Technique for Determining Bridge Displacement Response Using MEMS Accelerometers

Abstract

In bridge maintenance, particularly with regard to fatigue damage in steel bridges, it is important to determine the displacement response of the entire bridge under a live load as well as that of each member. Knowing the displacement response enables the identification of dynamic deformations that can cause stresses and ultimately lead to damage and thus also allows the undertaking of appropriate countermeasures. In theory, the displacement response can be calculated from the double integration of the measured acceleration. However, data measured by an accelerometer include measurement errors caused by the limitations of the analog-to-digital conversion process and sensor noise. These errors distort the double integration results. Furthermore, as bridges in service are constantly vibrating because of passing vehicles, estimating the boundary conditions for the numerical integration is difficult. To address these problems, this paper proposes a method for determining the displacement of a bridge in service from its acceleration based on its free vibration. To verify the effectiveness of the proposed method, field measurements were conducted using nine different accelerometers. Based on the results of these measurements, the proposed method was found to be highly accurate in comparison with the reference displacement obtained using a contact displacement gauge.

Keywords: bridge health monitoring; free vibration separation method; measurement error; microelectromechanical systems accelerometer; vehicle detection.

Figure 1

Test bridge used in field measurements (Units: mm).

Figure 2

Installation of accelerometers and contact displacement gauge.

Figure 3

Test setup for analyzing static characteristics of accelerometers listed in Table 2.

Figure 4

Static characteristics of accelerometers.

Figure 5

Displacement record at the longitudinal center of the lower flange of the main girder.

Figure 6

Numbers of girder bridges within different ranges of span lengths.

Figure 7

Displacement response spectrum at the longitudinal center of the lower flange of the main girder.

Figure 8

Filtered displacements obtained by separately applying a low-pass filter of 1.0 Hz and a bandpass filter between 1.0 and 20 Hz.

Figure 9

Application of proposed free vibration separation method of determining bridge displacement.

Figure 10

Installation of accelerometer for detection of vehicle exit.

Figure 11

Times of vehicle entry and exit based on acceleration record.

Figure 12

Displacement responses obtained using proposed free vibration separation method: (a) Accelerometer A; (b) Accelerometer B; (c) Accelerometer C; (d) Accelerometer D; (e) Accelerometer E; (f) Accelerometer F; (g) Accelerometer G; (h) Accelerometer H; (i) Servo-type accelerometer.

Figure 13

Displacement responses obtained using proposed free vibration separation method: (a) Accelerometer A; (b) Accelerometer B; (c) Accelerometer C; (d) Accelerometer D; (e) Accelerometer E; (f) Accelerometer F; (g) Accelerometer G; (h) Accelerometer H; (i) Servo-type accelerometer.