Ultimate Bearing Capacity Calculation Method for Steel Tube Concrete Lattice Columns

Literature Overview

This study by Chen Baochun and Ou Zhijing, published in the China Civil Engineering Journal (2008, Vol. 41, No. 1, pp. 55-63), addresses the fundamental question of how to accurately calculate the ultimate bearing capacity of steel tube concrete (SRC) lattice columns. Funded by the National Natural Science Foundation of China (Grant No. 50578042), the research originates from Fuzhou University and Fujian University of Technology. The authors propose a finite element analysis method using the ANSYS general-purpose program and validate it against experimental data, then compare the results with three Chinese design codes: CECS 28:90, JCJ 01-89, and DL/T 5085-1999. The research is significant because lattice columns composed of SRC limbs are widely used in high-rise buildings, industrial structures, and transmission towers, yet their stability calculation methods in existing codes contain notable inconsistencies.

Core Technical Content and Key Findings

The authors systematically investigate the influence of eccentricity ratio, slenderness ratio, and structural parameters on the ultimate bearing capacity of SRC lattice columns. The finite element model captures the nonlinear behavior of both steel tubes and concrete cores, including the confinement effect that enhances concrete compressive strength under lateral restraint. The analysis reveals that the eccentricity reduction coefficient formula in CECS 28:90 is the most reasonable among the three codes examined.

A critical finding concerns the material correction coefficient gamma for the equivalent slenderness ratio. The type of steel used for the column limbs and the concrete strength grade significantly affect the stability coefficient. Through numerical analysis of calculation examples, the authors propose a revised calculation method for gamma, which better accounts for the material-dependent stiffness degradation under combined axial and bending loads.

Comparison of Code Calculation Methods

Code Standard	Eccentricity Reduction Coefficient	Equivalent Slenderness Method	Material Correction	Overall Accuracy
CECS 28:90	Reasonable formula	Basic equivalent slenderness	Limited	Best among three
JCJ 01-89	Simplified approach	Simplified magnification factor	Not considered	Moderate
DL/T 5085-1999	Conservative estimate	Over-simplified	Not considered	Conservative but imprecise

Interpretation of Technical Points

The concept of equivalent slenderness ratio (lambda_eq) is central to this research. For lattice columns, the overall stability is governed not only by the individual limb stability but also by the interaction between limbs through lacing members. The magnification factor k applied to the equivalent slenderness ratio depends on the lacing configuration, including the crossing angle of diagonal lacing members. The authors verify that the simplified magnification factor algorithm is reasonable, as structural parameters such as the lacing member crossing angle have negligible influence on the overall bearing capacity.

The finite element model captures several critical nonlinear mechanisms: the progressive yielding of steel tube walls under combined stress states, the cracking and crushing of concrete cores, and the potential buckling of individual limbs before overall column failure. The good agreement between FE results and experimental data validates the modeling approach, particularly the contact interface between steel tubes and concrete cores.

Influence of Key Parameters on Ultimate Bearing Capacity

Parameter	Influence Level	Direction of Effect	Practical Implication
Eccentricity ratio	High	Capacity decreases with increasing eccentricity	Critical for design under eccentric loads
Slenderness ratio	High	Capacity decreases with increasing slenderness	Governs stability design
Steel grade	Moderate to High	Higher grade increases capacity	Affects material correction coefficient
Concrete strength	Moderate to High	Higher strength increases capacity	Affects stability coefficient
Lacing crossing angle	Low	Negligible effect on overall capacity	Can be simplified in design
Lacing member section	Low	Minor effect on overall capacity	Secondary design consideration

Integration with Engineering Practice

From a steel pipe manufacturing and welding perspective, this research has direct implications for the specification of steel tubes used in SRC lattice columns. The steel tubes must satisfy not only the mechanical property requirements for the specific grade (e.g., Q235, Q345, Q390 per GB/T 1591 or GB/T 700) but also dimensional tolerances that affect the confinement effectiveness. The wall thickness ratio (D/t) is particularly important: thinner-walled tubes provide less confinement to the concrete core but are more susceptible to local buckling under compressive loads.

For the welding connections between column limbs and lacing members, the quality of fillet welds and groove welds directly affects the load transfer efficiency. According to GB 50661 and JGJ 81, the weld design must account for the combined axial and shear stresses at these critical joints. Prequalification of welding procedures per ISO 15614-1 is recommended for high-strength steel grades.

Practical Recommendations for Steel Tube Specifications

Requirement	Specification	Rationale
Steel grade	Q235B minimum, Q345B preferred	Adequate ductility for confinement
Wall thickness tolerance	+/- 0.5 mm or +/- 10%	Ensures consistent confinement
Straightness	<= 0.5% of length	Prevents initial eccentricity
Surface condition	Free from cracks, laps, seams	Prevents stress concentration
Weld quality (if HSS)	Full penetration groove weld	Ensures full section utilization
NDT requirement	UT per GB/T 2970	Detects internal defects

Key Questions and Reflections

The research raises an important question about the applicability of equivalent slenderness methods to SRC lattice columns with non-uniform limb properties. When different limbs use different steel grades or concrete strengths, the material correction coefficient gamma becomes more complex. The proposed method assumes uniform material properties across all limbs, which may not reflect actual construction practices where material substitutions occur due to supply constraints.

Another reflection concerns the long-term behavior of SRC lattice columns under sustained loads. The finite element analysis presented is primarily elastic-plastic and does not account for concrete creep and shrinkage effects, which can significantly affect the load redistribution among limbs over time. For structures with design lives exceeding 50 years, this omission may lead to non-conservative estimates of long-term deflections and secondary stresses.

The comparison with three Chinese codes reveals a broader issue: the lack of harmonization between design standards for SRC structures. CECS 28:90 has since been superseded by GB 50935-2014, which incorporates many of the improvements suggested in this research. However, the material correction coefficient methodology proposed here remains relevant for situations where the new code's simplified approach may be insufficient.

Study Insights and Implications

This research demonstrates that the accurate calculation of SRC lattice column capacity requires careful consideration of material-dependent stability behavior. The proposed correction to the equivalent slenderness ratio method provides a more rational approach than the existing code provisions, particularly for columns with high slenderness ratios or when high-strength materials are used. For engineers involved in steel tube supply and welding fabrication, the key takeaway is that material properties and dimensional accuracy of steel tubes directly influence the structural performance of the entire lattice system, reinforcing the importance of strict quality control throughout the manufacturing chain.