四脚鉄塔支持型鋼製煙突の固有周期推定式の提案

抄録

 Chimneys with various shapes are built in the thermal power plant. In particular, super high chimneys exceeding 100m in height, it is common to support chimneys with a steel tower or concrete outer cylinder and so on. Estimation of natural period is indispensable for evaluating dynamic behavior such as wind induced vibration and earthquakes. However, it is complicated to estimation of natural period of truss tower supported steel chimney. One of the reasons is that it is impossible to uniquely determine the weight and stiffness of truss tower, because there are many members that making up truss tower, and also chimney body weight concentrating on the tower support point height depends on the number of parts to support and the height. This paper presents estimation formula of natural period of truss tower supported steel chimney except such kind of complicate process.

 The target chimney type is truss tower supported chimneys. In this paper, 21 chimneys data are used refer from previous measurement results and previous design data that evaluated eigenvalue. The chimney heights are 131m to 230m, aspect ratio of chimneys are 20.7 to 47.6, aspect ratio of truss towers are 2.80 to 5.14. Proposed method of the past for estimate 1st natural period is given regression equation by chimney height. In focus as 1st natural period at 200m in height, the estimation result is 2.0s by regression equation as mentioned. However, the correct values evaluated by measurement or analysis are 1.7s to 2.6s despite of same height, the values of regression coefficient by chimney height are 0.007 to 0.014. Because, even the chimney height is same, the chimney body number is different this means the total weight is influence to natural period. And also aspect ratio both chimney and truss tower influence to natural period as well. These influences must be taken into account in order to evaluate the natural period.

 In this paper, we proposed two estimation formulas of 1st natural period of truss tower chimney those influence included. One is experience formula, the other is theoretical formula. The experience formula isn’t necessary evaluate the weight and stiffness of whole chimney. Because such influence is taken into account by proposal parameters using the chimney diameter, the number of chimney and aspect ratio of chimney. Among the proposal parameters used in experience formula, since the aspect ratio of chimneys and the number of chimneys are determined from the functional conditions required for plant, the structure designer only examines the truss tower shape. Previously, there was no way to logically examine the basic shape of the tower shape. However, by using this proposed formula, it is possible to plan the structure of truss tower considering the dynamical vibration characteristics of the chimney. On the other hand, the theoretical formula is a method for estimating 1st natural period from total mass and the truss tower stiffness, assuming truss tower supported chimney to one-degree-of-freedom model. In order to obtain the total mass and the stiffness of the truss tower, it is necessary to examine the cross section of the member. However, it is possible to estimate 1st natural period with the same accuracy of eigenvalue analysis. These proposal formula performs more accurately than previously estimation method. Furthermore, it is possible to examine the influence of higher order modes by clarifying the relationship between the 1st natural period and the 2nd natural period. Therefore, by using this proposed formula, it is possible to study a reasonable chimney shape from a multilateral perspective.