一葉双曲面RC造冷却塔の構造特性に及ぼす形状不整の影響の分析 : 実測された形状の再現モデルを用いた損傷・耐力評価

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In the inland in Europe and America, construction case of many reinforced concrete cooling towers exists mainly on the power plant which is hard to utilize much cooling water. It is long in the history of such tower construction, and many towers constructed in 40〜70's are used at present. In China and India, the construction of towers in the power plants increases. In such situation, it is reoorted that the collapse accidents by the gust repeatedly arose in United Kingdom and France in a past. Investigation of the cause of those accidents and examination of the countermeasure are developing, and the evaluation of the safety of the existing facilities becomes important problem. On the thin RC shells for these towers, the much research analyzes buckling / vibration problem and effect of initial imperfection on structural properties, and the proposal of design guidelines has been made. To begin with, using the initial imperfection of the distribution idealized axisymmetrically and asymmetrically, a useful knowledge on fundamental phenomenon and analysis method were obtained, and the recommendations for the tolerance limit of the initial imperfection magnitude were proposed. It was analyzed that according to the many leading research results. the fluctuation of meridian direction in-plane stress by initial imperfection was inversely proportional to the shell thickness due to dead weight and wind load within elastic stress analysis, and that it largely affects meridian direction in-plane stress and circumferential direction bending stress for the circumferential curvature error. In addition. partial initial imperfections with small amplitudes play a major role in meridian and circumferential stresses. Concerning the characteristic of initial imperfection itself, the probability-statistical technique for simulating the spatial distribution property is examined, and the statistical analysis procedure to simulate the effect of imperfection on the stress and buckling load is proposed. Recently, the effect of material nonlinearity of RC members on the ultimate strength also became remarkable problem by some researches. In this study, the three-dimensional distribution of the geometric error based on the reported measured data in the shell surface is reproduced numerically in detail for the tower which caused the collapse accident in a past, and the spatial distribution characteristic is analyzed. Next, the effect of the geometric imperfection of reoroduced shape on elastic stress distribution due to dead weight is examined, and the effect on the wind resistance performance is examined by the elasto-plastic analysis. In addition, the evaluation on the performance of the tower by the thermal stress is made, and the numerical examination on the reinforcing method of the tower is tried. By this study, some results shown below were obtained. Reproduced geometric imperfection widely exists for the shell full face, and it becomes a waveform which recurs in circumferential and meridian direction, and the complicated distribution in which the component which recurs periodically overlaps with the local component is shown. The peak of the imperfection component exists from N=0(axisymmetric) to N=5 on the circumferential direction and from 0 to 10(cycle/100m)in the height direction. ・ The possibility of changing to tension from compression exists on hoop stress due to dead load within large area by the effect of the imperfection. ・Due to wind load, the bending cracks develop in the windward of the imperfect shell, and next, it leads to the failure of the tower by the tension which arises by the bending of the meridian direction. This situation is similar to the thing for the perfect shell. From the result shown in the model which reoroduced the imperfection, the turbulence of the remarkable section forces existed further than the case of the perfect shell, and the result of predicting the lowering of the load bearing capacity of largest about 18% was obtained. Still, the effect by the difference between the wind direction became a remarkable result. The effect of the comparatively small temperature loading on rigidity and ultimate strength lowering may be remarkable, and this fact is different from the behavior of the general RC member. In case of thin shell cooling tower, the temperature crack may become a factor of the ultimate strength lowering very much, and it seems to have to examine it in future prudently. Except for the cooling tower, some measures seem to be also necessity for temperature condition and the evaluation method in structural type of the similarity. ・The reinforcement plan by vertical ribs and level rings assumed in this study showed the effective result, and it showed the result of expecting the effect. when there is the imperfection. ・In design and safety assessment of existing towers, the necessity of the elasto-plastic analysis seems to be high. ・Shape measurement value for present analysis is estimated with that the crack has already developed by part and that it has also combined the accumulation of the creep deformation and the repetition of wind/thermal stresses. The examination which will be more detailed in future by concerning in respect above, is desired.
2004-10-30

一葉双曲面RC造冷却塔の構造特性に及ぼす形状不整の影響の分析 : 実測された形状の再現モデルを用いた損傷・耐力評価

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