## 5 Summary  
The following conclusions can be drawn based on the results, and within the limitations, of the research presented in this paper:  
1. The Measured Temperature Method can reflect the actual temperatures inside the cavities, but measurements on site are cost- and time-prohibitive. Therefore, the application range of the Measured Temperature Method is limited. When there is no measurement on site to obtain the temperature inside the cavity, the Ambient Temperature Method, Mean Temperature Method, or Air Element Method can be used as alternative methods. 2. The influences of different simulation methods for the internal thermal boundary conditions on the numerical hourly temperature curves of the parts of cross-sections far from the cavities are negligible. Compared with the measured hourly temperature curves, the numerical hourly temperature curves of the parts of the cross-sections near the cavities calculated by the Measured Temperature Method provide the closest agreement. When there is a lack of measured temperature inside the cavity, the numerical hourly temperature curves calculated by the Air Element Method provide a closer agreement with the measured curves than the curves calculated by the Ambient Temperature Method and Mean Temperature Method. When the Ambient Temperature Method is used, the trends of the numerical curves of the bottom flanges and the webs near the bottom  
Figure 27: Comparison of the vertical temperature gradients of the adjacent box girder cross-sections: **(a)** Positive vertical temperature gradient; **(b)** negative vertical temperature gradient. flanges are similar to the measured hourly ambient air temperature curve, because the temperature change in the hourly ambient air temperature curve was much larger than the temperature changes in the measured curves inside the cavities. 3. The comparisons of the temperature contour plots obtained from the FEMs considering different simulation methods for the internal thermal boundary conditions indicated that the temperature distributions on the parts near the cavities calculated by the Measured Temperature Method and Air Element Method were close to the measured values. 4. The influences of different simulation methods for the internal thermal boundary conditions on the highest hourly average effective temperature of concrete closed girder cross-sections and the trends of the vertical temperature gradients for the box girder and adjacent box girder cross-sections were small. The maximum vertical temperature gradients calculated by the Air Element Method on the top and bottom flanges were larger than those calculated by the Ambient Temperature Method and Mean Temperature Method for the small box girder cross-sections. 5. The Air Element Method can be used as a simulation method for the internal thermal boundary conditions in the FEM to predict the temperature distributions on concrete closed girder cross-sections under historically extreme temperature conditions. The longitudinal thermal movement of concrete closed girders calculated by the one-year measured average effective temperature of the cross-sections or by the Chinese-code-specified effective temperatures for the highway bridge structures would be smaller than those under historically extreme temperature conditions, which are thus unconservative for engineering applications. It is suggested that the average effective temperature of concrete closed girder cross-sections under historically extreme temperature conditions should be calculated for each city using the FEM with the Air Element Method, as described in this paper. The comparisons of vertical temperature gradients under historically extreme temperature conditions indicate that the Chinese-code-specified vertical temperature gradients are conservative for the bridge deck surfaces and unconservative for the bottom flanges. 6.