Zhuojin Pipe Fitting Co., LtdAxial Tensile Mechanism and Load-Bearing Capacity of Steel Tube Concrete Members
Literature Overview
This study by Cai Wenzhe, Shi Qingxuan, and Wang Bin, published in Earthquake Engineering and Engineering Dynamics (Vol. 38, No. 4, 2018, pp. 124-131), investigates the axial tensile behavior of steel tube concrete (SRC) members through validated finite element analysis. The research was supported by the National Natural Science Foundation of China (Grant No. 51478382) and the University Talent Science and Technology Fund (RC1811). While SRC members are extensively studied under compression and bending, their tensile behavior—particularly relevant for seismic design where members experience alternating tension and compression—remains insufficiently characterized.
Core Technical Content and Key Findings
The authors established a validated ABAQUS finite element model of SRC columns and conducted numerical simulations on 10 SRC column specimens under axial tensile loading. The model was verified against experimental test results, confirming its applicability and accuracy.
Load Transfer Mechanism Under Axial Tension
The fundamental finding of this study is that under axial tensile loading, the steel tube bears the majority of the tensile force, while the core concrete contributes minimally to the tensile capacity. This behavior can be understood through the following mechanism:
- Initial loading stage: Both the steel tube and core concrete carry tensile stress proportional to their respective stiffnesses. The steel tube, with its significantly higher tensile modulus (E_steel ≈ 200 GPa vs. E_concrete ≈ 25-40 GPa), carries the dominant share of the tensile force.
- Concrete cracking stage: As the tensile stress in the concrete reaches its tensile strength (typically 2-5 MPa for normal concrete), micro-cracks initiate and propagate through the concrete core. The bond interface between the steel tube and concrete begins to debond, progressively transferring the concrete's tensile share to the steel tube.
- Steel tube yielding stage: After the concrete has effectively lost its tensile contribution, the steel tube carries essentially the full tensile load. The steel tube yields when the average tensile stress reaches its yield strength.
- Fracture stage: The final failure occurs at either the member end (due to stress concentration at the loading interface) or at the mid-span (due to localized necking or buckling of the steel tube wall under the combined tensile and residual compressive stresses from the concrete).
Simplified Tensile Capacity Formula
Based on the finite element analysis results, the authors propose a simplified formula for the tensile load-bearing capacity of SRC members:
| Parameter | Influence on Tensile Capacity | Relative Weight |
|---|---|---|
| Steel ratio (ρ_s) | Strong positive effect | High |
| Steel yield strength (f_y) | Strong positive effect | High |
| Concrete compressive strength (f_c) | Negligible effect | Very Low |
| Steel tube diameter (D) | Moderate positive effect | Medium |
| Steel tube wall thickness (t) | Moderate positive effect | Medium |
The negligible influence of concrete compressive strength on tensile capacity is a critical finding. It means that using high-strength concrete in SRC members does not improve their tensile performance, a conclusion that has significant implications for material selection in seismic design.
Welding and Fabrication Implications for Tensile SRC Members
The tensile behavior of SRC members has direct implications for the design and fabrication of welded connections and joints:
Welded Connection Design for Tensile Loading
Since the steel tube carries the majority of the tensile force, welded connections between SRC members must be designed to transfer the full tensile capacity of the steel tube. This requires:
- Full-strength welds: The weld cross-sectional area must be at least equal to the steel tube cross-sectional area. For butt welds, full penetration is mandatory.
- Weld quality requirements: Tensile loading is particularly sensitive to weld defects. Any volumetric defect (porosity, slag inclusion, lack of fusion) in the weld creates a stress concentration that can initiate brittle fracture under tensile loading.
- Weld HAZ toughness: The heat-affected zone of the weld must maintain adequate Charpy V-notch impact energy (typically ≥47 J at the service temperature) to prevent brittle fracture initiation in the HAZ under tensile loading.
Material Selection for Tensile SRC Applications
| Material Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Impact Energy at -20°C (J) | Suitability for Tensile SRC |
|---|---|---|---|---|---|
| Q235 | 370–500 | 235 | 26 | 34 | Marginal |
| Q345 | 470–630 | 345 | 21 | 34 | Good |
| Q390 | 510–660 | 390 | 20 | 34 | Good |
| Q420 | 520–640 | 390 | 20 | 34 | Good |
| Q460 | 570–690 | 460 | 18 | 34 | Excellent |
The study's finding that steel yield strength strongly influences tensile capacity supports the use of higher-grade steels (Q390, Q420, or Q460) for SRC members subjected to significant tensile loading, such as those in seismic zones or in tension members of long-span structures.
NDE Requirements for Tensile-Critical SRC Welds
Given the criticality of weld quality for tensile SRC members, the following non-destructive examination (NDE) requirements are recommended:
- Volumetric NDE: Ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) of all full-penetration butt welds to detect internal defects such as lack of fusion, slag inclusions, and porosity clusters.
- Surface NDE: Magnetic particle testing (MT) or dye penetrant testing (PT) of all weld surfaces and weld toes to detect surface cracks, lack of fusion, and undercuts.
- Acceptance criteria: For tensile-critical welds, the acceptance criteria should be at least as stringent as ASME Section V Article 4 Level 1 or EN ISO 17637 Level B, with zero tolerance for planar defects (cracks, lack of fusion) regardless of size.
- Weld procedure qualification: Each welding process must be qualified through coupon testing that includes tensile tests of the weld metal and HAZ, macrograph examination of weld cross-sections, and Charpy impact tests at the minimum service temperature.
Study Insights and Conclusions
This study fills an important gap in the understanding of SRC member behavior under axial tension, a loading condition that is increasingly relevant in seismic design and in tension members of large-span structures. The clear finding that the steel tube dominates the tensile capacity, while concrete strength has negligible influence, simplifies the design process and provides clear guidance for material selection. For welding engineers, the study reinforces the critical importance of weld quality in tensile-critical SRC applications. Since the steel tube carries essentially all of the tensile load, any weld defect that reduces the effective cross-sectional area of the steel tube directly reduces the member's tensile capacity. The proposed simplified tensile capacity formula provides a practical tool for preliminary design, but detailed finite element analysis incorporating realistic weld geometry and material properties is recommended for final design verification of critical tension members.