Crack assessment of bridge structures is essential for maintaining safe transportation infrastructure. Traditional crack detection by manual visual observation has drawbacks, as it is expensive, time-consuming, and limited by the height and volume of bridges. Recently, unmanned aerial vehicles (UAVs) with image processing have been used to address these limitations. However, cameras on UAVs will generally not be perpendicular to the crack surface during actual measurements; therefore, deviation in the perspective angle can lead to inaccuracies in crack identification. In this work, we propose a robust and straightforward crack detection method based on geometric correction and calibration algorithms to address these issues. Four parallel laser emitters were installed on the UAV camera for crack image acquisition, and the laser-obtained images were geometrically adjusted using the four-point linear correction algorithm. After crack image processing, the object-to-image resolution was analyzed, and the crack information was extracted. Compared to the lens imaging concept method, the method based on the four-point lasers showed greater precision for crack width identification, with a measurement accuracy of over 95%. This indicated that the proposed crack identification system showed great potential for actual crack detection of bridges.

Unsteady aerodynamic forces are significantly critical to the safety and stability of trains traveling in high winds. This paper describes a study into unsteady crosswind forces of a moving train subjected to a three-dimensional stochastic wind field with longitudinal, lateral, and vertical turbulences. Initially, a three-dimensional computational fluid dynamic (CFD) model is established to calculate the aerodynamic coefficient of a moving train, and then the wind velocity time histories at the position of the train are generated. Finally, the quasi-steady theory and weighting function method are used to model the unsteady crosswind forces of a moving train in a three-dimensional turbulence field. The results demonstrate that a generalized sine form is useful for predicting the aerodynamic coefficient that varies with the resultant wind yaw angle, and an adequate modeling of unsteady crosswind forces with complete wind turbulences can produce a greater force fluctuation and peak. Particularly when the flow direction of crosswind deviates from 90°, consideration of only a portion of the turbulence components may underestimate the dynamic response of trains.

Train–bridge dynamic interaction analysis is critical for the dynamic design of bridges and the safety and comfort assessment of trains. This study introduces a train–bridge dynamic model of a straddle monorail train and a steel–concrete composite track beam to investigate the dynamic performance of the bridge and train. It explores the influence of track irregularities and passenger loads on the dynamic response of train–bridge systems at various traveling speeds. The numerical results indicate that there is no significant resonance between the straddle monorail train and the steel–concrete composite bridge. However, track irregularities and train speed significantly amplify the responses of the train and bridge, including displacement, acceleration, and impact coefficient. Additionally, increased passenger load leads to a substantial rise in the vertical displacement of the bridge while reducing the vibration of the train, thereby improving riding comfort. The findings of this study provide valuable scientific insights and have significant practical applications for the use of steel–concrete composite bridges in straddle monorail systems.

This study develops an analytical framework for investigating in-plane nonlinear subharmonic resonance in fixed–fixed circular arches under a vertical base-excitation, a phenomenon not adequately addressed in previous research. Based on Hamilton’s principle, the governing partial differential equation for in-plane nonlinear motion is first derived. The tangential displacement is then expressed as a modal superposition, and the system is reduced to a set of second-order ordinary differential equations via the Galerkin method. Using the method of multiple scales, the nonlinear 1/2-subharmonic resonance is solved, yielding closed-form, steady-state amplitude–phase relations and corresponding stability conditions. Validation against finite element simulations and Runge–Kutta analyses confirms the accuracy of the proposed approach. Dimensionless fundamental frequencies match finite element results exactly, with discrepancies in critical base-excitation below 2.5%. A close agreement is observed in both the amplitude–frequency and force–response curves with numerical predictions and Bolotin’s method, accurately capturing the characteristic hardening nonlinearity and three distinct dynamic regions spanning negligible vibration, stable resonance, and instability. Parametric studies further reveal key trends. Larger included angles intensify the vibration amplitude and promote saddle-node bifurcation, while narrowing stable operating regions. Higher slenderness ratios enhance structural flexibility and nonlinearity, shifting resonant peaks toward higher frequencies. Increased damping suppresses the response amplitude and raises the thresholds for vibration initiation and bifurcation.