 25th of May 2005 until the 18th of June 2006 (Daily maximal values of average windspeeds in records 100 s intervals).

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pytwelve months are greater that those induced by other causes such as wind load. In general, those deforma-tions were not causing any significant stresses or strains.

3.3. Wind induced dynamic deformations

The analysis of the dynamic data recorded on the 29th June 2005 regarding displacements and stresses isgiven in this paragraph. The deformation of the outer perimeter of the ridge purlin with a sensor length of1500 mm caused by vertical bending vibrations is shown in Fig. 13. The deformation amplitude, d, was±0.01 mm. Therefore, the resulting strain, e, was ±6.7 · 10�6 and the stress, r, was ±1.4 N/mm2, which isdeemed insignificant. Similarly the deformation, d, of the same structural element, but in horizontal directionwas ±0.005 mm. The strain, e, was therefore ±3.3 · 10�6 and the resulting stresses, r, were with ±0.7 N/mm2

even lower than the stresses in vertical direction.The sensor length of the strain sensors on the purlin (secondary beam) was 2000 mm. The sensors both

measured deformations due to vertical displacements of the purlin. The sensors were placed perpendicularto the strain sensors of the ridge purlin (see Fig. 14). The deformation, d, in the western purlin were±0.015 mm giving thus strains, e, of the order of ±7.5 · 10�6 and stresses, r, of approximately ±1.6 N/mm2. The vertical deformation, d, in the eastern purlin were ±0.005 mm, with the strain, e, being in the rangeof ±2.5 · 10�6 and resulting in very low stresses, r, being of the order of ±0.53 N/mm2. Due to the very lowstresses found in the structure it was safe to assume that the stress levels caused by strong winds would be wellbelow any fatigue limit and insignificant regarding structural integrity.

The rotation at midspan of the ridge purling # [rad], was used to obtain the axial torsion as follows:

smaxð#Þ ¼ 4� #� G� D=2� 1=L

where G, is the shear modulus of steel, D is the outer diameter of the ridge purlin and L is the clear span of theridge purlin. The rotation #, which is also shown in Fig. 15, was ±0.1� which resulted in axial torsion smax(#)

Fig. 11. Correlation between deformations of ridge purlin and temperature; peak wind speeds (80 km/h) indicated by dashed vertical lines.

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Fig. 13. Deformations of outer perimeter of ridge purlin at mid span during the occurrence of the peak wind speed of 80 km/h (see dashedline; data measured on the 29.06.2005 at 16:36).

Fig. 12. Correlation between deformations of purlin (secondary beams) and temperature; peak wind speeds (80 km/h) indicated by dashedvertical lines.

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pyof ±20 N/mm2. This was about 10% of the design axial torsion capacity. The maximum amplitude of verticaldisplacement at the roof edges was calculated from the double integration of the baseline corrected and filteredacceleration signal (Fig. 16; bandpass filter between 0.1 H and 10 Hz). The vertical displacement, d, was±0.25–0.5 cm.

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Fig. 14. Deformation of outer perimeter of purlin at the connection point during the occurrence of the peak wind speed of 80 km/h (seedashed line; data measured on the 29.06.2005 at 16:36).

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Fig. 15. Axial inclinations of ridge purlin which cause torsion during the occurrence of the peak wind speed of 80 km/h (see dashed line;data measured on the 29.06.2005 at 16:36).

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Fig. 16. Displacement of the four corners of the glass roof during the occurrence of the peak wind speed of 80 km/h (see dashed line; datameasured on the 29.06.2005 at 16:36). Graphs are based on the double integration of the base line corrected and filtered raw accelerationdata.

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3.4. Side effect: snow load induced deformations

Although the system was aimed at recording the roof behaviour due to wind load, it was also used to assesthe impact of snow. After a sunny and warm period, in March 2006 very heavy snowfalls were recorded inSwitzerland. Albeit being well within the range of standardised snow falls, the effect of this snow load shouldbe clearly distinguishable by the monitoring system. The snow heights at a meteorological station at Bern Kir-chfeld during the winter period 2005/2006 are shown in Fig. 17. The very clearly distinguishable snow period(a) and period (b) were used to trace back the behaviour of the roof under snow load. Period (a) was from the13th of January 2006 until the 14th of February 2006; period (b) was chosen from the 20th of January 2006until the 11th of March 2006.

Fig. 17. Snow heights measured at Bern Kirchfeld. Investigated periods of time are indicated by a and b in the graph.

Fig. 18. Deformation of outer perimeter of ridge purlin and purlin at midspan at temperatures of �0.1 �C from the 13.01.2006 until14.02.2006 (Period of time a). Time on a non-linear scale.

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For the deformation analysis it was assumed that with identical temperatures and otherwise similar bound-ary conditions, any deformations could be put down to snow load of the roof. Deformations were regarded inisolation for temperatures of �0.1 �C, 0 �C and +2.7 �C. The deformations of the ridge purlin and purlin dueto snow load was analysed carefully for two periods of time and for each temperature.

Fig. 19. Deformation of outer perimeter of ridge purlin and purlin at midspan at temperatures of �0.1 �C from the 20.02.2006 until11.03.2006 (Period of time b). Time on a non-linear scale.

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Fig. 20. Resonance frequency of the glass roof (eastern side).

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Period (a) and period (b) are identical with times of the highest snow. The temperatures were chosen in sucha way as to obtain a large sample size and also to regard relevant periods of snowfall and defrosting. However,the analysis is shown only for deformations recorded at 0 �C as this temperature was found to be representa-tive of deformations registered at the other two temperatures.

The deformations of the outer perimeter of the ridge purlin and purlin for period a are shown in Fig. 18.Note, that the time scale is non-linear due to the different number of data points of the same temperatureavailable for any period of time. During this period there was a base level of snow of about 60 mm on theroof, which periodically grew up to 180 mm. The deformations can be put down to a load equivalent to a snowheight of 60 mm. The peak deformation registered at the end of period a is also in good agreement with thesnowfall registered on the 12th of February 2006.

The baseline deformation of the eastern and western purlin (both in vertical direction) was of the order of�0.510 mm/2000 mm. The vertical deformation due to snow were approximately 0.150 mm/2000 mm for60 mm of snow. The baseline deformation of the ridge purlin in vertical direction during the same periodwas approximately �0.160 mm/1500 mm. The snow loads caused very small deformation of approximately0.020 mm/2000 mm for 60 mm of snow. These deformations were even lower than the horizontal snow loadinduced deformations of the ridge purlin, which are approximately 0.030–0.040 mm/1500 mm per 60 mm ofsnow.

The deformations of the ridge purlin and purlin for period b (February/March 2006) shown in Fig. 19 aresimilar or slightly smaller than those described above although during this period the snow height increasedwithin a few hours from 0 mm to 180 mm. The lack of coherence with the deformations observed during per-iod a can be explained with different density of snow during both periods. The fresh snow was less dense thanthe snow that had accumulated over a couple of months during the first period (a). In summary, the influence

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Fig. 21. Resonance frequency of the glass roof (western side).

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of snow load on the roof was clearly distinguishable. However, it could not be quantified further due to thelack of data on snow density.

3.5. Cross check with theoretical model – analysis of natural frequency

It was important to check the validity and plausibility of the monitoring results and thus the entire mon-itoring system by comparison with theoretically expected results. This was done by comparison of rotationand displacements, from which the natural frequency can be obtained and compared to the theoreticallyexpected natural frequencies. The maximum dynamic rotation, neglecting the influence of temperature,recorded on the 29th June 2005 was �±0.08� or 0.00139 rad. Under the assumption that the purlin (secondarybeam) is stiff the dynamic displacements of the roof due to wind loads are approximately 9.1 mm. This con-firms the theoretically expected displacements of the order of a few millimetres.

In Figs. 20 and 21 the natural frequency of the eastern and western side of the glass roof is shown. It wasobtained by a Fourier transformation of the acceleration data. The results of the monitoring regime are there-fore in line with the natural frequencies from the structural design (between 2.22 Hz and 3.00 Hz). Further, asecond natural frequency band at approximately 5.00 Hz could be confirmed.

In addition, the system showed frequencies at 0.8 Hz and 1.6 Hz. However, these lower frequencies wereonly observed on the western side. It is assumed that the zero crossing of the natural frequency should besomewhere on the eastern side, nearby the location of the accelerometer. This would explain why it couldnot be picked up by the sensors on the east side of the roof.

4. Conclusions

The monitoring regime on the Wave of Bern has demonstrated that a monitoring system can be used notonly on old and unsafe structures, but also on new structures to corroborate the design statements regardingtheir integrity and safety and confirm the agreement with theoretical engineering models.

In principal, the monitoring system was suitable to detect resonance effects on the Wave of Bern. Theresults showed that most likely there will not be any dynamic effects due to wind loads. During the measure-ment period from the 25th of May 2005 until the 18th of June 2006 no exceptionally strong wind speeds, in linewith those measured at official meteorological stations had been registered.

However, it is very likely that the wind speeds observed during the measurement period will not be exceededduring a storm. This can be put down to the surrounding environment of the train terminal which most likelyshelters it. During the measurement period no dynamic effects such as resonance vibrations or critical stresseswere observed. The stresses and strains during the measurement period were by far below the design stresses.

The deformations due to snow were of the order of 0.020 mm/2000 mm and 0.150 mm/1500 mm. Thestrains due to these deformations were of the order of 0.01&. Deformations due to snow load were also clearlyvisible using the system.

The maximum stresses were those due to axial torsion in the ridge purlin. The stresses were of the order of±20 N/mm2. The maximum stress was only about 10% of the design capacity.

References

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