| According to Chinese Specification for Structural Design of Reinforced Concrete Buildings, a series of piecewise expansion joints must be designed. However, the existence of expansion joints not only make difficulty for the structures’ design and construction, increase the construction cost, reduce the ability of anti-seismic and fire protection, but also increase the cost for operation and maintenance steeply. At the same time, it will reduce the safety performance and the lifetime of the structures if the expansion joints are not properly designed or constructed. Therefore, large jointless concrete structures beyond the specification limit are often explored during the recent years. Those jointless structures have advantages of easy design, low maintenance cost, and so on. However, due to lack of the thermal stress release effect of expansion joints, the thermal stress may exceed the tensile stress limit when the temperature of the jointless concrete structures is changed, and then there will be some micro cracks, which may lead to a breakdown of the structures. Based on finite element analysis, this thesis studies distribution of the temperature and the thermal stress in a large underground jointless concrete structure in ShenZhen City (the length is 1000m, the maximum width is 80m).Before the mulch is backfilled, the top surface of the large underground concrete structure is exposed to the sunlight. At this time, the structure surface gives a largest temperature change which in turn causes a largest change in the thermal stress. Based on this circumstance, this thesis considers two extreme cases of air temperature in summer and winter respectively to simulate the temperature distribution and the thermal stress of this structure, and then calculates the structure’s fatigue life. Firstly, this thesis simulates the temperature distribution of the large underground concrete structure without expansion joints. It is found that whether in summer or in winter, within the depth 0.4m from the surface, the temperature near the surface changes over time drastically. However, the temperature shows only a very slight variation beyond the depth of 0.4m and tends towards a constant. Secondly, the thesis analyzes the structure’s thermal stress. Since concrete belongs to brittle materials, we study and compare the principal stress. We focus on studying the top concrete plate whose temperature is changed obviously. It is found that:(1) the minimum principal stress is-12.5MPa when the temperature on the top surface reaches its highest value of 70℃ in the summer day time; (2) the maximum principal stress reaches its highest value of 2.8MPa when the top surface temperature reaches 12℃ in the winter night. Finally, we calculate the fatigue life during the structure is subject to an alternating temperature load. The fatigue life is 112 days in the summer and it is about 87 days in the winter. We found that before the mulch is backfilled, the top surface of the large underground concrete structure is subject to a largest temperature stress. The main conclusions are as follows:(1) In the summer, the thermal stress is compressive stress which decreases with the depth from the surface, and thus the structure is safe. However, the thermal stress is tensile stress in the winter. When the temperature of the surface is much lower than the normal, the structure stress may exceed the tensile stress limit and results in some micro cracks. Therefore, some warm keeping methods are suggested for the structure in the winter; (2) Comparing the stress in different locations of the structure, it is easy to find that a large thermal stress occurs where the structure size is suddenly changed and where the geometry corners are concentrated; (3) If the structure can safely pass the period before the mulch is backfilled, the design of the large underground concrete structures without expansion joints is a bold innovation, which is completely feasible in the practical engineering. |
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