Experimental study on torsional behaviour of square concrete filled CFRP-steel tube (S-CF-CFRP-ST)

Abstract

In this study, the torsional behaviour of square concrete filled carbon fiber reinforced polymer (CFRP) steel tube is revealed by experimental study. Taking the confinement factor of transverse CFRP, strengthening factor of longitudinal CFRP and the compressive strength of cubic concrete as principal factors, total 18 specimens were designed and tested under torsional load. Based on the tests, a numerical simulation method is firstly proposed to estimate the torsional performance of square concrete filled CFRP steel tube (S-CF-CFRP-ST), and then validated against the representative tests results, such as the failure modes, torque-rotation angle curves, and torque-shear strain curves. The test results reveal that the CFRP sheets are all under tension during loading and can cooperate well with steel tube as strains measured on CFRPs are almost same with the ones on steel tube. The failure mode of S-CF-CFRP-ST, such as cracked concrete, ruptured CFRP and torsional deformation of steel tube, belongs to ductile failure. The obtained torque-rotation angle and torque-shear strain curves of tested specimens have similar characteristics and can be divided into elastic stage, elasto-plastic stage and plastic stage. Additionally, the torsional deformation of S-CF-CFRP-ST is in line with plane section assumption before the specimen gets into plastic stage. The good consistence between numerical results and experimental ones reveals that the proposed simulation method can predict the torsional behaviour of S-CF-CFRP-ST reasonably and accurately. Using this verified numerical models, parametric study is conducted to explore the influence of principle factors (e.g. CFPR layers, strength of concrete and steel, and steel ratio) on torsional behaviour of S-CF-CFRP-ST. It is found that the torsional bearing capacity of S-CF-CFRP-ST is slightly improved with the increase of CFRP layers and significantly affected by the strength of the used square steel tube. Additionally, the increase of steel ratio for CF-CFRP-ST member enhances not only the torsional strength but also the initial stiffness.