Indirect structural health monitoring in bridges: scale experiments

In this paper, we use a scale model to experimentally validate an indirect approach to bridge structural health monitoring (SHM). In contrast to a traditional direct monitoring approach with sensors placed on a bridge, the indirect approach uses instrumented vehicles to collect data about the bridge. Indirect monitoring could offer a mobile, sustainable, and economical complementary solution to the traditional direct bridge SHM approach. Acceleration signals were collected from a vehicle and bridge system in a laboratory-scale experiment for four different bridge scenarios and five speeds. These signals were classified using a simple short-time Fourier transform technique meant to detect shifts in the fundamental frequency of the bridge due to changes in the bridge condition. Results show near-perfect detection of changes when this technique is applied to signals collected from the bridge (direct monitoring), and promising levels of detection when one uses signals from sensors on the vehicle (indirect monitoring) instead of those recorded on the bridge itself. To test the detection capability of the different approaches, we created four experimental scenarios. The first scenario considers the structure in a pristine while in each of the other three, we added mass to the bridge to simulate damage. Two different scenarios are compared in each test. A pristine one, and a second one with the induced change to modify its dynamic properties. The second block in Figure 1 shows the two scenarios considered. The data from the two scenarios is classified by means of a short-time Fourier transform-based change detection scheme aimed at detecting changes in the fundamental frequency of a bridge. We report the results of this work in terms of the detection rate. This quantity reflects the fraction of cases in which an actual change is detected. These two steps are represented in the last two blocks in Figure 1. Previous work includes a theoretical solution for the simplified case of a single degree of freedom oscillator travelling over a beam structure (Yang et al. 2004). In a subsequent paper, Lin et al. (2005) were able to experimentally determine the natural frequency of an actual bridge structure by analyzing the acceleration signals of a passing vehicle. An experimental setup was used by Kim to simulate the vehicle-bridge interaction and identify damage scenarios (Kim et al. 2010). A particular methodology, referred as the “pseudo static approach”, was used to identify damage using vibration data taken from the bridge structure at different locations along the bridge span (1⁄4, 1⁄2 and 3⁄4 of span length). This damage identification approach shows good accuracy at determining a change of stiffness of the bridge. Being inspired by this experimental work, we decided to further pursue the identification of changes in the bridge structure using the indirect approach. In this paper, we concentrate on studying the influence of different vehicle velocities and sensor locations on the classification accuracy of different scenarios. The scenarios are produced in a laboratory using a scaled physical model of a moving vehicle over a simply supported bridge. The data is obtained through multiple runs of the vehicle over the structure. Hereafter, we refer to this particular experimental setup as the “scale bridge structure”. The following section contains a description of the experimental setup. This description includes the structural model, the vehicle model, the vehicle motion control system, and the data acquisition equipment. The third section contains a description of the different scenarios that were compared in the detection experiments. In the fourth section, we describe the Fourier transform-based change detection approach, and in the fifth section, we present and discuss preliminary results. Our initial conclusions are given in the last section. Figure 2. Structural components and vehicle from experimental setup 2 EXPERIMENTAL SETUP The experimental setup simulates a passing vehicle over a simply supported bridge structure. The vehicle-bridge interaction is studied by recording accelerations at four different locations on the vehicle as well as at the midspan of the bridge. The whole system consists of several components: 1) the mechanical components consisting of the bridge, its approaches, and the vehicle, 2) the vehicle motion control system and 3) the data acquisition system. 2.1 Mechanical components An overview of the mechanical components is given in Figure 2 (a). It consists of an acceleration ramp and a deceleration ramp that provide the running path for the vehicle, and a simply supported bridge. The acceleration and deceleration ramp are made from C Shape aluminum extrusions (2 x 1 x 1/8 in). They are supported on each end by aluminum slotted extrusions. The slotted extrusion shown in Figure 2 (b) allows one to fix the ramps at different locations along the slots. This flexibility will allow further research that will explore placing the ramps in the right or left lanes to study the effect of traffic-induced torsion. 2.1.1 Vehicle The vehicle used on the experiment has two axles. A scheme of the vehicle is shown in Figure 2(c). The vehicle has four independent wheel suspensions. In this paper the vehicle properties are maintained at constant values shown in Table 1. The dynamic properties of the vehicle were obtained by capturing the dynamic response after an impulse force is applied. Table 1. Vehicle properties ________________________________________ Properties with added mass ________________________________________ Bouncing frequency 5 Hz Front damping 5.9% Rear damping 5.9% Extra weight at midspan 5 lb) ________________________________________ The suspension system is designed so it can be easily modified to simulate different vehicle characteristics. For example, a heavily loaded 2 axle vehicle can be simulated by replacing the spring in the suspension shown in Figure 2(c) with a stiffer spring. 2.1.2 Bridge The bridge structure is composed of an aluminum plate and two aluminum angles that act as beams. A cross-sectional view of the bridge is shown in Figure 3. . Figure 3. Bridge section The bridge deck also has two angles on top that are used as rails for the vehicle. The properties of the bridge are shown in Table 2. Table 2. Bridge properties ______________________________________ Deck dimensions 8 x 2 x 1/8 in Beams dimensions 8 x 1 x 1/4 in Fundamental frequency 7.18 Hz Damping 1.35% ______________________________________