Static Stability And Local Failure Sensitivity Analysis Of Kiewitt-Lamella Single Layer Reticulated Shells

Kiewitt-lamella single layer reticulated shell is a form-active structure system,which is formed by combining the grids of Kiewitt and lamella,and owing to its load bearing reasonability,it has become one of research focuses in the field of large span spatial structure.This structure is taken as the research object in the thesis and its static stability with initial geometrical imperfection and local failure sensitivity are studied.And the main contents are as follows:(1)Static stability of the ideal structures of Kiewitt-lamella single layer reticulated shells and its imperfect structures which merely considers overall imperfections are comparatively studied.The double-control index is used to evaluate whether the bearing condition of the structure is changed.Based on this,the determination of reasonable maximum value of overall imperfections is discussed.The result shows that,the double-control index including the statistical average value of bending-axial stress ratio of the bar mainly bearing axial stress not exceeding 0.3,and the statistical average value of percentage of the bar mainly bearing axial stress in effective bars not exceeding 60%,is the criterion to estimate the Kiewitt-lamella single layer reticulated shells are in bearing condition mainly of membrane internal stress.Based on the double-control index,it is suggested when considering the overall imperfections of Kiewitt-lamella single layer reticulated shells,the maximum value of the imperfection should be defined as 1/500 of the span in engineering practice.(2)Multiple-segments-of-beam method is used to simulate the initial curvature of bars,the shape of a bar is set as a semi-sine wave,and the amplitude and rotation angle of the bar is simulated with extreme value type I and uniform distribution random parameters,respectively.Combined with examples,the method is verified in applicability.With stochastic imperfection mode method as the introduction pattern of initial curvature of bars,its influence on static stability of single layer reticulated shells is evaluated.On the basis,the reasonable amplitude of initial curvature of bars is suggested.The study indicates that combining the overall imperfection and initial curvature of bars at the same time,the stability bearing capacity coefficients obtained from stochastic imperfection mode method are normally distributed,and the statistical average value of the coefficients are suggested as the final value.The overall imperfection and initial curvature of bars have coupled influences on structural stability bearing capacity so as to weaken the detrimental influence to some extent,of two kinds of imperfections when introduced separately.In engineering practice,it is suggested to choose 1/400 of the maximum length of rods as the amplitude of initial curvature of the type-a-section bar(rolled circular tube)which is commonly used in single layer reticulated shells to consider the unfavorable influence of initial curvature.(3)The local failure sensitivities of the ideal structure and overall imperfection structure of Kiewitt-lamella single layer reticulated shells are comparatively studied,and with alternative path method,the sensitivity index is used to evaluate the influence of single bar failure and nodal area failure on the structural stability bearing capacity and to measure the importance of single bar and nodal area in the structure.The results show that for the ideal structure,the distribution of sensitivity indices of single bar failures is quite discrete,that is the difference between the sensitivities of different bars is rather great,but the nodal area failure results in the great change of force distribution path so that the sensitivity index of the structure for nodal area failure is more intense.For overall imperfection structures,as the rise-span ratio becomes larger,both the locations of the bar and nodal area of the most intense sensitivity transfer from the outer part of the lamella to the middle part of Kiewitt.Compared with the ideal structure,all sensitivity indices of the overall imperfection structure for single bar failure decrease significantly,thus the introduction of overall imperfection weakens the influence of single bar failure on the stability bearing capacity of the left structure.In engineering practice,it is suggested that for the structure of small rise-span ratio,focus on the diagonal bars in the outer part of the lamella,and for the structure of large one,focus on the bars in the middle part of Kiewitt connected with the main radial rib,and properly improve the safety margin of the structure.