A comprehensive review of bridge influence lines: from identifications to applications in bridge health monitoring

Structural health monitoring (SHM) systems (SHMSs) have been extensively installed on bridge structures to guarantee their safety and functionality during their service life. These SHMS collected various types of data, based on which structural characteristics could be extracted for bridge condition assessment and/or damage detection. Though vibration-based dynamic characteristics are the most popular family, static characteristics have attracted increasing attention in recent years. This article, dedicated to bridge influence lines (ILs), one of the representative bridge static characteristics, attempts to provide the first state-of-the-art review of IL-related issues in bridge engineering, from IL identification to applications. The concept and analysis indices of IL are first introduced, along with the detailed merits of IL-related methods. Subsequently, the bridge IL sensing technologies are presented and divided into single-point, multiple-point, and distributed sensing. With distributed strain and displacement sensing, constructing the recently emerged special influences surface based on multiple ILs becomes technically viable. Various types of IL identification algorithms were reviewed. The least square method does not consider the noise and dynamic disturbances; the regularization method minimizes the noise effect during the identification process; the basis function method is claimed to be able to eliminate the noise and dynamic effects. These IL identification methods show their own pros and cons. Then, the IL-related applications are summarized, especially for damage detection and bridge weight-in-motion, considering diverse IL types. Finally, the current challenges and potential future research directions of IL-related issues are discussed. This study should be marked as the first comprehensive review of bridge ILs.