Abstract
Direct displacement-based design (DDBD) is currently a widely used displacement-based seismic design method. DDBD accounts for the torsional response of reinforced concrete (RC) frame buildings by using semi-empirical equations formulated for wall-type buildings. Higher-mode responses are incorporated by using equations obtained from only a few parametric studies of regular planar frames. In this paper, there is an attempt to eliminate torsional responses by proportioning frames’ secant stiffnesses so that the centers of rigidity and supported mass (the mass on and above each story) coincide. Once the torsional rotations are significantly reduced and only translational motions are achieved, higher-mode responses are included using a technique developed by the authors in their recent paper. The efficiency of the proposed design procedure in fulfilling the intended performance objective is checked by two plan-asymmetric 20-story RC frame building cases. Case-I has the same-plan configuration while Case-II has a different-plan configuration along the height. Both cases have different bay widths in orthogonal directions. Verification of the case studies by nonlinear time history analysis (NTHA) has shown that the proposed method results in designs that satisfy the performance objective with reasonable accuracy without redesigning members. It is believed that a step forward is undertaken toward rendering design verification by NTHA less necessary, thereby saving computational resources and effort.
Highlights
A reliable Performance-Based Seismic Design (PBSD) uses a probabilistic design approach to achieve performance objectives (Bertero and Bertero [1], Collins and Stojadinovic [2], and Castaldo et al [3])
The displacement-based design is found to be a better choice than force-based design since problems with force-based design have been clearly identified by Priestley [4], and damage is correlated with displacements than forces
From Nonlinear Time History Analysis (NTHA) outputs, it is observed that the allowable story drift of the governing performance level (i.e., 4 percent) is not exceeded for all ground motions (Figure 7)
Summary
A reliable Performance-Based Seismic Design (PBSD) uses a probabilistic design approach to achieve performance objectives (Bertero and Bertero [1], Collins and Stojadinovic [2], and Castaldo et al [3]). Amaris and Priestley [31] found that for cantilever wall buildings ranging from two to 20 stories, reducing the elastic modal force effects by the behavior factor (force-reduction factor) and produces an unsafe design They recommended using Modified Modal Superposition (MMS). DDBD does not provide design procedures to account for torsional and higher-mode responses of plan-asymmetric RC frame buildings having a mass and stiffness irregularity along the height. A seismic design procedure is proposed to incorporate the torsional and higher-mode responses in DDBD of plan-asymmetric buildings with the above-mentioned irregularities. Torsional responses are significantly reduced by assigning frames’ lateral stiffness so that eccentricity between centers of supported mass and rigidity is eliminated Once this task is performed, higher-mode responses are included by a design procedure developed by Abebe and. The goal of the proposed method is to render design verification by NTHA less mandatory though more case studies should be investigated
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