Calculation methods for axial load bearing capacity of steel fiber reinforced geopolymer concrete filled square stub steel tubular columns

CFST is extensively utilized in practical engineering due to its superior structural characteristics. To mitigate the environmental damage associated with the use of ordinary silicate cement, GC replaces OPC within the square steel tubes. Additionally, to enhance the brittleness of the core GC, steel fibers are incorporated to improve the mechanical properties of the components. Although numerous studies have investigated the axial compressive properties of square CFST columns, research on SFRGCFST stub columns remains sparse. The mechanisms through which square steel tubes constrain the core SFRGC and the axial compressive damage mechanisms of SFRGCFST columns are not fully understood. Moreover, SFRGC differs from traditional OPC in its cementitious material composition, and the absence of specific standards and regulations for geopolymer concrete raises concerns about the reliability of the axial compressive capacity of SFRGCFST columns. Therefore, it is crucial to study the axial compressive performance of SFRGCFST stub columns and develop a corresponding method to calculate their axial compressive capacity. This research will provide essential data support for further studies.

Based on prior research, 12 sets of 24 SFRGCFST stub columns were designed for an axial compression test to examine the effects of GC strength grade, steel tube thickness, steel fiber volume fraction and aspect ratio on the failure morphology, ultimate capacity, load-displacement curves, and strength indices of the specimens. The study aimed to determine the axial compressive capacity of the SFRGCFST stub columns using the multiaxial strength criterion and the theory of cross-sectional equilibrium of the square steel tube. Furthermore, by integrating experimental data and numerical analysis, a strengthening index influenced by steel fiber was proposed. Ultimately, these formulae were compared and analyzed in relation to existing codes and regulations.

Through axial compressive performance tests on SFRGCFST stub columns, the research findings reveal several key outcomes: (1) Failure characteristics of specimens transition from shear damage to bulging damage with increasing coefficient index, specifically noted greater than 0.88 (SGT4-1.2-60). Lower indices (SGT3-0.9-60) show typical shear damage with bulging concentrated in the middle and upper quarter of the steel tube; (2) As GC strength grade increases, the descending section of the curve deepens, indicating reduced deformability and ductility of the SFRGCFST specimens. Furthermore, the load-displacement curve for SGT4(5)-0.9-60 shows a marked improvement compared to SGT4(3)-0.9-60. Increasing the volume fraction of steel fibers from 0.6% to 1.2% smoothens and moderates the descending section of the curve; (3) Compared to ordinary steel tube concrete (SCT4-0-0), SFRGCFST exhibits an average and maximal increase in ultimate load capacity by 35.71% and 45.55% respectively. The substitution of GC for OPC, coupled with a 0.9% volume of steel fiber, effectively boosted the ultimate load capacity by 21.85% compared to SFRCFST; (4) All specimens showed a Strength Index (SI) greater than 1.0, with GCFST showing a 26.32% higher SI than CFST and a 27.98% increase in SFRGCFST compared to SFRCFST under the same steel fiber volume fraction. The strength index demonstrates minimal change with increased steel fiber content, peaking at a 0.6% volume fraction; (5) A new formula for calculating the axial compressive load capacity of SFRGCFST stub columns is proposed, showing a fitting average value of 0.986, which significantly surpasses the DJB/T13-51-2010 standard (0.936) and other scholarly models. The variance of the new formula is smaller at 0.002, indicating more stable calculation results. Moreover, calculated results under ultimate load conditions consistently underscore the safety of the test by being lower than the measured load capacity.Conclusions: The impact of altering the volume fraction of steel fiber and the aspect ratio significantly affects the SI of the specimens. Increasing the volume fraction of steel fiber not only enhances the mechanical properties of the core GC but also yields a smoother axial compressive load-displacement curve for SFRGCFST. Additionally, as the volume of steel fiber and the aspect ratio increase, the downward slope of the load-displacement curve becomes more gradual. A formula for calculating the axial compressive capacity of SFRGCFST stub columns has been developed based on the multiaxial strength criterion and the equilibrium theory of the cross-section. This formula yields results with an error margin within 5% when compared to experimental data. These findings can provide valuable insights for the analysis of force performance, engineering design, and the broader application of such structural components.