direct displacement-based design method

In Direct Displacement-Based Design (DDBD), the performance objective is to achieve the design displacement (target displacement) and the stiffness and strength of the structure is determined in such a way that the maximum displacement of the structure in an earthquake reaches this displacement. For this purpose, the design base shear is determined based on 3 key parameters of equivalent damping, damping modification factor and P-Delta effect. Due to the variation of relationships for each of these parameters, in this study the influence of using different relations on achieving performance objectives is investigated. In this study, 8 bridge piers with 2 different heights, 2 different span lengths and 2 seismic hazard levels were selected. Then, each of these piers was designed for 27 different design paths resulting from 3 distinct relationships for each of the 3 key parameters and a total of 216 bridge piers were designed by DDBD approach. Then, to evaluate the seismic performance of the piers, each of the 216 piers was modeled in OpenSees software and subjected to 14 earthquake records scaled on the design spectrum. After determining the maximum displacement of each pier, the proximity of this displacement to the target displacement was studied as a performance objective indicator. The results of analysis show that the use of different design relations has a significant effect on the maximum displacement of piers and their performance in earthquakes. Finally, comparing the results for 27 selected design paths, the best combination of relations for direct displacement-based design was proposed.

Direct displacement-based design method which established by Priestley et al. is one of the best methods for performance based design of structures. In this method, the structures are designed to achieve a targeted performance displacement instead of restricting it to a story drift. One of the important parameter in direct displacement-based design method is the estimation of yield displacement based on geometric characteristics of the structure. Incorrect estimation of this parameter may result in an error in determination of ductility and finally design base shear. Assessments performed on various steel structures show differences between non-linear analysis results and those presented by direct displacement-based design Regulation. A portion of this difference relates to equations that estimate the yield displacement values. In this study it is intended to investigate the available yield displacement equation of steel structures with moment-resisting frames in direct displacement-based design by performing various non-linear static and dynamic analyses on 36 moment-resisting frames with different number of stories including 3,6,9,12,15 and 20 stories considering two different numbers of spans (3 and 6 spans). Results showed that the equation used in direct displacement-based design method, is acceptable for structures with less than six stories, while it is recommended to decrease this equation by 25% for structures taller than 6 stories.