Pure Bending Behavior of Rectangular Hollow Concrete Filled Steel Tube Beams

Structural Behavior and Failure Mechanisms

The finite element study on pure bending behavior of rectangular hollow concrete filled steel tube beams addresses the interaction between the steel tube, the concrete core, and the interface under constant bending moment.

In pure bending, the top and bottom faces of the rectangular tube carry compression and tension, while the side walls contribute to shear lag, local buckling restraint, and confinement.

The concrete core increases stiffness and prevents premature inward buckling, but the steel tube still controls ductility, weld integrity, and local stability.

The most important failure mechanisms are local buckling of the compression face, concrete crushing at the compression zone, interface slip, weld cracking at corners, and excessive curvature before concrete confinement becomes effective.

Finite Element Modeling Essentials

A reliable finite element model must capture geometric nonlinearity, material nonlinearity, contact or bond slip, and realistic boundary conditions in the pure bending region.

The steel tube is usually modeled with shell elements or solid elements depending on thickness, while the concrete core is modeled with solid elements and a damaged plasticity or compression damage model.

The interface can be modeled with tied contact, surface to surface contact with friction, cohesive zone elements, or simplified slip models, but the choice must match the physical interface condition.

Mesh convergence is critical because local buckling and concrete crushing are sensitive to element size and aspect ratio.

Modeling item	Recommended approach	Typical pitfall
Steel tube elements	Shell for thin walls and solid for thick walls	Overly coarse mesh misses local buckling
Concrete core	Damaged plasticity or compression damage model	Pure elastic model overestimates stiffness
Interface	Contact with friction or slip law	Perfect bond may overestimate capacity
Boundary condition	Constant moment in central span	Shear contamination in loading zone
Imperfections	Introduce geometric imperfection	Perfect geometry delays buckling unrealistically
Validation	Match strain, curvature, and load deflection	Model tuned only to peak load

The model should reproduce the curvature distribution, strain profile, and moment rotation response, not only the ultimate load.

If the model matches the initial stiffness but not the post peak behavior, the interface slip and concrete damage parameters are likely wrong.

If the model matches the ultimate load but shows unrealistic buckling wavelength, the mesh or imperfection amplitude is questionable.

Design and Inspection Insights

The engineering value of the study lies in identifying which parameters most influence bending capacity and ductility.

The width to thickness ratio of the steel tube is often the dominant stability parameter, while concrete strength and interface condition control confinement and post peak energy absorption.

For fabricated members, weld quality at corners and at longitudinal seams can be more critical than nominal material strength because cracking can initiate local loss of confinement.

Design parameter	Effect on bending response	Control measure
Width to thickness ratio	Controls local buckling	Limit ratio per design standard
Concrete strength	Improves compression capacity	Match strength to tube capacity
Hollow ratio	Affects stiffness and weight	Balance efficiency and stability
Transverse stiffeners	Delay local buckling	Place near high stress zones
Internal ribs	Improve confinement	Avoid congestion and casting defects
Weld quality	Maintains tube continuity	Visual, UT, or MT as applicable

A practical reflection is that rectangular concrete filled tubes are efficient but detail sensitive, and the weakest point is often not the bulk material but the weld, corner, or interface.

The finite element model should therefore be used to guide detailing, not merely to calculate a capacity number.

The study conclusion is that pure bending performance of rectangular hollow concrete filled steel tube beams is governed by a coupled stability and bond problem, so design must control wall slenderness, interface transfer, concrete confinement, and weld integrity together.