Experimental Investigation of Crushing Ultimate Bearing Capacity of Grouted Square Steel Tubes

Overview of the Study

The paper by Zheng Wenzhong and colleagues from Harbin Institute of Technology addresses a critical gap in the design of grouted square steel tube (GSS) chord members in steel truss-concrete composite beams. The existing crushing capacity formulas for grouted chord members fail to account for the influence of axial force in the chord member on the joint crushing bearing capacity. This omission can lead to non-conservative or overly conservative design estimates in practical engineering applications. The research team conducted both physical model tests and numerical simulation tests to develop a more accurate empirical formula for predicting the crushing ultimate bearing capacity of grouted square steel tube joints.

Experimental Program and Parameters

The model test program involved 15 grouted square steel tube specimens, systematically varying key geometric and loading parameters. The primary variables examined in the physical tests included the ratio of the brace side length to the chord side length (denoted as the side length ratio) and the nature and magnitude of the axial force applied to the chord member. The numerical simulation extended the parametric study to include additional variables that are difficult to vary in physical testing, such as chord wall thickness, cement mortar strength, and steel grade.

The experimental setup followed standard testing protocols for steel tube joints, with careful attention to load application symmetry and strain measurement at critical locations including the chord wall, brace-chord weld interface, and the grout-steel interface. Strain gauges were arranged in both longitudinal and circumferential directions to capture the full stress state during crushing.

Crushing Mechanism and Failure Mode Analysis

The analysis of both test and simulation results revealed that the crushing mechanism in grouted square steel tubes is fundamentally different from that of hollow square steel tubes. In hollow tubes, crushing is primarily governed by local buckling of the chord wall. In grouted tubes, the presence of cement mortar provides significant confinement and load distribution, transforming the failure mode from local buckling to a more distributed yielding pattern. The grout effectively prevents premature local buckling and allows the steel tube to reach higher stress levels before failure.

The axial force in the chord member plays a crucial role in modifying the crushing behavior. Compressive axial force in the chord reduces the crushing capacity because it pre-stresses the chord wall and accelerates yielding at the brace-chord intersection. Conversely, tensile axial force increases the crushing capacity by providing additional resistance to local deformation. This interaction effect is not captured by existing formulas that treat crushing capacity as independent of chord axial force.

Derived Empirical Formula

Based on the combined results of model tests and numerical simulations, the authors proposed a modified empirical formula for the crushing ultimate bearing capacity of grouted square steel tube joints. The formula incorporates the side length ratio, the chord axial force ratio, the chord wall thickness, the cement mortar strength, and the steel yield strength as key parameters. The formula takes the following general form:

F_c = A · f_y · t_c · (1 + α · η) · (1 + β_m · f_cm / f_y) · g(β)

where F_c is the crushing ultimate bearing capacity, A is a coefficient derived from regression analysis, f_y is the steel yield strength, t_c is the chord wall thickness, η is the axial force ratio, α is a modification factor accounting for axial force influence, β_m is a grout contribution factor, f_cm is the cement mortar compressive strength, and g(β) is a function of the side length ratio.

The formula was validated against both the 15 model test specimens and the additional simulation cases, showing good agreement with mean error within acceptable engineering tolerances. The derived formula provides a more rational basis for the design of internal steel truss-concrete composite beams, where grouted square steel tubes are used as chord members in the truss system.

Engineering Practice Implications

From a steel pipe manufacturing and welding perspective, this study has several important implications. First, the quality of the grouting process directly affects the crushing capacity, which means that grout density and compaction must be tightly controlled in fabrication. Second, the weld quality at the brace-chord intersection is critical, as the stress concentration at this location governs the onset of yielding. Third, the wall thickness of the square steel tube must be selected with consideration of the axial force level in the chord, as thinner walls may lead to premature crushing under combined loading. For welding engineers, ensuring full penetration welds at the brace-chord junction and avoiding weld defects such as undercut and porosity is essential for achieving the predicted crushing capacity.

Summary

This study provides a significant advancement in the understanding and design of grouted square steel tube joints by incorporating the effect of chord axial force on crushing capacity. The combination of physical testing and numerical simulation offers a comprehensive parametric study that would be impractical with either method alone. The derived empirical formula is a valuable tool for engineers designing composite truss-concrete beam systems, and the insights into the crushing mechanism inform both fabrication quality control and welding process specifications for these structural components.