Zhuojin Pipe Fitting Co., LtdFull-Process Stress Analysis of Steel Tube Reinforced Concrete Arch Surface
Overview of the Literature
This study presents a comprehensive experimental investigation of the stress development throughout the entire loading process of a steel tube reinforced concrete (STRC) arch structure. The arch surface is a critical structural element in tunnel linings, bridge arches, and underground cavern supports, and understanding its stress evolution under various loading stages is essential for safe and economical design.
The literature distinguishes between three loading stages: the construction stage (during concrete pouring and curing), the service stage (under operational loads), and the failure stage (beyond the ultimate load). The study uses a full-scale STRC arch specimen with extensive strain gauge instrumentation to capture the stress distribution at each stage.
Experimental Setup and Instrumentation
The test specimen is a half-scale model of a typical tunnel arch with the following dimensions and material properties:
| Parameter | Specification |
|---|---|
| Arch span | 6.0 m |
| Arch rise | 1.5 m |
| Cross-section width | 400 mm |
| Cross-section thickness | 200 mm |
| Steel tube outer diameter | 114 mm |
| Steel tube wall thickness | 6 mm |
| Steel grade | Q345 (fy = 345 MPa) |
| Concrete grade | C35 (fc = 35 MPa) |
| Reinforcement ratio | 1.8% |
The instrumentation includes:
- 24 strain gauges on the steel tube surface (top, bottom, and sides at 8 stations along the arch).
- 16 concrete strain gauges embedded in the concrete at key locations.
- 8 displacement transducers at the arch crown, haunches, and springings.
- 4 load cells at the loading points and 4 reaction points.
Stress Development Analysis
The study divides the loading process into five stages based on the load-displacement curve:
| Stage | Load Range | Description | Key Observations |
|---|---|---|---|
| Stage 1 | 0-30% of ultimate load | Elastic stage | Linear stress-strain relationship; steel and concrete share load proportionally |
| Stage 2 | 30-60% of ultimate load | Cracking stage | Concrete cracking initiates at the intrados; steel tube stress increases |
| Stage 3 | 60-80% of ultimate load | Plastic deformation stage | Steel tube yields at critical sections; stress redistribution begins |
| Stage 4 | 80-95% of ultimate load | Hardening stage | Concrete crushing; steel tube enters strain hardening region |
| Stage 5 | 95-100% of ultimate load | Failure stage | Progressive failure of concrete; steel tube local buckling |
Stage 1 (Elastic Stage). In this stage, the steel tube and concrete act as a composite section with a load-sharing ratio of approximately 35:65 (steel:concrete) based on the transformed section method. The stress distribution follows the flexure formula, with maximum compressive stress at the intrados and maximum tensile stress at the extrados.
Stage 2 (Cracking Stage). Concrete cracks initiate at the extrados (tension face) at approximately 40-50 percent of the cracking load predicted by the plain concrete modulus of rupture. After cracking, the steel tube and reinforcement carry the tensile forces, and the stress distribution becomes nonlinear.
Stage 3 (Plastic Deformation Stage). At approximately 60-70 percent of the ultimate load, the steel tube begins to yield at the haunch region, where the bending moment is maximum. The stress redistribution is significant, with the steel tube carrying 50-60 percent of the total load.
Stage 4 (Hardening Stage). The concrete at the intrados begins to crush, and the steel tube enters the strain hardening region. The load-displacement curve flattens, indicating reduced stiffness but continued load capacity.
Stage 5 (Failure Stage). Failure occurs through a combination of concrete crushing at the intrados and local buckling of the steel tube at the haunch region. The ultimate load is 15-20 percent higher than the theoretical plastic hinge capacity, indicating strain hardening and arch action effects.
Stress Distribution Patterns
The study identifies several important stress distribution patterns:
Crown Region. The crown of the arch experiences primarily compressive stress with minimal bending. The steel tube stress at the crown is uniformly distributed across the cross-section, with values of 120-180 MPa at the ultimate load.
Haunch Region. The haunch region experiences the maximum bending moment and shear force. The steel tube stress varies linearly across the cross-section, with maximum compressive stress of 380-420 MPa (exceeding yield) at the intrados and maximum tensile stress of 350-380 MPa at the extrados.
Springing Region. The springing experiences primarily shear and axial compression. The steel tube stress is lower than at the haunch, with values of 80-150 MPa. The concrete at the springing remains largely uncracked until the failure stage.
Engineering Practice Implications
The most significant finding for engineering practice is the stress redistribution mechanism that occurs after concrete cracking. The steel tube acts as a tension reinforcement, and the arch action provides additional load-carrying capacity through membrane action. This means that the actual ultimate load capacity of STRC arches is significantly higher than what would be predicted by simple beam theory.
The study recommends the following design considerations:
- Steel tube placement. The steel tube should be placed at the extrados (tension face) of the arch to maximize its effectiveness as a tension reinforcement.
- Concrete cover. The concrete cover to the steel tube should be at least 50 mm to ensure adequate bond and prevent corrosion.
- Load combination. The design should consider the combined effect of vertical loads (dead load, live load, snow load) and horizontal loads (earth pressure, seismic load), as the stress distribution is highly sensitive to the load ratio.
- Construction sequence. The stress state during construction (before concrete curing) should be considered, as premature loading can lead to excessive cracking.
Key Reflections
The study provides excellent experimental data on the full-process stress behavior of STRC arches, but it has several limitations that should be noted. First, the test is conducted on a single arch specimen, and the variability between specimens is not assessed. Second, the loading is quasi-static, and the dynamic effects of seismic loading are not considered. Third, the long-term effects of creep and shrinkage on the stress distribution are not addressed.
From a personal perspective, the most valuable insight is the quantification of the steel tube's contribution to load-carrying capacity at each stage. Engineers often underestimate the role of the steel tube in STRC arches, treating it as a formwork rather than a structural element. The data clearly shows that the steel tube carries a substantial portion of the load, particularly in the post-cracking and plastic stages.
In summary, this literature provides comprehensive experimental evidence on the stress evolution of STRC arch surfaces throughout the entire loading process. The findings are directly applicable to the design and assessment of tunnel linings, bridge arches, and underground supports. The recommended design considerations and stress distribution patterns should be incorporated into design codes and practice guidelines for STRC arch structures.