Pure Torsion Behavior of Steel Tube Concrete Members

Literature Overview

The paper by Han Linhai and Zhong Shantong from the Research Institute of Engineering Mechanics, China Seismological Bureau, and Harbin University of Architecture (Industrial Construction, Vol. 25, No. 1, 1995, pp. 7-13) presents an experimental and numerical study of four steel tube concrete (STC) members subjected to pure torsion loading. The study demonstrates that steel tubes and core concrete deform compatibly throughout the loading process, exhibiting excellent plastic performance. Full-process finite element analysis was performed and showed good agreement with experimental results. The authors defined a torsional capacity index for STC members and provided simplified calculation formulas for both torsional capacity and the composite shear modulus.

Core Technical Findings

The experimental results reveal several important behaviors of STC members under pure torsion:

Loading Stage	Steel Tube Behavior	Concrete Behavior	Overall Response
Elastic stage	Linear torsional response	Linear shear response	Combined linear behavior
Cracking stage	Elastic, hoop stress developing	First cracks at 45 degrees	Slight stiffness reduction
Post-cracking stage	Plastic yielding at critical sections	Cracks propagate and widen	Gradual stiffness degradation
Ultimate stage	Large plastic deformation	Crushing at compression diagonals	Significant ductility maintained

The composite shear modulus is a critical parameter that governs the torsional stiffness of the member. The simplified formula proposed by the authors accounts for the interaction between the steel tube and concrete, recognizing that the composite behavior is not simply additive but involves a coupling effect due to the confining action of the steel tube on the concrete.

Torsional Capacity Formula and Its Components

The torsional capacity of an STC member can be expressed as a function of the individual contributions of the steel tube and concrete core, modified by an interaction factor. The key parameters influencing torsional capacity include:

Parameter	Symbol	Typical Range	Influence on Capacity
Steel tube yield strength	f_y	235-460 MPa	Directly proportional
Steel tube wall thickness	t	4-20 mm	Directly proportional
Steel tube outer diameter	D	100-600 mm	Proportional to D^2
Concrete compressive strength	f_c	20-60 MPa	Moderate influence
Concrete shear strength	f_t	2-5 MPa	Significant influence
Section shape factor	alpha	1.0-1.2	Accounts for composite action

Welding and Fabrication Considerations for Torsional Members

From a fabrication standpoint, STC members subjected to torsional loading require special attention to weld quality and geometric accuracy. Torsional loads generate shear stresses that are perpendicular to the member axis, and any weld defects can act as stress concentrators that initiate failure. The following considerations are critical:

Longitudinal weld seams in seamless or welded steel tubes should be oriented perpendicular to the principal torsional stress direction to minimize the risk of weld crack initiation.
For ERW or HFW welded tubes, the weld seam quality directly affects the torsional performance, particularly at the microstructural level where the heat-affected zone may have reduced toughness.
Connection welds at member ends must be designed to transfer full torsional moments, requiring either full-strength welds or mechanical connections with adequate shear capacity.
The straightness and concentricity of the steel tube are critical for torsional members, as geometric imperfections can lead to torsional-flexural buckling at loads below the pure torsional capacity.

Quality Control for Torsion-Critical Members

Given the demanding performance requirements of torsion-critical STC members, the following quality control measures are recommended:

Inspection Method	Purpose	Acceptance Criteria
RT (Radiographic Testing)	Internal weld defects	No cracks, no incomplete fusion
UT (Ultrasonic Testing)	Wall thickness uniformity	Within +/-10% of nominal
MT (Magnetic Particle Testing)	Surface cracks	No linear indications
Dimensional inspection	Geometric accuracy	Straightness within L/1000
Hardness testing	HAZ properties	Within 350 HV for carbon steel
Torsional test (coupon)	Weld joint strength	Meets or exceeds base metal

Study Insights and Reflections

This pioneering study from 1995 established fundamental knowledge about the torsional behavior of STC members that remains relevant today. The finding that steel tube and concrete deform compatibly throughout the loading process is particularly important for design engineers, as it validates the use of composite design approaches. However, the study is limited to pure torsion, and in practice, STC members are often subjected to combined torsion, bending, and axial loads, which may produce different failure modes and lower capacities.

From a welding perspective, the long-term durability of STC members under torsional loading is a concern, particularly in seismic applications where cyclic torsional loading can lead to low-cycle fatigue at weld connections. The microstructural changes in the heat-affected zone of welded joints, including grain coarsening and potential tempering of martensitic phases in high-strength steels, can reduce the fatigue resistance and ductility of the weld region. Future research should focus on cyclic torsional loading of STC members with various weld configurations to establish fatigue design curves specific to these composite members.