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measurement the displacements of bridges. It is a difficult task, since bridges are designed

to overpass some distance, mostly over the sea or the river. The use of accelerometer base

measurement technique for evaluating bridge forced vibrations or to perform bridge

modal analysis is already established. The value of acceleration amplitude can be very

low at low frequencies. The limitations with accelerometer application are linked with

transducer types. Installation of the conventional devices for displacement measurement

is difficult and expensive. In addition to already presented global positioning system

(GPS) development and experience for bridge structural health monitoring, Laser Doppler

Vibrometer (LDV) have been suggested as an alternative for displacement measurement

on bridges, producing valuable information for structural integrity assessment.

LDV includes: a laser, a mirror, three beam splitters, a Bragg cell, and a photo

detector, Fig. 13. The mirror reflects the reference beam of laser frequency

which

passes through the upper beam splitter and adds to the beam from Bragg cell,

+ f

and

adds also to the beam obtained from the target,

+ f

. The mixture is detected by the

photo detector and analysed to give velocity as the result /18/.

Figure 13: Scheme of Laser Doppler Vibrometer (LDV) /18/

Digital image processing techniques as innovative, highly cost-effective and easy to

implement, still maintaining the advantages of dynamic measurements and resolutions. At

first, the measurement point is marked with a target panel of known geometry. A com-

mercial digital video camera is installed on a fixed point beyond the bridge (e.g. on the

coast) or on a pier (abutment), which can be regarded as a fixed point. Then, the video

camera takes a motion picture of the target marked at the measurement point. Meanwhile,

the displacement of the target is calculated using the image processing techniques, which

require the texture recognition algorithms, projection of the captured.

The displacement measurement is more appropriate. Used locations of a bridge for

this purpose can be different, depending on the bridge design, e.g. the lower deck of

bridge can be used, Fig. 14. LDV detects relative vibrations as opposed to the absolute

measures of accelerometer. Some results in field measurements on a highway little bridge

excited by traffic under normal operating systems will be presented through some

pictures. The accelerations measurements were performed by accelerometer for civil

engineering structures and by LDV with a fringe counter board in terms of velocity and

displacement, and have been compared as calculated from LDV signals and the one

directly measured by the accelerometer. The advantages of the laser devices at lower

frequencies are clear and new possibilities are opened to analyse bridge loads in operating

conditions, monitor bridge health, predict fatigue life and improve bridge design.