Cross-Sectional Stress Analysis of Dumbbell-Shaped Steel Tube Concrete Arch Ribs During Concrete Injection

Literature Overview

This paper published in the China Journal of Highway and Transport in 2005, authored by Chen Baoshun and colleagues from Fuzhou University and Fuzhou Planning and Design Institute, addresses a critical construction safety issue: the risk of pipe bursting during concrete injection into dumbbell-shaped steel tube concrete (STC) arch ribs in bridge engineering. Funded by the Fujian Provincial Higher Education Science and Technology Project (Grant No. JA03016), the study uses finite element analysis to evaluate cross-sectional stresses under various concrete injection sequences and conditions, aiming to prevent catastrophic bursting accidents during construction.

Core Technical Content

The research establishes that during concrete injection into dumbbell-shaped STC arch ribs, the junction between the steel tube and the web plate generates extremely high stresses, which is the primary cause of bursting accidents. The injection sequence is identified as a critical factor: injecting the web cavity first and then the two outer tubes creates the most unfavorable stress condition, while injecting the two outer tubes first and then the web cavity provides a more favorable stress distribution.

Injection Sequence Stress Comparison

Injection Sequence	Stress Level at Tube-Web Junction	Relative Risk	Recommendation
Web first, then tubes	Very high (most unfavorable)	High bursting risk	Not recommended
Tubes first, then web	Moderate (more favorable)	Reduced bursting risk	Recommended
Simultaneous injection	Intermediate	Moderate risk	Requires careful control

Recommended Countermeasures Against Bursting

Countermeasure	Effectiveness	Implementation Difficulty	Cost Impact
Tension rods at web	High	Moderate	Low
Steel section stiffeners on web	Very high	Moderate	Moderate
Sequential cavity injection	High	Low	Low
Controlled injection pressure	Moderate	Low	Low
Combined approach	Very high	Moderate	Moderate

Welding and Steel Pipe Fabrication Analysis

The dumbbell-shaped STC arch rib is a complex structural component where steel tube fabrication and welding quality are critical to structural integrity:

Structural Configuration Analysis

The dumbbell-shaped cross-section consists of:

Two outer circular steel tubes (typically 400-600 mm diameter, 12-20 mm wall thickness)
A central web plate connecting the two tubes (typically 8-16 mm thickness)
Butt-welded joints between tubes and web plate
Circumferential butt welds at arch segment connections

Critical Weld Locations and Quality Requirements

Weld Location	Weld Type	Criticality	Required NDT
Tube-to-web junction	Fillet or T-butt weld	Critical (bursting zone)	100% UT + 100% MT
Arch segment butt welds	Full penetration butt	High	100% RT
Stiffener attachment welds	Fillet weld	Moderate	10% UT + 100% MT
Tension rod anchor welds	Full penetration	High	100% RT

Welding Process Considerations

For the tube-to-web junction welds, which are identified as the critical bursting zone:

Process selection: GTAW (TIG) for root pass followed by GMAW (MIG) or SAW for fill and cap passes is recommended for high-quality results. The root pass must achieve complete fusion at the tight junction geometry.
Preheating: Preheating to 100-150°C is recommended for wall thicknesses exceeding 16 mm to reduce hydrogen-induced cracking susceptibility in the HAZ.
Interpass temperature control: Maximum interpass temperature of 250°C to prevent excessive grain growth and maintain toughness in the HAZ.
Weld sequence: Symmetric welding sequence from the center outward to minimize angular distortion at the tube-web junction.
Post-weld treatment: Local PWHT to 550-620°C for stress relief of the critical junction welds.

Engineering Practice Integration

The bursting accident risk during concrete injection is a well-recognized hazard in STC bridge construction. Several practical measures should be implemented:

Construction Sequence Optimization

Pre-injection inspection: Verify all welds are NDT-qualified before any concrete injection begins.
Pressure monitoring: Install pressure gauges at injection points with alarm thresholds set at 60% of the calculated bursting pressure.
Sequential injection protocol:

Phase 1: Inject both outer tubes simultaneously from bottom to top
Phase 2: Wait for initial set (minimum 4 hours or 5 MPa compressive strength)
Phase 3: Inject web cavity from center outward

Injection rate control: Maximum injection rate of 2 m³/h for standard-sized dumbbell sections to limit hydrostatic pressure buildup.

Case Study Insight

The paper's finding that web-first injection is most unfavorable is consistent with the mechanical principle that injecting the web first creates a constrained condition where the expanding concrete pushes outward against the still-empty tubes, generating high radial stresses at the junction. Conversely, injecting the tubes first allows the concrete in the tubes to provide lateral restraint before the web is filled, distributing stresses more evenly.

Key Questions and Reflections

From a metallurgical perspective, the bursting phenomenon at the tube-web junction involves complex stress states: multiaxial stress concentration, strain rate effects from rapid concrete placement, and potential interaction with weld HAZ microstructure. The HAZ at these junctions, particularly if the base material is a high-strength steel (Q345 or Q390), may have reduced ductility compared to the base metal, making it more susceptible to cracking under the combined stresses of concrete injection.

A question that warrants further investigation is: how does the temperature of the injected concrete affect the stress state? Warm concrete (above 30°C ambient) has lower viscosity and may flow more rapidly, creating higher dynamic pressures during injection. Conversely, cold concrete may have higher viscosity and require higher pumping pressures, also increasing stress levels. Optimal concrete temperature for injection should be studied systematically.

Additionally, the long-term fatigue behavior of the tube-web junction welds under traffic loading on the completed bridge should be considered. The residual stresses from concrete injection, combined with traffic-induced cyclic loading, could initiate fatigue cracks at the weld toe if not properly addressed through welding procedure optimization.

Study Insights and Implications

This research provides essential guidance for the safe construction of STC bridge arch ribs with dumbbell-shaped cross-sections. The identification of injection sequence as a critical factor, combined with specific recommendations for stiffening and sequential injection, provides a practical framework for preventing bursting accidents. For welding engineers, the critical importance of the tube-web junction welds cannot be overstated—these welds must be designed, executed, and inspected to the highest standards, as they are the primary failure location during construction. The finite element methodology presented can be adapted for specific project geometries to optimize the injection sequence and stiffening design for each unique bridge structure.