Cable force is an essential indicator for evaluating the health status of a bridge. To realize the real-time and accurate cable force monitoring of the whole bridge, models were constructed using backpropagation neural networks combined with a finite element model of a cable-stayed bridge. This strategy obtained the cable forces in the stay cables without sensors, the elastic moduli of the stay cables, and the elastic modulus of the bridge girder concrete. The results showed that the average differences in the forces in the 75 stay cables without sensors obtained from our identification model and those measured in 21 stay cables with sensors presented a maximum discrepancy of 0.17%. Then, the structural parameters from measured data were used to update the finite element model. All the results calculated via the cable force formula presented an error of about ±1% compared to the measured results. This research demonstrated that the models for identifying cable forces and bridge parameters provide a valuable and novel approach to force identification in stay cables without sensors.

In this study, a transient heat flow model has been established for parallel wire or strand cables at high temperature by employing the lumped thermal mass approach, and the numerical solution of the surface temperature as a function of time in each layer of the steel wire or strand inside the cable was calculated. Accuracy of the theoretical method is verified through uniform heating test of 73Φ15.7 mm steel strand cable. The results calculated show that temperature field inhomogeneity of cable section is overestimation on the condition that heat conduction inside the cable is not considered. Considering heat conduction or not, the maximum temperature difference of the core steel stand at the same time point is 373 . Unprotected 73Φ15.7 mm cable is damaged in only 12 min in UL1709 fire, and arrangement of fire protection layer around cable can effectively retard the temperature rise of cable surface. The experimental results are in good agreement with the theoretical calculation values in early stage of fire, and the temperature difference between the two is within 10%. Besides, the numerical calculations were analyzed in accordance with the fire protection requirements limiting the surface temperature of cables. It was observed that the minimum thickness of the fire protection layer required to meet the PTI DC45.1-12 standard was linearly related to the numerical value of the section factor of the outermost layer of steel wires or steel strands in the equivalent model, and the slope of the function was approximately equal to the conduction coefficient of fire protection layer, as the cable was subjected to fire for 30 min. Further, based on this, a simple method was proposed to calculate the minimum thickness of the fire protection layer for parallel wire or strand cables.