Comparison between damage detection methods applied to beam structures

Damage detection methods based on the dynamic measurements of structures are one of the most important techniques for damage evaluation in bridges. The methods considered in this study have been recognized as the most promising tools for damage detection in these structures. Some of these methods were applied during the deliberate damage of Z24 Bridge in Switzerland and I-24 Bridge in USA. These methods have been evaluated in different cases and therefore it is difficult to decide which method is the best for the particular purpose. The comparison of these methods will be done using simulations performed in a composite simply supported bridge. Almost all the methods provided good results when noise was not taken into account. However, accuracy of these methods is still limited when noise is present during acquisition of the dynamic response. first time with this method. Another procedure for identification of damage uses the Wavelet Packet. Chang et al (2005) proposed a method that consists of decomposition of the dynamic response into wavelet packet components. The energy for a given component and level of decomposition is obtained for all the measuring points. Finally, the curvatures of these energies are calculated and the damage is detected as a local disturbance along the beam. The damage detection methods evaluated in this study were: COMAC, curvature, damage index, discrete wavelet analysis (details), continuous wavelet transform, wavelet packet transform and Holder exponent methods. All of them are level 2 methods. Also, level 1 methods were used as comparison between principal frequencies and MAC method. The comparison was done on the first five mode shapes of the structures. 2 BRIDGE STRUCTURE The bridge considered for the evaluation of the damage detection methods was designed following the AASHTO bridge code 1994, Salgado (2000). It is a composite simply supported bridge with two steel I beams (HE800B) and concrete slab with 300 MPa of compressive strength and 215 mm of depth. Its total length is 20 m. Figure 1 illustrates the geometry of the bridge. Figure 1. Geometry of the bridge adopted as example. 3 DAMAGE SCENARIOS In this study, damage scenarios representing fatigue damage caused by heavy traffic were considered. This damage was simulated with open cracks located in the mid-length region of the steel I beams. For this purpose, two general cases were evaluated. Firstly, the severity of damage was considered with four crack depths. The first crack has a depth of 8 mm appearing along the bottom flange of the steel beam trying to simulate a light damage scenario. In the second case, the crack propagates to 17 mm along the bottom flange. In the third case, the crack covers the entire bottom flange. Finally, in the last case, the crack has propagated to the half of the total depth of the steel I beam and it represents a severe damage scenario. In the second general damage case, the extension of damage was evaluated using three crack patterns. One crack in the middle length of the steel beam was simulated for the first damage pattern. Two cracks appear equidistant 500 mm to the first crack in the second damage pattern. Finally, additional six cracks which are equidistant 500 mm appear over the steel I beams. The proposed damage scenarios are shown in Figure 2. 20.0