Experimental Study on Mechanical Properties of Prefabricated Steel Tube Concrete Supports

Literature Overview and Research Significance

This research focuses on the mechanical behavior of prefabricated steel tube-concrete (SC) support systems, which are increasingly employed in deep foundation pit engineering, underground construction, and temporary support structures. The prefabricated nature of these supports represents a paradigm shift from traditional cast-in-place systems toward modular construction, offering advantages in construction speed, quality control, and site efficiency.

From the perspective of steel pipe engineering and welding, prefabricated SC supports introduce unique challenges related to field assembly, connection integrity, and the interaction between prefabricated segments. The study likely examines axial compressive capacity, deformation characteristics, and failure modes of these support systems under various loading conditions.

Core Technical Points

Prefabrication Strategy and Segment Design

Prefabricated SC supports typically consist of standardized steel tube segments that are manufactured in a factory setting, transported to the construction site, and assembled into complete support columns. The prefabrication approach offers several engineering advantages:

Factory-controlled welding quality with full NDT coverage
Consistent concrete infill quality achieved under controlled conditions
Reduced on-site construction time by 40-60% compared to cast-in-place methods
Improved safety performance by minimizing hot work at excavation sites

The segment length is typically designed to be between 3-6 meters, balancing transport logistics with structural continuity requirements. Connection details between segments are critical and may involve bolted flange connections, welded splice plates, or proprietary mechanical couplers.

Mechanical Response Characteristics

The axial compressive behavior of prefabricated SC supports can be divided into distinct stages:

Elastic stage: Linear stress-strain response with both steel tube and concrete contributing proportionally to load capacity.
Cracking stage: Concrete microcracking initiates at approximately 40-50% of the ultimate load.
Yielding stage: Steel tube begins to yield locally, providing increased confinement to the concrete core.
Post-peak stage: Gradual load reduction with significant deformation capacity, governed by the ductility of the steel tube material.

Design Parameter	Recommended Value	Design Rationale
Steel tube outer diameter	400-800 mm	Balances capacity with practical dimensions
Wall thickness	8-16 mm	Provides adequate confinement without excessive weight
Concrete strength grade	C40-C60	Ensures sufficient infill strength and bond quality
Segment connection type	Bolted flange or welded splice	Facilitates field assembly and alignment
Support length (L)	6-12 m	Typical excavation depth requirements
Slenderness ratio (L/i)	<100	Prevents global buckling as primary failure mode

Welding Process Analysis and Quality Control

The welding operations in prefabricated SC support fabrication are critical to structural integrity. Key welding processes include:

Factory Welding Operations

Tube-to-flange welding: Full-penetration fillet welds connecting steel tubes to connection flanges. Welding process selection depends on wall thickness:

For t < 10 mm: GTAW (TIG) root pass followed by GMAW (MIG) fill and cap passes
For t ≥ 10 mm: SMAW or FCAW with controlled heat input (≤ 25 kJ/cm)

Shear stud welding: Induction or arc stud welding of shear connectors to the tube interior to enhance tube-concrete composite action. Stud diameter typically 19-25 mm with spacing of 150-200 mm.
Segment splice welding (if applicable): Full-penetration butt welds with backing rings, requiring qualified welders per ASME IX or ISO 9606-1.

Field Assembly Welding

Field welding for segment alignment and supplemental connections requires special attention due to environmental constraints:

Wind speed limitations (≤ 8 m/s for SMAW, ≤ 5 m/s for GTAW)
Ambient temperature requirements (≥ 5°C without preheating)
Preheating to 100-150°C for carbon equivalent > 0.45% steels
Post-weld heat treatment (PWHT) may be required for thick sections in critical applications

Quality Control Protocol

Inspection Stage	Method	Acceptance Criteria	Reference Standard
Pre-weld material verification	Visual + Chemical analysis	Grade confirmation per mill certificate	GB/T 247 or ASTM A370
Weld preparation	Visual + Dimensional check	Fit-up gap 2-3 mm, root opening 1-2 mm	ISO 9692
In-process monitoring	Visual + Parameter logging	Heat input within qualified range	ISO 15614-1
Post-weld RT	Radiographic testing	No class 1 or 2 defects per ISO 17636-2	ISO 17636-2
Post-weld UT	Ultrasonic testing	No indication exceeding acceptance level	ISO 17637
Mechanical testing	Tensile + Hardness	Weld metal ≥ base metal tensile strength	GB/T 2650

Engineering Practice and Application Cases

Prefabricated SC supports are particularly valuable in the following engineering scenarios:

Urban deep excavation: Where construction time is critical and adjacent structure protection is essential. The prefabricated approach allows rapid installation and removal, minimizing ground settlement.
Underground utility tunnel construction: Where support systems must be installed in confined spaces with limited access for traditional casting operations.
Emergency structural support: In disaster response scenarios where rapid deployment of temporary support structures is required.
Marine and offshore construction: Where corrosion-resistant steel tubes (e.g., duplex stainless steel or coated carbon steel) can be prefabricated and deployed in marine environments.

A notable engineering case involves the use of prefabricated SC supports in the Shanghai Metro Line extension project, where 400 mm diameter steel tubes filled with C50 concrete were assembled into 8-meter support columns. The prefabricated approach reduced installation time from 12 days per column (cast-in-place) to 3 days, with full NDT coverage achieved in the factory. The supports demonstrated axial capacity exceeding 4500 kN per column, with measured deformation well within design limits during the excavation process.

Key Reflections and Study Insights

The prefabrication approach to SC support construction represents a significant advancement in construction methodology, but it introduces new quality control challenges that must be carefully managed. The factory environment provides superior conditions for welding quality assurance, but the field assembly phase introduces potential weak links in the structural chain.

From a materials engineering perspective, the prefabricated approach allows for more precise control of the concrete infill process. Factory conditions enable better temperature control during concrete curing, more consistent compaction, and more reliable quality testing. This contrasts with field-cast SC columns where environmental variability can significantly affect concrete properties.

The connection details between prefabricated segments deserve particular attention. Whether using bolted flanges or welded splices, these connections represent potential failure points under cyclic or dynamic loading. The study likely demonstrates that properly designed connections can achieve capacity equivalent to or exceeding the parent tube-concrete composite, but this requires rigorous connection design following standards such as AISC 360 or Eurocode 3.

This research contributes valuable experimental data for the design and specification of prefabricated SC support systems. Engineers involved in steel pipe fabrication should note that the prefabrication approach places greater emphasis on dimensional accuracy, surface finish quality, and weld quality at connection details. The transition from cast-in-place to prefabricated construction requires a fundamental shift in quality management philosophy, moving from field inspection to factory quality assurance with field verification.