Laser Scanning Technology for Bridge Monitoring

After the collapse of the Minnesota I-35 bridge (August 1, 2007), there has been a renewed interest in the US to enhance bridge infrastructure monitoring (Liu et al., 2009).Other than developing traditional inspection and material testing techniques, there has been also increased discussions about possible applications of Commercial Remote Sensing (CRS) technologies for civil infrastructure monitoring (Al-Turk & Uddin, 1999, Shinozuka & Rejajaie, 2000, Chen et al., 2011).Laser scanning techniques are one of the remote sensing technologies that play significant role in environmental and infrastructure evaluation and monitoring.However, there are different sensing requirements for monitoring physical structures such as bridges, than conventional geospatial applications such as air quality, environmental impact and transportation operations, etc.The most important of which is the sensor resolution requirement.This chapter discusses critical bridge monitoring issues and provides examples of applications of two laser scanner technologies that are currently being developed for bridge monitoring: 1) range finding laser (static) and 2) scanning laser vibrometer (dynamic).Both laser systems are terrestrial and single point systems that utilize mechanical or optical scanning mechanisms to create a field of view (FOV) of the optical receiver.The range finding laser, also called LiDAR (for Light Detection and Ranging), is based on the transmission and receiving of pulsed lights.By determining the heterodyne laser beam phase shifts, scanning LiDAR can detect the distance information from a plane of data points, called point cloud.The point cloud information, which basically consists of the physical positions of any surface that the laser "sees", can then be used to detect useful critical information about a structure including the elevation (underclearance), surface (damage quantification) and deformation under loading (deflection), etc. Contrast to conventional analysis of photographic images, relatively simple algorithms can be used to manipulate the geometric point cloud data to retrieve the afore-mentioned information.Other bridge-related issues including validation of new constructions and comparisons before-and after critical event, can also be extracted from LiDAR scans. www.intechopen.comLaser Scanner Technology 72 Based on the measurement of Doppler effects of a returning continuous laser beam from a moving target, the scanning laser vibrometer (SLV) is a laser system that can detect the vibration of a subject.By covering the entire surface of a subject, SLV can not only detect the vibration frequencies but is able to separate the different vibration mode shapes of the subject (Oliver, 1995).This makes the SLV a very useful tool in isolating vibration-induced problems and in some cases, detect system or component level damages.Because of the non-contact nature and the ability of sensing from a distance away, scanning lasers have the advantages of limited disruption to traffic, low labor requirements and providing permanent electronic documentations of the temporal changes of a structure.Scanning Laser is ideal as a bridge field inspection tool and can help reduce the costs of inspection and at the same time, enhance the accuracy in field inspections.However, to implement laser scanning systems into bridge evaluation, one needs to understand the basics of bridge inspection practices and issues, in particular, recognizes the fact that some bridge issues are not necessarily associated with condition assessment, but with serviceability requirements such as adequate bridge underclearance, excessive bridge movements or traffic-induced vibrations.Scanning lasers alone will not provide the critical information associated with bridge inspection, additional evaluation methodologies usually are needed to extract the necessary information associated with the bridge problems.The examples provided will demonstrate some additional physical theories that can be used to identify critical bridge information that affiliate with actual structural conditions.