Failure mechanism and bearing force of CFRP strengthened square hollow section under compressive load.

Carbon fibre-reinforced polymer (CFRP) plates can efficiently repair or enhance the mechanical properties of the square hollow section. However, the loading end of such a CFRP-strengthened member is prone to local bearing failure under compressive load. Given this limitation, an innovative CFRP-plate-strengthened square hollow section composite member (CFRP-SHSCM) was raised, and the thick-walled section was welded on both ends of the thin-walled steel column. The mechanical properties of CFRP-SHSCMs were investigated through parameter finite element (FE) analysis, focusing on the influence of the amount of CFRP layers (nc), the slenderness ratio (λ), the initial geometric imperfections (v0), the CFRP layouts (2S and 4S) and the length of the exposed steel column (Le). The load-displacement curves, the bearing force, and typical failure modes were also acquired. Results indicated that with increasing nc and v0, and decreasing λ, the conventional CFRP-SHSCMs were prone to local bearing failure with poor ductility, leading to the insufficient use of the CFRP plate, in contrast, the improved CFRP-SHSCMs primarily underwent overall buckling failure and exhibited better bearing force and ductility. Finally, the modified Perry-Robertson formula was put forward to predict the ultimate load of the CFRP-SHSCMs. The coefficients of variation between the FE simulation and the theoretical results were 0.00436 and 0.0292, respectively.

Experimental and Analytical Research on Flexural Behavior of Concrete-Filled High-Strength Steel Tubular Members.

Using high-strength steel (yield strength fy ≥ 460 MPa) in concrete-filled steel tubes is expected to provide a superior bearing capacity by achieving light weight and efficient construction, but the existing design limitation on diameter-to-thickness (D/t) ratios for concrete-filled high-strength steel tubular (CFHST) members inevitably obstructs its wide application. In this study, aiming at the application of circular CFHST members using Q690 steel (fy ≥ 690 MPa), a total of 15 CFHST beams were examined using a three-point loading test to investigate the failure mode, bearing capacity and plasticity evolution. Subsequently, finite element models (FEMs) were established to analyze the full-range curves, composite effect, failure mechanism and influences of key parameters including material strengths, D/t ratios, and shear-span ratios. A simplified calculation method for bearing capacity was finally proposed and verified. The results indicate that the full-range performance of tested CFHST members with out-of-code D/t ratios have ductile behavior, though they fail through the mode of steel fracture and concrete cracks in the tension zone as well as through local buckling in the compression zone; out-of-code CFHST members (e.g., D/t = 120) can perform reasonable composite behavior because of contact pressure larger than 2.5 MPa, where a thin-walled steel tube experiences an arch failure mechanism similar to core concrete at a trussed angle of 45°; the simplified bearing capacity model achieves a mean value of 0.97, and can be accepted as a primary tool to perform structural design and performance evaluation.