Experimental Studies on Seismic Performance of UHPSFRC-Filled Square Steel Tubular Columns

The excellent seismic performance of concrete-filled steel tube (CFST) structures has been widely recognized, but there is a paucity of research on composite columns using UHPC with added steel fibers. This paper presents the experimental studies and numerical analyses with OpenSees on seismic performance of ultra-high performance steel fiber-reinforced concrete (UHPSFRC)-filled square steel tubular columns. Five half-scaled specimens of UHPSFRC-filled square steel tubular columns were tested under a combination of constant axial compression and cyclic horizontal load, with parameters of width-to-thickness ratio (28.5, 19.9 and 14.7) and axial compression ratio (0.133, 0.266 and 0.399) of the steel tube. With the decrease in width-to-thickness ratio, the maximum bending moment capacity increased by 33.5% and 15.3%, and the energy dissipation capacity and ductility increased, while the strength degradation and stiffness degradation reduced. With the increased axial compression ratio, the loading capacity increased from 55.3 to 70.2 kNm (26.94%). The results indicate that UHPSFRC-filled square steel tubular columns improve seismic performance by decreasing the axial compression ratio and by increasing the width-to-thickness ratio. When the width-to-thickness ratio was reduced, the steel tubular was able to provide higher lateral restraint to the internal UHPC; thus, seismic performance was improved. With the increase in the axial compression ratio, the second-order damage effect of the members was greatly affected, and it accelerated the plastic damage. A modified UHPSFRC model considering steel tubular constraints was adopted, and the nonlinear dynamic modeling of the column response using OpenSees led to good agreement with the tested response of the column under cyclic motion. The theoretical calculation model can better predict the bending capacity of the UHPSFRC-filled square steel tubular columns. However, the calculation formulas of initial stiffness and yield bending moment need further research.