Moment-Rotation Curve Calculation Method for Steel Tube Concrete Lattice Column-Composite Box Beam Nodes

Literature Overview

This paper develops a calculation methodology for determining the moment-rotation curves of nodes connecting steel tube concrete lattice columns with composite box beams. These nodes represent critical structural elements in high-rise and long-span structures where the lattice column system provides high axial capacity while the composite box beam delivers efficient flexural performance. The accuracy of node behavior prediction is essential for seismic design and serviceability verification.

Structural Configuration and Behavior Mechanism

The lattice column consists of multiple steel tube concrete (CFST) sub-columns connected by cross-bracing, forming a hollow structural system with high axial load capacity and favorable lateral deformation characteristics. The composite box beam integrates steel box sections with concrete infill, providing enhanced bending stiffness and shear capacity. The node region where these two systems connect experiences complex stress states involving bending, shear, axial force, and torsion simultaneously.

Stress Distribution Characteristics

The moment-rotation behavior of the node is governed by the interaction between several deformation mechanisms. The steel box beam flange and web undergo bending and shear deformation at the connection. The concrete infill within the box beam contributes to shear transfer and provides additional stiffness. The CFST sub-columns experience localized deformation near the connection point, where the steel tube may buckle inward under compressive forces transmitted from the beam.

Moment-Rotation Curve Stages

The calculated moment-rotation curve exhibits distinct stages that correspond to progressive damage accumulation. The initial elastic stage follows a linear relationship with a stiffness value determined by the composite section properties. The yield stage begins when the beam flange reaches yield stress, after which the stiffness decreases progressively. The plastic stage is characterized by strain hardening of the steel tube and concrete crushing in the infill zone. The ultimate stage represents the failure condition where excessive deformation or weld fracture limits further load increase.

Calculation Methodology

The proposed method employs a component-based approach where the total node rotation is decomposed into contributions from individual deformation mechanisms. Each component is modeled using appropriate constitutive relationships that capture the material non-linearity and geometric effects.

Key Calculation Parameters

The method incorporates a degradation function that reduces stiffness as damage accumulates, calibrated against experimental data from similar node configurations. The stiffness degradation rate depends on the axial load ratio on the column, which intensifies local buckling and accelerates damage progression.

Engineering Practice Implications

For practical design applications, the method provides engineers with the ability to predict node behavior under both gravity and seismic loading conditions. The moment-rotation curves can be directly integrated into non-linear frame analysis models, enabling accurate assessment of structural performance under extreme loading. Engineers should pay particular attention to the weld details at the node, as these represent potential weak links that may govern the ultimate behavior.

The research also highlights the importance of concrete infill quality within the box beam. Poorly compacted or contaminated infill concrete significantly reduces shear capacity and accelerates stiffness degradation. Quality control measures should include slump testing, density verification, and non-destructive evaluation of the infill concrete after placement.

Study Insights and Conclusions

This calculation method provides a practical and theoretically grounded approach for predicting the moment-rotation behavior of complex lattice column-composite beam nodes. The component-based decomposition strategy allows engineers to identify critical deformation mechanisms and target design improvements at specific locations. For seismic design of structures employing lattice columns, this method enables more accurate assessment of drift capacity and energy dissipation potential. The key insight is that the node behavior is dominated by the interaction between beam flange bending and sub-column local deformation, and that proper design of both elements is essential for achieving adequate seismic performance. Engineers should utilize this methodology during the preliminary design phase to optimize node geometry and connection details before detailed fabrication planning.