Axial compression performance of pultruded GFRP tube-self-compacting concrete-steel tube composite medium-long column

Pultruded glass fiber reinforced polymer(GFRP)tube-self-compacting concrete (SCC)-steel tube composite column has excellent mechanical properties, light weight, high strength, good corrosion resistance and convenient construction. It has wide application potential in bridges and high-rise buildings. At present, the research work of scholars on DSTC is carried out around the winding GFRP tube. Most of the research contents are DSTC short columns, and there are relatively few studies on pultruded GFRP tubes and DSTC medium-long columns. In this paper, the pultruded GFRP tube-SCC-steel tube composite medium-long column is studied to clarify its axial compression mechanism and derive the bearing capacity calculation model, which provides theoretical support for the design and application of the new composite medium-long column.

Based on the three important parameters of SCC strength grade(C40, C50, C60), GFRP tube thickness(4mm, 5mm) and end CFRP cloth reinforcement length(50mm, 100mm), axial compression tests were carried out on five composite medium-long column specimens. The loading phenomenon and failure characteristics, load-displacement curve, GFRP tube load-strain curve and steel tube load-strain curve were analyzed in depth. The performance indexes such as ultimate bearing capacity, constraint effect coefficient, ductility coefficient, initial stiffness, strengthening stiffness and stiffness degradation rate are systematically studied. Based on the twin shear unified strength theory, based on the bearing capacity calculation model of DSTC composite short column, the stability coefficient is introduced to establish the bearing capacity calculation model of DSTC composite medium-long column.

①Specimens with different concrete strength grades (C40, C50, C60) exhibited shear failure; the composite medium to long column with a 5mm thick GFRP tube and a 100mm CFRP fabric reinforcement at the ends showed local splitting failure.②Higher concrete strength grades resulted in greater ultimate load capacity, with similar ultimate displacements; however, the constraint effect was diminished, leading to reduced ductility and increased stiffness degradation rates.③Increasing the thickness of the GFRP tube enhanced the constraint effect, thereby improving the ultimate load capacity and displacement of the composite medium to long columns, as well as increasing the ductility coefficient, initial stiffness, and reinforced stiffness.④Lengthening the CFRP fabric reinforcement at the ends significantly improved the initial stiffness of the composite medium to long columns, with minimal impact on the constraint effect coefficient, reducing stiffness degradation.⑤As the SCC strength grade increased, the initial strain of the GFRP tube transitioned from rapid development at both ends to rapid development in the middle. In the later stages of loading, specimens with local splitting at the failure location exhibited rapid strain development, while the rest showed slower strain growth; specimens experiencing shear failure exhibited rapid overall strain development.⑥Based on the double shear unified strength theory, three stability coefficients were introduced to the load-bearing capacity calculation models of two DSTC composite short columns, leading to the establishment of six load-bearing capacity calculation models for the DSTC composite medium to long columns. The optimal model exhibited an average load capacity coefficient of 0.992 and a standard deviation of 0.047.Conclusions: ①Similar to composite short columns, the failure characteristics of composite medium to long columns are primarily determined by the constraint effect of the GFRP tube. Failure modes are mainly shear and splitting failures. Shear failure resembles that of short columns, while splitting failure is localized, without full-length failure.②Higher SCC strength grades correspond to greater ultimate load capacities but smaller constraint effect coefficients and more significant stiffness degradation. Thicker GFRP tubes increase ultimate load capacity and ductility while enhancing the constraint effect, although their impact on stiffness degradation is limited. The ultimate load capacity, ultimate displacement, ductility coefficient, and constraint effect coefficient of specimens with a 5.5mm thickness increased by 10.59%, 15.97%, 23.02%, and 10.32%, respectively, compared to those with a 4mm thickness. Increasing the length of the end CFRP fabric reinforcement can mitigate stiffness degradation; for example, the stiffness degradation rate of specimen DZ4-1 was reduced by 6.26% compared to DZ2-1, although its influence on ultimate load capacity and constraint effect was limited.③The load-bearing capacity calculation model for composite medium to long columns was established by correcting the short column model with stability coefficients based on the double shear unified strength theory. Comparative analysis showed that the corrected model exhibited good accuracy and stability, with the optimal model achieving an average load capacity coefficient of 0.992 and a standard deviation of 0.047, providing reliable theoretical support for practical engineering applications.