Performance-based seismic design of eccentrically braced frames using target drift and yield mechanism as performance criteria

Abstract

A direct performance-based design approach that basically requires no assessment such as nonlinear static or dynamic analysis after initial design has been presented. Based on energy-balance criterion, the proposed approach gives the design base shear by using the elastic design spectral value for a given hazard level, a preselected global structural yield mechanism, and a predesignated target drift. In addition, the design lateral force distribution employed in the proposed method is based on nonlinear dynamic analysis results using a number of SAC ground motions. The shear links are designed according to the AISC Seismic Provisions for Structural Steel Buildings (AISC, 2005), while the members outside the links are designed by using a capacity design concept. The following conclusions can be drawn from this study: 1. Overall, the EBFs designed by the proposed method can be expected to satisfy the required performance objectives when subjected to a major earthquake. That is because the selected performance objectives in terms of the yield mechanism and maximum drift are explicitly built into the determination of design lateral forces and design of the frame members. 2. All the inelastic activity was confined to the shear links and the column bases in PBPD frames as intended. On the other hand, inelastic activity in the IBC frames occurred in a somewhat less controlled manner among the frame members including columns, depending on the ground motion used. 3. The maximum link plastic rotations in PBPD frames were generally within the AISC limitation, and well below the maximum rotations reached in the University of Texas-Austin tests. Moreover, the 10-story PBPD frame showed more evenly distributed plastic rotations along the height. On the other hand, in the IBC frame, although most maximum link plastic rotations were below the AISC limitation, the plastic rotations at upper floors tended to be higher and exceeded the 0.08 radian limit during some of ground motions. The roof level links exhibited only minor yielding. 4. The maximum interstory drifts in the PBPD frames were within the 2% preselected target drift for all the selected 10% in 50 years ground motions, signifying that the seismic performance of the deformation-sensitive components can be controlled by the proposed design procedure. 5. The maximum absolute floor accelerations were generally below the code-specified floor design acceleration, indicating that the seismic performance of the acceleration-sensitive components can also be assumed to be satisfactory. It was also evident that the acceleration-sensitive components in a low-rise EBF are more vulnerable than in a high-rise EBF. 6. Generally, the proposed design story shear distribution represents the envelope story shear distribution of the structure due to the ground motion records used in this study very well, because it is based on inelastic behavior. On the contrary, the code-specified force distribution, although it includes higher mode effect, does not represent realistic maximum story shear distribution during major earthquakes which causes the structures to respond inelastically. 7. It was shown that the proposed procedure can be used to achieve the multilevel design goals as required in PBEE. 8. The results also showed that the EBFs can be designed by using the proposed methodology to determine the preselected target performance without increase in the material weight.