Overview
The temperature variations in the structures cause lengthening, shortening or curvature of the beams due to a non-linear distribution of temperature. Such circumstances create stresses if conditions do not allow free deformations.
Temperature changes in the sections of bridge girders depend on many parameters related to the materials used, the shape and dimensions of the bearing elements, their coating, exposure and sunlight conditions.
The purpose of the work is firstly to clarify the role actually played by the effects of temperature in the modern design of structures and to provide practical and simple rules to justify these effects. On the other hand, a better understanding of the behaviour of composite structures under thermal effects should identify the type of facilities most sensitive to the effects of temperature. This new knowledge leads to design structures that are less sensitive to thermal effects.
Field tests, modelling, statistical computations.
Funding
Results
Measurements on the viaduct des Vaux helped to highlight the daily cycles of temperature on the thermal behavior of structures. In a cross section, sunshine and the changes in ambient temperature cause uniform temperature variations and temperature gradients essentially in the slab. The daily variation of temperature at a point below the slab to the right of the beams can be worth more than twice in a mixed section in a concrete section, due to the higher thermal mass of concrete structures. Out of all the measurements made on the viaduct des Vaux, the temperature in the slab varies from -7.5 ° C in December to 34 ° C in August. In general, the slab is warmed up with the advantage that metal beams in summer and winter cools to advantage.
Modelling sections of the two mixed concrete structures helped to highlight the decisive role of some meteorological and physical parameters on temperature distribution in bridges. Temperature, solar radiation and the radiation emitted by the soil are the main environmental parameters. The absorbency of the material, the film coefficient and emissivity of the surface are the characteristics of the most influential materials.
A three-dimensional model was developed to generalize the study to other types of composite bridges and concrete to determine the effects of temperature on these structures. In this digital model, environmental parameters considered were defined using extremal curves of temperature and sunshine based on meteorological records in Switzerland for the period from 1980 to 2000. The results of this model have been validated comparing the measurement results performed on the viaduct des Vaux.
The results of the simulation of the behaviour of composite bridges under extreme weather conditions have shown that for a slab thickness and given coating, the shape and dimensions of the cross section have little influence on the distribution of constraints. Therefore, a calculation constraints becomes unnecessary and simple standard values may be allowed to reflect the effects of temperature in the mixed sections:
- extreme tensile stresses in the slab are between 0.8 and 1.7 N / mm2, in the lower half to three quarters of the slab,
- extreme compressive stresses are between -10 and -22 N / mm2 in the centre of metal beams and -5 N / mm2 in the bottom flange beams and 15 to -20 N / mm2 in the bottom flange of the boxes on supports.