Evaluation of Stress and Strain in Structural Components Using LiDAR Scan Data

Abstract

Assessing structural behavior with conventional field monitoring techniques is a challenging task that can be time consuming and expensive. This is especially true when the structure being investigated is a highway bridge where access issues can make it difficult and dangerous for personnel to install and maintain conventional monitoring components, such as strain gauges, that must be physically attached to the structure. Even digital image correlation (DIC), an optical method that is often thought of as a noncontact approach, does not avoid this problem since a speckled pattern must be applied directly to the component to facilitate the tracking of surface displacements. Access to the structure was a primary consideration during a recent Auburn University research project investigating the behavior of horizontally curved steel girders on a large bridge (Ramp C) over I-565 in Madison, Alabama. The 650 ft bridge, consisting of three continuous spans over six lanes of interstate traffic, presented instrumentation challenges because of its size, its proximity to traffic, and the number of strain gauges needed to capture relevant data. Additionally, the relatively small web slenderness ratio of the girders would likely prevent or severely limit the web breathing behavior we had hoped to study on this project. Because of the multiple challenges and the real possibility that the desired behavior would not be observed, the decision was made to forgo conventional field measurement methods and to pursue a novel structural monitoring technique based on geometric data (point cloud data, scan data) obtained through the use of a conventional, terrestrial LiDAR scanner. The LiDAR structural monitoring (LSM) method explored here requires two LiDAR scans of the structural component being evaluated -- one before and one after an applied load or an enforced displacement. The raw point data for each scan is transformed from a random collection of points into a regular, gridded representation of the scanned component. These discretized representations provide the foundational geometry for the mathematical descriptions of the undeformed and deformed component that are needed to determine the change in surface strains that accompany the change in shape. Cross-section properties and constitutive relationships are used in combination with the change in surface strains to determine the change in the state of stress at any point on the surface of or within the component. The proposed LSM method is evaluated through the use of three analytical problems, two laboratory test problems, and field monitoring of the Ramp C bridge in Madison, Alabama.