PERFORMANCE EVALUATION OF TWO NEW SEISMIC RESISTANT DIAGRID FRAMING SYSTEMS
Moghaddasi Bonab, Nasim Sadat
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The diagrid system offers a visually appealing and structurally efficient structural system for gravity load bearing. The architectural elegancy and high structural redundancy of the diagrid structure makes it a desirable choice for tall building design. However diagrid structure is prone to high inelastic deformation demand during strong earthquakes. To address this issue of limited ductility and energy dissipation capacity in conventional diagrid framing, two new types of seismic resistant diagrid structural systems termed highly energy-dissipative ductile (HED) diagrid and hybrid diagrid framing systems are proposed in this research and their seismic performance is assessed. The proposed HED diagrid framing system provides a competitive design option in high seismic regions with its high ductility and improved energy dissipation capacity enabled by incorporating replaceable shear links interconnecting the diagonal members at their nodes. A parametric study has been conducted to investigate the effect of different design parameters on the seismic performance of this system. A new type of composite brace comprised of glass fiber reinforced polymer (GFRP)-tube confined concrete, steel core and post-tensioned tendons, is developed for self-centering diagrid members. The hysteretic behavior of a self-centering chevron assembly comprised of two inclined composite braces is subsequently examined. Constitutive modeling of GFRP-tube confined concrete with high confinement volumetric ratio is conducted with experimental data calibration under monotonic and cyclic compression. The constitutive model is implemented into a finite element analysis platform OpenSees to enable nonlinear analysis of complex structures utilizing this type of confined concrete elements. The self-centering chevrons are implemented in the lower stories of the hybrid diagrid framing system to form base diagonals with large stiffness, enhanced ductility and energy dissipation capability and enable a rocking behavior for the diagrid system. The structural characteristics and seismic behavior of these two new seismic resistant systems are demonstrated with a prototype 21-story building subjected to nonlinear static and dynamic analysis. The findings from nonlinear time history analysis verify that satisfactory seismic performance can be achieved by these structural systems subjected to design basis earthquakes in California, specifically showing re-centering behavior while all main structural elements remain elastic in both systems.