Dynamic Response in Tall Timber Structures

Abstract

Modern building tend to strive towards more slender and lightweight constructions. That is to be more provident with space and materials as well as for aesthetic reasons. The effect of these lightweight slender buildings is an increased sensitivity to lateral loads with regard to the dynamic behaviour of the structure. Since the european union changed to more function based standards, the development of timber and timber products have increased during the past 20 years. It is now both in the interest of and feasible to build taller and larger buildings with the primary load bearing system made of timber. Timber have a relatively low mass compared to other construction materials which can result in larger deformations and discomfort if the dynamic response in the structure is too large. The purpose of this report is to make a parametric study on how mass, stiffness and damping affect the feasible building height of a tall timber structure with regard to dynamic effects caused by wind. This is performed via simulations and analyses of a planned timber structure above 10 floors. The general design parameters are modified in order to fulfill the acceleration requirements for a structure with an increasing number of floors. The initial structure is composed of load bearing Cross Laminated Timber (CLT) walls and floors that acts in diaphragm action. A FE-model is used to determine the eigenfrequencies of the structure and the swedish Annex, EKS 10, is used to calculate the peak acceleration. The determined eigenfrequency and acceleration curve is compared with the requirements of horizontal acceleration according to ISO 10137. If the structure fulfills the requirements, the structure is successively increased with 2 storeys at the time. If the structure does not fulfill the demands, it is improved with mass, stiffness and/or damping in an iteration process until it fulfills the requirements. The result of this study is divided into a "general behaviour" and a "structural behaviour" chapter, to make it possible to understand the impact of each individual parameter separately and the combined impact on the structure. The improvements of adding mass and stiffness separately did not result in dramatic improvements of the acceleration. But by combining mass, damping and stiffness, considerable improvements with respect to the dynamic response is achieved and a building height of 26 storeys was feasible. Improvements of mass and damping combined made it possible to fulfill the demands on a 22 storey timber structure. This study conclude that the most feasible solution is to add mass and damping in forms of a concrete top storey (floor and walls) together with a TMD (Tuned Mass Damper) on the top floor. Key words: Dynamic, Timber, Multi storey structure, Wind load, Acceleration