Important facts when using friction dampers in buildings

Denis Lefebvre, eng., M.A.Sc.

The utilization of friction dampers may considerably reduce the base shear, axial forces in the columns, and

vertical reactions on foundations. However, a good calibration of the dampers is necessary to have adequate control over interstorey deflections. Engineers who wish to use friction dampers (Pall) are compelled to analyze the structure with a non linear model in order to study the behavior under earthquake loading. The choice of an appropriate accelerogram is required and represents a second difficulty. The method proposed by Ibrahimbegovic et Wilson (FNA : Fast Nonlinear Analysis) and used by SAP2000 is based on a modal superposition technique that is very fast and requires only one computation of eigenvalues and eigenvectors. However, the friction dampers vertical components that are transferred to the columns are included in the results only if a sufficient number of modes have been considered in the analysis. It is recommended to use a sufficient number of modes (or all the modes), but it is a difficult task for the engineer to do so or to choose the good ones. Buildings having thousands of degrees of freedom generally possess a vertical stiffness that is greater than the translation stiffness. The result of this is that the modes providing an important contribution to the behavior in translation are listed at the beginning, while modes acting on vertical forces (ex. : friction

dampers vertical component) are listed at the end. To get round this problem, the engineer is forced to ask a great number of modes, which is not always possible for it costs much time and memory. Elements that increase the vertical component on one column are the calibration of friction dampers, their number, and their vertical components. For a building having a few friction dampers (1 or 2) attached to the columns, the error is quite small (10%) and passed unnoticed. However, if there are many friction dampers, the error may be important. If you are using the standard FNA method, important erroneous results may occur in the calculation of axial forces in the columns and vertical reactions on the foundations, because it is impossible to choose a great number of modes for an analysis, except for small structures. The method presented in VisualDesign is a variation of the original FNA method. With this method, engineers will only need to study two or three modes per storey as they would do when using a standard linear analysis in order to get a minimum of 90% of the participating mass. In this non linear analysis, we simultaneously apply the FNA method for the calculation of horizontal forces applied to the structure and a static method is used for the calculation of the friction dampers vertical components. The method suggested in VisualDesign offers an alternative that will allow the engineers to get more accurate results (correct axial forces in the columns and vertical reactions on foundations) while choosing fewer number of modes.

(A french copy of this paper is available on our web site)