Improved Method Of Calculating Target Displacement In DBSD Considering The Effect Of Higher Modes And Inelasticity

With the continuous research on structural seismic response and the proposing of performance-based seismic design notion, people are more and more aware the important role of the indicator-displacement in earthquake engineering design and evaluation. Nowadays, displacement-based seismic design is the most convenient and effective way to achieve performance-based design and it is expected to become practical seismic design method and a future trend of seismic design.Displacement-based seismic design takes displacement (deformation) of structure as the parameter controlling structural performance. The first step is to determine the target displacement of structure according to the seismic performance level, then in turn to determine the equivalent mass, equivalent damping ratio, equivalent stiffness and earthquake force of the equivalent single degree of freedom system. Finally, complete strength design of components. Therefore, in the displacement-based seismic design, the computation of the target displacement is very important.There are various methods to determine the target displacement, but all of these methods are based on the assumption that the response of structure is controlled by the fundamental mode and that the mode shape remains unchanged after the structure yields. So they can not consider the effect of higher modes. According to the existing research results, we can see that, the effect of higher modes on structure increases gradually along with the growth of period of structure. So in the design for structures with longer period, the methods considering only the first modal effect will have greater error, even will be failure in the deformation controlling.Based on the platform-OpenSees, which is a foreign newly developed tool used for structural nonlinear analysis, this paper analyzes two parts: the contribution of each modal to the roof displacement of structure in nonlinear seismic response, and the effect of different values of R on the roof displacement of structure in nonlinear seismic response, to research the method that may consider the effect of higher modals in the computation of target roof displacement. The specific contents include as follows:First, with two frames designed according to Chinese 7 degree, 0.15g area for example, which are 5 layers and 11 layers, respectively, the paper introduces a method of decomposing roof displacement to each modal in elastic response. Further depth to the nonlinear seismic response of structure, the paper introduces three methods to decompose roof displacement to each modal, which are the uncoupled modal response history analysis method proposed by Chopra, the method using the characteristic values at the time of maximum roof displacement appearing, and the method considering reduction of rigid of beams and columns. Based on these methods, this paper decomposes roof displacement to each modal of the structures to research the effect of higher modes other than the first mode on roof displacement of structure. The main conclusions include as follows:â‘ The maximum roof displacement of structure in nonlinear seismic response increased with the growth of height of structure and the earthquake level.â‘¡In the same earthquake level, with the growth of structure height and period, the participation ratio of the second and third modal increased significantly; and for the same structure in different earthquake levels, the participation ratio of the second and third modal have no obvious rule.â‘¢For the structure with short period, the effect of earthquake levels for participation ratio of higher modes is obvious, and the rule is that, higher the earthquake level is, larger the participation ratio of higher modes are.Second, with three frames which seismic force reduction factor Rd were respectively taken 2.5, 3.25 and 4.0 for example, input 10 ground motion acceleration records for the nonlinear dynamic response analysis, a statistics of the maximum roof displacements is done and conclusions are as follows:With the value of Rd increasing, the roof displacement of structure in nonlinear response increases, and larger the Rd is, more obviously the roof displacement increases.It shows that, in displacement-based seismic design, we also should improve the target roof displacement computing method based on the value of R.Third, in view of these two conclusions, this paper propose a preliminary suggest on the improving of the target roof displacement computing method based on the subentry coefficient method in FEMA356, which can consider effect of higher modes and the value of R.