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1 Introduction

With the development of calculation theories, materials and construction technologies, more and more sea-crossing bridges have been constructed or being under construction in China. Compared with bridges on land, these sea-crossing bridges are subjected to tide and wave, which may result in structural vibration and foundation scour. In these sea-crossing bridges in China, Hangzhou Bay Bridge is one of the most wonderful and famous with a total length of 36 km across the Hangzhou Bay, located in the southeast of China [1], as in Fig. 1. In this project, curved continuous steel box girder ramp bridges are designed and constructed to connect the main bridge and the offshore platform, as in Fig. 2. The main span arrangement of ramp bridges is 6 × 20 m, with single column pier of single pile, as in Fig. 3.

Fig. 1
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Location of the Hangzhou Bay Bridge and the offshore platform

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Fig. 2
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Ramp bridges connecting the main bridge and the offshore platform

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Fig. 3
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The ramp bridges with single column pier of single pile, a elevation view, b side view photo

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As a result of complex hydrological conditions of the Hangzhou Bay, the ramp bridges have a severe foundation scour problem. To makes matters worse, the land reclamation on both shores of the Hangzhou Bay will make the hydrological situation more complicated and result in more serious scour problem. Meanwhile, tide-induced vibration was found during construction and exists during operation. Therefore, a structural health monitoring system (SHMS) is specially designed and implemented on the ramp bridges for further study of the structural behavior.

2 Design and Implement of the SHMS

In the SHMS, both environmental parameters and structural response are monitored by integration of sensor technology, measurement technology, communication technology and computer technology [2]. The SHMS consists of four subsystems, which are the sensor system, the data acquisition and transmission system, the data processing and management system, and the structural evaluation system.

The sensor system includes ultrasonic anemometer, temperature transducer, displacement transducer, accelerometer and inclinometer. Integrated circuits of low noise amplifier and filter are adopted in signal conditioning module.

Distributed data acquisition system is designed to meet the need of wide range monitoring. And those adjacent sensors are connected to the same data acquisition instrument on-site through wires. All the instruments on-site are connected to the central server by optical fiber local area network considering transmission capacity and system expansion. Data is synchronized by GPS timing module on data acquisition instruments.

The hardware part of the data processing and management system is a central server with a large capacity hard disk array, while the software part is specially developed for data collection, processing and storage. For data processing, various algorithms are used including time domain analysis, frequency domain analysis, and correlation analysis. Both original and processed data can be displayed on the monitoring interface, which is based on Web page.

The structural evaluation system is specially developed by comparing the identified results and the theoretical analyzed results. For the ramp bridges’ response, both displacements of bearing and girder vibration amplitude are critical to structural evaluation.

3 Structural Performance Based on Long-Term Monitoring Data

The monitoring content includes wind, temperature, support displacement and girder vibration. There are totally 1 ultrasonic anemometer, 25 temperature transducers, 40 displacement transducers, 50 accelerometers and 18 inclinometers.

3.1 Wind Parameters

A 3D ultrasonic anemometer is installed above the deck to monitor the wind parameters, which include wind speed, wind direction, wind attack angle. Based on long-term monitoring data, the actual wind field characteristics are obtained, including distribution of wind speed and direction, turbulence intensity and gust factor. In order to study wind effect on the bridge, wind parameters during a typhoon are analyzed in detail, which happened on 8th August, 2012.

Distribution of horizontal average wind speed and direction at interval of 10 min is shown in Fig. 4a. Along wind gust factor and mean wind speed changing with time are shown in Fig. 4b. The mean wind speed during typhoon is over 10 m/s and the maximum value is about 13.9 m/s. The wind direction changes from northeast to southeast, which matches with the change of location of typhoon center relative to the bridge from southeast to southwest. The gust factor decreases below 1.5 as the wind speed increases and remains stable.

Fig. 4
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Examples of the monitored wind parameters during a typhoon, a mean wind speed and direction map, b along wind gust factor and mean wind speed

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3.2 Temperature

For continuous girder bridge, expansion and contraction of main girder according to the change of temperature will lead to displacement at sliding bearings and horizontal thrust on fixing supports. As for the ramp bridge, the temperature effect is more complicated and may result in disadvantageous structural response. In order to study the actual temperature effect, monitoring of temperature is carried out. Platinum resistance temperature transducers are adopted to monitor both air temperature and steel box girder temperature. Through 20 temperature transducers on a section of the box girder, the structural temperature distribution is monitored. Based on long-term monitoring, temperature distribution under different conditions are acquired and the most unfavorable distribution under high temperature is shown in Fig. 5. The most unfavorable high temperature distribution happens at 15:05 on 4th July, 2012, and the external air temperature is about 35.4 °C.

Fig. 5
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Structural temperature distribution on a section of steel box girder (°C)

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It is found that the temperature distribution of the steel box girder is sensitive to solar radiation. The temperature on top plate is obviously higher than other plates because the top plate is directly subject to solar radiation. The temperature on web plate decreases from top to bottom as a result of temperature conduction in steel material. And the temperature on bottom plate is the lowest but a little higher than the external air temperature, because it is mainly affected by temperature conduction on both internal and external boundaries of air and steel material.

3.3 Displacement of Bearing

As the length of continuous girder bridge increases, the total temperature deformation at both ends will certainly increase. Therefore, the bearing at both ends must function well to adapt to the requirement of temperature deformation. Otherwise, the constrained deformation will inevitably cause considerable thrust and bending moment on bridge piers. It is important to monitor the working condition of bearings. There is a pair of bearings at each end of the continuous steel box girder bridge. One is longitudinal sliding bearing, while the other is bi-directional sliding bearing. Therefore, 5 displacement transducers are installed at every bearing pair, which consists of 2 longitudinal ones, 2 vertical ones and 1 lateral one.

To study bearing deformation, monitored displacement values are analyzed with monitored air temperature. In Fig. 6, both longitudinal displacement of one sliding bearing and the air temperature are averaged by interval of 10 min. Obviously, bearing deformation has certain linear correlation with air temperature at long time scale. Long-term monitoring data shows that the bearings function well.

Fig. 6
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Statistics of longitudinal displacement versus air temperature

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3.4 Girder Vibration

Numerous accelerometers are installed on steel box girder to monitor girder’s vibration in longitudinal, lateral and vertical directions. Original acceleration data shows that vehicle-induced vibration is prominent in all three directions, as in Fig. 7a. Although vehicle-induced signal is prominent in amplitude, it is high-frequency and decays rapidly. So a low pass filter is designed to eliminate the interference of vehicles. After filtering vehicle-induced vibration signal, it is found that lateral vibration is the most obvious, which is caused by tidal wave in the Hangzhou bay. There are four obvious vibration peaks corresponding to two cycles of ebb and flow in a day [3], as in Fig. 7b.

Fig. 7
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Typical acceleration curve in a day, a original vibration data, b filtered vibration data

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Through frequency domain analysis, the structural vibration frequencies are distributed in a wide range. When looking at the dominant frequency, one interesting phenomena is observed that the dominant frequency changes in 24 h, as in Fig. 8. This frequency variation is mainly due to the change of water depth, which leads to variation of structural associated mass [4].

Fig. 8
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Variation of dominant frequency in 24 h

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4 Finite Element Analysis

4.1 Finite Element (FE) Modeling and Prediction of Scouring Depth

FE method is introduced to analyze structural dynamic characteristics and to evaluate the structural status by use of the ANSYS software. Shell63 element is used to simulate the steel box girder and Solid95 element is used to simulate the concrete pier, as in Fig. 9.

Fig. 9
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FE model of curved continuous bridge

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4.2 FE Model Optimization

Before using the FE model, it is important to modify the original model. In this case, scouring depth is crucial because it has a great influence on the basic modal frequencies [5], as in Fig. 10. It is evident that the modal frequencies decrease as the scouring depth increasing, because the increase of free length of piers reduces the structural integral stiffness inevitably. Since material parameters and structural geometric parameters are relatively precise, measured modal frequencies and shapes can be used to determine the actual scouring depth. Based on the measured results, the current estimated scouring depth is about 13 m from completion of the structure 4 years ago. And the estimated results match quite well with the results given by annual flush detection on site.

Fig. 10
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Calculated basic modal frequencies versus scouring depth

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4.3 Structural Performance Evaluation

Using the modified FE model, various calculations can be carried out to evaluate the structural status.

First of all, the temperature effect is calculated by means of FE analysis and the results are compared with the measured ones. The girder is divided into top plate, bottom plate, left web, right web and diaphragm plate. Every part is applied with actually measured temperature independently. Then displacements at both ends of the continuous bridge are obtained and compared with measured ones, as in Fig. 11. It shows that the theoretical results agree with measured ones very well. This confirms the accuracy of the FE analysis and its availability in evaluation.

Fig. 11
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Comparison of the calculated and the measured temperature displacements, a one end of the bridge, b the other end of the bridge

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Then, the effect of tide-induced vibration is evaluated by focusing on material stress according to different amplitudes, as in Fig. 12a. Stresses under other kinds of load are also calculated, such as wind, temperature and vehicle, as in Fig. 12b. The most disadvantage condition of the structure is evaluated under different load combinations.

Fig. 12
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Stress distribution under different loads, a stress under tide-induced vibration, b stress under vehicle load

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It is found that lateral vibration will lead to slight tensile stress in local area of some piers. However, the tensile stress is still within the acceptable range.

5 Conclusions

Long term monitoring of the bridge offers the opportunity to observe structural responses under different environmental parameters. When combined with theoretical analysis, it is useful to evaluate the structural status.

  • Steel box girder is sensitive to solar radiation and the difference of extreme temperature distribution is very large, which should be paid more attention during designing.

  • Bearing displacements are mainly affected by overall temperature change and have a significant correlation with the temperature.

  • The structural vibration has a direct relationship with regular tides in the Hangzhou Bay. The lateral modal frequency is as low as about 0.5 Hz which is in the range of the tidal frequencies. Meanwhile, foundation scour caused by tides will lower the structural integral stiffness, which may make the vibration problems worse.

  • Although the ramp bridges under current condition are still in security status, continuous monitoring is suggested and more attention should be paid to foundation scour detection on site.

  • Finally, the influence of tides should be taken good care of for bridges under marine environment, including foundation scouring and structural dynamic responses.