Climate Related Thermal Actions for Reliable Design of Concrete Structures

Sammanfattning: When the temperature changes in the environment the temperature will also change in a structure. Complex interactions of climate factors affect the temperature changes. When the temperature is varied in a material it will expand or contract, and movements will be produced. If the movements are prevented, stresses may be induced which can contribute to cracks in the structure. Severe cracks have been found in box-girder bridges in Sweden, with the most cracks on the south side. This indicates a large impact of solar radiation. Thermal actions or climate situations used in design and analysis of temperature loads in concrete structures are often based on approximations or generalisations of climate data which are more or less accurate compared to real temperature distributions. Assumptions concerning the independence of the actions during certain time periods have been made; in the European code three day intervals were assumed to be mutually independent. The thermal actions stated in the code are based on climate data from Germany, which may differ from the climate situation in other parts of Europe. Not much information and guidelines are given concerning thermal actions at cross-sectional level, where significant stresses may occur due to these effects. The climate situations used for analyses gives the largest thermal actions, but may or may not give the most unfavourable stress field. Large differentials are assumed to occur either during a day with a large amount of incident solar radiation and a large difference in daily minimum and maximum air temperature or during a night with no cloud-cover and a similar but reversed difference in air temperature. In this thesis a detailed approach for simulating and predicting thermal actions is presented, where all the input factors are real climate data with a high resolution. The developed model has been validated against temperature measurements performed on both a simple slab and the concrete arch of the New Svinesund Bridge. The model is well suited to use for predicting temperature distributions in concrete, since it can capture temperature variations with high accuracy. Long-term climate data from meteorological stations were used with the model to identify extreme events of thermal actions based on annual maxima. The results show that the design values in the European code may be an underestimation of Swedish conditions. The resulting thermal stresses from the long-term temperature simulations were also calculated and analysed for box cross-sections. The results showed that the recommendation in the Eurocode concerning temperature differentials at cross-sectional level may overestimate the thermal stresses in the transverse directions. An inclusion of the non-linear component of the temperature distribution reduces the calculated tensile stress level. The most important geometrical factor governing the transverse thermal stresses is the wall and slab thickness. The largest tensile stresses will appear in the thinner members, independent of the members being horizontal or vertical. A simulation using a full-scale 3D-model of a bridge confirms the influence of varying thickness, but shows the importance of the boundary conditions when using full-scale analysis. The largest thermal tensile stresses appear on the inside surface on the south web wall, which confirms the impact of solar radiation on the stress field in a structure. A short-term simulation using climate data from just a few days can be useful for estimating the thermal stresses. It is important however to have knowledge of the input data, since the results show that a three or four days intervals cannot always be assumed to be mutually independent events.