Experimental evaluation of composite and non‐composite columns and braces in special concentrically braced frames

Hollow structural section (HSS) and concrete‐filled tube (CFT) cross‐sections have been widely employed in the columns and braces of special concentrically braced frames (SCBFs). Square‐HSS cross‐section widely used in multistory frames is filled with concrete and converted to square‐CFT cross‐section to enhance the behavior of this cross‐section. However, some investigations indicated that circular‐HSS cross‐section filled with concrete (circular‐CFT) showed better behavior in comparison with square‐CFT cross‐section due to more uniform and larger concrete confinement in circular‐CFT cross‐section. The current study was experimentally undertaken to evaluate (1) the seismic performance and the global and local hysteresis responses of HSS and CFT members with various cross‐section shapes from initial elastic range to collapse in the system level of multistory SCBFs, (2) the behavioral differences between square cross‐section and circular cross‐section, and (3) the behavioral differences between HSS cross‐sections and CFT cross‐sections employed in the columns and braces of SCBFs. Four full‐scale one‐bay, two‐story SCBFs with four various cross‐sections, namely, square‐HSS, circular‐HSS, square‐CFT, and circular‐CFT, for columns and braces were subjected to cyclic lateral loading. Evaluating base shear–roof drift hysteretic loops of SCBF specimens demonstrated that SCBF specimens with CFT columns and braces (CFT‐SCBFs) experienced respectively around 107%, 58%, 28%, and 152% higher stiffness, post‐yielding and post‐buckling strengths, ductility, and energy dissipation capacity than SCBF specimens with HSS columns and braces (HSS‐SCBF). In addition, the experimental observations indicated that CFT braces experienced local buckling initiation, crack initiation, and fracture at respectively 2.22, 2.35, and 2.32 times of roof drifts of those exhibited by HSS braces. Moreover, assessing braces with various cross‐sections indicated that CFT braces showed an increase in compression strength, post‐buckling strength, compression axial deformation, and out‐of‐plane buckling approximately by 83%, 152%, 127%, and 100%, respectively, in comparison with HSS braces. Finally, square‐HSS/CFT braces sustained rupture propagation better than circular‐HSS/CFT braces.