Real time monitoring of Dalian Beida Bridge

Wang Jun, Yi Xiaodong, Wei Erhu

There are many traditional surveying methods used for the large-scale bridge structure deformation monitor such as the accelerometer measure, the total station surveying and the laser collimation, but these methods are limited by its function of which the continuity, timeliness and automaticity can not meet the need of the large-scale construction dynamic monitor. In recent years, with the GPS hardware and software technology developed, especially the GPS receiver with the high data-collection frequency (for example 10Hz even 20Hz[1][4]) appearanced as well as the GPS data processing was improvement, the GPS-RTK technology applied in large-scale bridge dynamic deformition monitor with real-time or quasi-realtime has become true[2][3]. Further, with the Fourier transformation tool the bridge base frequency could be obtained, the data of bridge vibrational state in spatial frame and frequency range distributed characteristic may provide the key to understand whether the bridge structure is health under the load drive environment. Structure health monitor flowchart [7] is shown in fig 1.

Fig 1 Health monitor flowchart

Fig 2 monitor points distribution

Testing plan and process

Installment of GPS sensor antana

In the bridge structure health examination, the sensor type, quantity and the structure testing position will guarantee the monitor effective implementation. Unsuitable sensor disposition would affect the precision of recognition parameter, one good sensor disposition plan aims to achieve:
(1) If testing is in the noise environment, entirety and
accuracy structure parameter should be acquired with as few sensors as possible;
(2) Correlationship should be able to establish between the obtained testing modality and the model analysis result;
(3) The interested modalities data could be gather through adding sensors with reasonable means;
(4) The time interval testing record should be most sensitive to the modality parameter change.
Beida bridge is a suspension bridge with large-scale. Because the bridge is symmetrical, only half bridge were used to carry on the testing point arrangement.
According to finite element modality analysis result, the measuring points had been arranged at six sites including the 1/2 scale, the mid-scale, the 1/4 scale along two sides of the bridge and with the names of s1, s2, s3, n1, n2, and n3, the measuring points distribution and the bridge testing coordinate frame are shown in Fig 2:

Testing process with GPS-RTK

The entire test had been implemented by TRIMBLE-5700 of the dual-frequency GPS receiver, S0, the GPS base station, located in spacious place nearby the bridge as seen in Fig 3 (a) where the error of multipath effect should be weaken, and rover station placed at s1, s2, s3, n1, n2, n3 separately, its main error source is the receiver system noise. The GPS receiver data sampling frequency was set 10HZ during testing process and data gathering time in each testing point will sustain more than 1h.

Spatial distribution of dynamic deformation for bridge structure

Generally, with the external force (such as typhoon, earthquake and heavey-load car), the large-scale bridge vibration takes on simple harmonics nature, and its amplitude and frequency are changed too when the external force are changed, therefore, two kind of situations exists in the bridge dynamic deformation characteristic distribution .
The vibration of large-scale bridge satisfies the simple harmonic motion equation, but its amplitude and frequency has a sudden change during bridge amplitudes (or vibrates).The simple harmonic motion equation is:

Where, A is an amplitude; f is a frequency; y is the deformation quantity; φ0 is the beginning phase; t is the time.
2)The vibration frequency of largescale bridge maintains invariable, but the amplitude becomes the exponential decay. The equation is:

Where, α is the coefficient of attenuation; is the initial time; Other variables meaning is as same as Eq (1).
In order to obtain Beida bridge vibration characteristic, through the long time interval GPS dynamic monitored in each testing point, the massive monitor data have been acquired with their coordinates(N, E) in WGS-84 reference frame. For the convenience of data processing and comparison, using the coordinate transformation equation with three parameters(Eq 3.), changed these monitor data of (N, E) into testing coordinates system of (X, Y) .

Because along the direction of x and y axial, bridge vibration are smaller than that of z axial, therefore the structure vibration time interval curve obtained through computation is along the z axis.
With abscissa axisis x of time (s) and axis Z of ordinatesis amplitude (m), Fig 4 shows the structure vibration time interval chart of the bridge at first 4 testing points.