A Method Based On Modal Analysis For The Research Of Irregular Torsion Of Structures

The aim of structural concept design is to consider the primary scheme of structures using global concept under specifically architectural space and environment conditions, then construct, compare and choose the structural system quickly and effectively using an approximation method in programme conceptual stage. At the present time, designers always adopt qualitative methods.Modal analysis is mainly applied in the problem of structural dynamics issues, it is seldom occurred to use it in general theoretical research of structures. Actually, the quantities of the frequency vector relate to the distribution of the absolute stiffness and mass of a structure, and the mode vector reflect the distribution of the relative one. Comparing with the strain and stress parameters, which reflect the mechanical characteristics of the members of a structure, the frequency and mode parameters of a structure which not involving load combination grasp the mechanical behaviors of structures wholly or partly in macroscopic view, which suit to guide concept design and constitution design of structural system.For these reasons, supported by the National Natural Science Fund Project "A Research of Designing Method of the Structural Engineering Controlled by Characteristic Parameters"(Grant No.50978232),this paper starts with the study of irregular torsion of structures, and concentrates efforts on quantitative analysis of the vibration modes and research of structural obscure concept demarcation based on vibration modal analysis, in order to provide a new way for the method of structural concept design and to provide theoretical support for establishing a method to structural concept design which based on inherent characteristics of structures.The main work is as following:1) This paper proposes a new method of describing vibration modes quantitatively. The author summarizes the current methods of descriptions about vibration modes, and gives a new method to describing vibration modes quantitatively named modal directional coefficient borrowing from Vibration Modes' Direction Factor and Modal Participating Mass Ratios, which makes it much easier to judge the principal direction of vibration modes and describe vibration modes quantitatively.2) This paper studys the correspondence between structural irregularities and the order of ill modes. A three-dimensional finite element model of frame shear wall structure was built in the work and using single factor analysis method. Causing the center of mass and stiffness misalignment through a variety of methods such as tuning the plane's position of shear wall for different storey height and length-width ratio of frame shear wall structures.This paper explores a new method that using modal analysis for judging irregular torsion of structures.3) This paper explores the relations between the structures that have long and narrow plane or changing unevenly in the same plane and the order of ill modes. Examples show that the length-width ratio of the structures that have long and narrow plane is significantly associated with torsional effect of structures, and the level of the structures that changing unevenly in the same plane is significantly associated with the order of partial modes. Which prove that the limit values for the two types of irregular structures given by national norms and standards are right.4) The results of the study show that the method based on vibration modal analysis are not very different from the limit values given by national norms and standards in judging irregular plane structures,which show the method is feasible.Furthermore, the idea based on the connatural parameters of structures which not involving load combination is vivid and easily implemented on computer comparing with displacement ratio and period ratio that given by national norms and standards. It is worthwhile to popularize the quantitative method in structural concept design.