| چکیده انگلیسی مقاله |
Air temperature variations due to daily, seasonal, and annual changes can affect the bridges. The deformation and stress caused under solar radiation should not neglected in bridge design and their effects can be compared with dead and live loads. Thus, the components of bridges such as bearings, dampers and so on, are seriously affected by the combination of external loads and thermal stress. Different countries have provided temperature gradients in their codes. Almost in all of the codes, the vertical temperature gradient is specified, but unfortunately in none of them, the lateral temperature gradient is presented. Furthermore, in the vertical gradient, there are numerous lacks exist in different codes. For an example, most of the codes do not involve either temperature variations due to annual changes, or not considered the longitude or latitude of the location of the designed bridges. These problems lead the engineers to do the precise study beside the codes provided by each country, for temperature effects on the bridge structures. This paper investigates the effect of vertical and lateral thermal gradient loads for concrete box girder designed based on Iranian Standard Loads for Bridge (ISLB) code, using experimental test and three-dimensional finite element analysis. The ISLB code has two main problems in the field of thermal gradients. Firstly, the vertical temperature gradient provided in ISLB code, cannot used for all bridges in Iran, because each bridge has its unique geographical environment, latitude, longitude, and axis of orientation. Secondly, it does not contain any models for the lateral temperature gradient. To handle these problems, the experimental test is done and thermocouples are installed in different parts of the segment to get the thermal gradients and investigate their effects. In the case of predicting vertical gradient, the recorded data show that, the maximum temperature difference occurs in 1430 hrs with the value of 8.8 °C , while by considering the lateral gradient, the maximum temperature difference occurs at 1130 hrs with the value of 2.9 °C . In this paper, comparison between vertical and lateral gradients leads to consider only vertical gradients in further investigations. Moreover, by applying vertical gradient in the finite element model of the Jenah bridge, maximum thermal stress is occurred in the intersection of the web and top flange with the value of 1.96 MPa and maximum deflection of 4.36 mm in the midspan of the bridge. As a solution for mitigating the negative effects of the thermal gradients, using polyurethane insulation is proposed and modeled in the FE model. Results of simulation reveal that utilizing insolation can reduce the top slab temperatures to 30.5 °C , 29.16 °C , 27.5 °C and 26.4 °C from 33.6 °C in the case of using 2, 3, 5 and 6mm polyurethane insulations, respectively, which results in stress reduction from 1.96 MPa to 1.65, 1.35, 0.63 and 0.38 MPa in the case of using 2, 3, 5 and 6 mm polyurethane insulations, respectively. Furthermore, using insulation can reduce the deflection of the bridge, which in this study, the maximum deflection of the 48 m span is reduced from 4.36mm to 3.57, 2.86, 1.63 and 1.07 mm, by utilizing 2, 3, 5 and 6 mm polyurethane insulations, respectively. |