Experimental Demonstration of a Mode Shape-Based Scour-Monitoring Method for Multispan Bridges with Shallow Foundations

Abstract

This paper experimentally investigates a vibration-based scour monitoring approach applicable to bridges with multiple simply supported spans on shallow foundations. A monitoring strategy based on the relative changes in pier mode shape amplitudes due to scour is postulated. The first global mode shape of a bridge structure with multiple spans is extracted from acceleration measurements using an output-only approach, Frequency Domain Decomposition (FDD). The relative changes of the pier mode shape amplitudes under scour are then tracked. Here, each pier mode shape value is compared with the mean values of the remaining piers in a process that creates a Mean-Normalised Mode Shape (MNMS). The approach is demonstrated on a scaled model of a bridge with four spans, supported on sprung foundations, where scour is simulated by the replacement of springs with springs of lower stiffness corresponding to a reduction in foundation stiffness. It is shown that at a given 'scoured' pier, significant increases in the MNMS value occur, suggesting that the location of the scour can be identified. The magnitude of the MNMS at a given pier also increases with an increase in stiffness loss due to scour. In practice, the approach would work best by carrying out a visual inspection of the

Laboratory investigation of a bridge scour monitoring method using decentralized modal analysis

“…Using the decentralized approach to detect scourrelated foundation stiffness loss at bridge foundations is experimentally investigated in this section. A scaled bridge model with four simplysupported spans on three-piers (see Figure 8) has been developed at Kyoto University and is used to test the approach 6,56 . The model piers are supported on springs to simulate the behavior of shallow foundations, and the abutments are considered as pinned.…”

“…Each spring has a stiffness of 49000 N/m for the healthy case, providing a total stiffness of 196000 N/m at each pier support. The spring stiffness was chosen so as to model an equivalent 4 m × 2 m shallow foundation on a loose to medium dense uniform sand deposit 6 . The geometric and material properties of one of the bridge beams are shown in Table 5.…”

“…This phenomenon is responsible for a majority of bridge failures worldwide [2][3][4] and causes significant economic and travel disruption 5 . Scour erosion causes a local reduction in soil elevation in the vicinity of bridge foundations, which affects the capacity and stiffness of the foundation system 6 . Losses in stiffness of the foundation system can lead to serviceability issues such as differential settlement, inclination of piers and abutments, and cracking in decks as well as partial or complete structural collapse 7 .…”

“…Scour was detectable by analysing the covariance of measured accelerations along the foundation but was not quantifiable. Malekjafarian et al 6 proposed a mode shape-based scour monitoring method for bridges with shallow foundations, based on measuring changes in the modal amplitude at a given pier relative to the mean of the amplitudes at remaining piers as detected from output-only modal analysis. Using sensors located at each pier, the mean-normalised mode shape exhibited sensitivity to scour-related foundation stiffness loss at given piers.…”

Wavelet-based operating deflection shapes for locating scour-related stiffness losses in multi-span bridges

“…Scour is responsible for a majority of bridge failures worldwide (Maddison, 2012;Melville & Coleman, 2000;Wardhana & Hadipriono, 2003) and can cause significant economic losses and travel disruptions (Lagasse et al, 1995). Scour reduces the elevation of the soil surface profile in the vicinity of bridge foundations, which affects the capacity and stiffness of these systems (Malekjafarian et al, 2020). The detection of scour and scour-related damage in structures is a problem that has received increasing research attention in recent years Fitzgerald et al, 2019aFitzgerald et al, , 2019bFoti & Sabia, 2011;Giordano et al, 2020;Ju, 2013;Kong & Cai, 2016;Li et al, 2020;Malekjafarian et al, 2020;Prendergast et al, 2016aPrendergast et al, , 2016bPrendergast et al, , 2017Xiong et al, 2018).…”