Zhuojin Pipe Fitting Co., LtdShear Resistance Performance of Square Steel-Concrete Filled Columns Under Combined Bending and Shear Loading
Literature Overview
The paper by Cai Jian, Liang Weisheng, and Lin Hui from South China University of Technology investigates the shear resistance behavior of square steel-concrete filled columns under combined bending and shear loading conditions. Funded by the National Natural Science Foundation of China (50878087), this research was published in the Journal of Shenzhen University (Science and Engineering Edition) in 2012. The study combines physical testing of full-scale specimens with nonlinear finite element analysis to develop an empirical formula for the shear capacity of square steel-concrete filled columns that accounts for the influence of axial compression.
Core Technical Content
Research Motivation and Scope
Square steel-concrete filled columns are widely used in building frames, particularly in multi-story and high-rise structures, where their compact cross-section provides efficient load-bearing capacity and good seismic performance. While the axial compression and pure bending behavior of these members has been extensively studied, their shear resistance under combined loading conditions remains less well understood. The shear behavior is particularly important for columns in the plastic hinge region, where the combined action of bending and shear can lead to premature shear failure if not properly designed.
Experimental Program
Six full-scale square steel-concrete filled column specimens were tested under combined bending and shear loading. The test matrix was designed to vary two primary parameters: the shear span ratio (the ratio of the shear span to the column height) and the axial compression ratio (the ratio of the applied axial load to the column's axial compression capacity). The specimens were loaded using a four-point bending configuration with a constant axial load applied throughout the shear test, simulating the combined loading conditions that occur in actual structural frames under seismic or wind loading.
Test Results and Failure Modes
| Parameter | Range Tested | Effect on Shear Capacity |
|---|---|---|
| Shear Span Ratio | Varying | Decreasing shear span ratio increases shear demand and reduces shear capacity |
| Axial Compression Ratio | Varying | Moderate axial compression enhances shear capacity; excessive axial compression may reduce ductility |
The failure modes observed in the specimens included diagonal shear cracking of the concrete core, local buckling of the steel tube faces, and shear yielding of the steel tube. The specimens with lower shear span ratios exhibited more pronounced shear failure characteristics, with the diagonal cracks developing at steeper angles and the steel tube experiencing more severe local buckling. The specimens with moderate axial compression ratios showed enhanced shear capacity due to the confining effect of the axial load on the concrete core, which increased the concrete's contribution to shear resistance.
Shear-Displacement Response
The shear-displacement curves exhibited an initial linear elastic region followed by a nonlinear hardening phase and eventual softening after the peak shear load. The stiffness degradation was more pronounced in specimens with lower shear span ratios, indicating a transition from flexure-dominated to shear-dominated behavior. The energy dissipation capacity, measured as the area under the shear-displacement curve, was higher for specimens with moderate axial compression ratios, suggesting that these specimens provided better seismic performance.
Nonlinear Finite Element Analysis
The nonlinear finite element model was developed using shell elements for the steel tube and solid elements for the concrete core, with appropriate contact conditions at the steel-concrete interface. The model incorporated the damage plasticity model for concrete and the von Mises yield criterion for steel, with strain rate effects neglected for the quasi-static loading condition. The finite element results were compared with the experimental results to validate the model and to conduct a parametric study on the effects of various geometric and material parameters on the shear capacity.
Proposed Empirical Formula
Based on the finite element analysis results, an empirical formula was developed to predict the shear capacity of square steel-concrete filled column specimens under combined bending and shear loading. The formula incorporates the shear span ratio as the primary variable and includes a correction factor for the axial compression ratio. The formula was validated against the experimental results and showed good agreement, with calculated values slightly on the conservative side of the measured capacities.
Welding and Fabrication Implications
Square steel-concrete filled columns are typically fabricated from square steel tubes, which can be produced through cold-formed or hot-rolled processes. For larger cross-sections, welded construction using steel plates is common, with the longitudinal and circumferential welds being critical structural elements.
Welding Considerations for Square Steel Tubes
The fabrication of square steel tubes for steel-concrete filled columns requires careful attention to weld quality, particularly at the corners where stress concentrations are highest. For welded square tubes, the corner welds must be designed for full penetration to ensure structural continuity and to prevent crack initiation under cyclic loading. The welding procedure should minimize residual stresses and distortion, which can affect the dimensional accuracy of the tube and the subsequent concrete filling operation.
The welding sequence for square tubes should be planned to minimize angular distortion at the corners. A symmetric welding sequence that alternates between opposite corners would help maintain the square geometry and prevent twisting. The weld metal should be selected to match the base metal properties, with attention to the impact toughness requirements for seismic applications.
Quality Control for Seismic Applications
For columns intended for use in seismic zones, the quality control requirements are more stringent due to the demand for ductile behavior under cyclic loading. The steel material must have adequate elongation and reduction of area to accommodate the large plastic deformations that occur during seismic events. The weld quality must be verified through non-destructive testing, with particular attention to the corner welds and any T-joints where braces or beams connect to the column.
The concrete filling process must be controlled to ensure full filling of the tube with adequate compaction and bonding. The concrete mixture should be designed for adequate workability to allow complete filling of the tube, particularly at the corners and near the weld seams. The concrete strength and durability properties should comply with the applicable design codes, with consideration given to the exposure conditions and the required service life.
Key Questions and Reflections
The study provides valuable insights into the shear resistance of square steel-concrete filled columns, but several aspects merit further investigation. The test specimens were subjected to monotonic loading, and the cyclic loading behavior, which is more representative of seismic conditions, was not examined. The hysteresis characteristics, energy dissipation capacity, and cumulative damage under cyclic loading would provide additional information for seismic design applications.
The proposed empirical formula, while showing good agreement with the experimental results, was developed based on a limited number of test specimens and finite element analyses. The formula would benefit from validation against a larger database of test results from different research programs and from consideration of additional parameters such as concrete strength, steel grade, and tube thickness ratio.
Summary
This research contributes valuable experimental and analytical data on the shear resistance of square steel-concrete filled columns under combined bending and shear loading. The proposed empirical formula provides a practical design tool for predicting shear capacity, with results that are slightly conservative and therefore suitable for engineering applications. For steel pipe manufacturers and welders, the study highlights the importance of producing high-quality square steel tubes with well-executed welds, particularly at the corners, to ensure that the columns can achieve their designed shear capacity under combined loading conditions. The combination of physical testing and finite element analysis offers a reliable approach for understanding and predicting the structural behavior of steel-concrete composite members in practical applications.