# The Current Development of Structural Health Monitoring for Bridges: A Review
## 5 Damage Identification Methods
### Methods under Varying Temperature Conditions  
In the BHM process, not only damage will cause changes in the dynamic characteristics of the structure, but also environmental uncertainties, especially varying temperature. Figure 9 illustrates the variation of natural frequency with temperature in a study. If these effects cannot be eliminated, the accuracy of the damage assessment cannot be guaranteed. [PERSON] et al. [186] proposed a GA-based damage detection technique, in which the damage parameters affected by temperature changes are variables in the numerical model. Experiments were conducted on a three-span continuous beam and a two-span steel grid, achieving Level 3 SDI while considering the variations in material properties and boundary conditions with temperature changes. [PERSON] et al. [187] analyzed the sensitivity of the parameters affected by temperature in FEM, and proposed a correction method for modal parameters that consider the temperature field. The experimental result showed that the SDI performance was improved after the parameters were ameliorated by this method. In the previously mentioned PSO-CS algorithm [152], the effect of temperature was eliminated by establishing a functional relationship between ambient temperature and the elastic modulus of materials. [PERSON] et al. [188] introduced a new concept, equivalent damage load (EDL), to approximate local damage treatment. In the simulation experiment of a two-span continuous bridge, it was proved that this method can distinguish between the structural response caused by damage and temperature effectively. [PERSON] et al. [175] studied the relationship between temperature and natural frequency in SBL. To solve the problem that it was difficult to directly establish the SDI baseline when considering the temperature gradient, [PERSON] et al. [189] obtained the damage reference value based on a single temperature condition. Damage was identified by the deviation of the measured value from the reference value. The effectiveness of this method was verified in model and field tests. [PERSON] et al. [190] studied the change rule of the natural vibration frequency of RC simply supported beams with varying temperatures, and the experimental results showed the linear negative correlation between the natural frequency and temperature in a specific temperature range. [PERSON] et al. [191] firstly applied PCA combined with a Gaussian mixture method (GMM) to bridge damage detection under varying temperatures. All the damage features were projected into both principal and non-principal component directions; then, a PCA based method was used to extract the loss features in the non-principal component, and GMM was used to classify the damage features and the effects of temperature in the principal component. In addition, they found that there was not always a linear relationship between the natural frequency of the actual bridge and the ambient temperature. [PERSON] et al. [192] refined the above approach to address the piecewise effects from changing temperature conditions. They first used PCA for the observations, then applied GMM to the first principal component and temperature conditions. The results of Z24 Bridge in Switzerland demonstrated the validity of this method, but it was only limited to Level 1 SDI. [PERSON] et al. [193] combined ICA with moving principal component analysis (MPCA) to identify structural anomalies under the influence of temperature. The blind source separation of thermal response was realized by ICA, and then sent to MPCA for processing to reveal abnormal changes. [PERSON] et al. [194] incorporated temperature change into [PERSON]'s modulus of materials and proposed a mathematical model for damage identification considering varying temperatures. They used the support vector machine (SVM) to determine temperature changes and possible damage locations, and then used MF optimization methods to accurately locate and quantify damage based on the established mathematical model.