Cable-Sling Construction Optimization for Dumbbell-Shaped CFST Arch Bridges

Literature Overview

The paper by Liu Shiming, Liu Yongjian, Yao Xiaorong, and Long Gang, published in the Journal of Highway and Transportation Research in 2012 (Vol. 29, No. 7, pp. 70-75), addresses the optimization of cable-sling construction sequences for multi-rib CFST arch bridges. Using the Wangpogou South Bridge as the engineering background, the study employs MIDAS/CIVIL spatial finite element analysis to investigate the effects of concrete pumping sequence and concrete age on structural behavior during construction. This work is particularly relevant to bridge engineers and steel pipe fabricators who need to ensure structural safety and serviceability throughout the construction process of complex CFST arch systems.

Engineering Background and Structural Configuration

The Wangpogou South Bridge features a dumbbell-shaped through-type CFST arch bridge composed of four arch ribs. The dumbbell configuration, where two outer arch ribs are connected by a central web or cross-beam system, provides enhanced lateral stability and torsional rigidity compared to single-rib or parallel-rib configurations. From a steel pipe fabrication perspective, the dumbbell arrangement requires precise dimensional control of the outer ribs and the connecting elements, as any dimensional deviation can lead to assembly difficulties and stress concentrations at connection nodes.

The construction method involves cable-sling erection, where each arch rib segment is lifted and positioned using a temporary cable system. The key construction challenges include:

Finite Element Analysis Methodology

The finite element model developed using MIDAS/CIVIL incorporates several critical material behaviors that are essential for accurate construction simulation:

1. Concrete shrinkage and creep: These time-dependent deformations significantly affect the long-term stress state of the CFST member, particularly during the construction phase when different sections are at different ages.
2. Concrete strength development: The time-dependent growth of concrete compressive strength affects the load-bearing capacity of each section during subsequent construction stages.
3. Steel-concrete interaction: The bond between the steel tube and the infill concrete, including potential slip at the interface, governs the composite action and load transfer mechanisms.
4. Nonlinear material behavior: Both the steel tube and the concrete exhibit nonlinear stress-strain relationships under high stress levels, particularly during the ultimate limit state.

The consideration of these factors in the finite element model is essential for predicting the actual structural behavior during construction. In practice, engineers must ensure that the construction sequence does not create unfavorable stress states that could compromise structural safety or lead to premature cracking of the concrete or yielding of the steel tube.

Key Optimization Findings

The study identifies several critical optimization measures that are directly applicable to construction practice:

Sling Sequence Optimization: Lifting two arch ribs simultaneously in a single cable-sling operation significantly improves labor efficiency and shortens the construction schedule. This approach requires careful planning of the temporary cable system to ensure that both ribs are lifted in a balanced manner, avoiding asymmetric loading that could induce torsional stresses in the already-erected ribs.

Cable Release Timing: The temporary cables should be released after the arch rib is fixed at the arch foot but before concrete pumping begins. This timing is critical because the steel tube must be able to bear its own weight and construction loads without temporary support, while the concrete pumping operation introduces additional loads that could be detrimental if the cables are still in place.

Concrete Pumping Sequence: The pumping order from bottom to middle to top ensures that the lower sections of the arch rib, which bear the greatest compressive loads, gain strength before the upper sections are loaded. This sequence also minimizes the risk of concrete segregation and bleeding, which could compromise the quality of the concrete-steel interface.

Concrete Age Requirement: The requirement that the preceding batch of concrete must have an age of not less than 7 days before the next batch is pumped is a conservative but prudent measure. This ensures that the earlier-placed concrete has achieved sufficient strength to bear the construction loads associated with subsequent pumping operations.

Welding and Fabrication Considerations

From a welding and fabrication standpoint, the construction of dumbbell-shaped CFST arch ribs involves several critical welding operations:

The on-site splicing welds are particularly critical because they must be performed in potentially adverse environmental conditions (wind, rain, temperature variations) and must achieve quality equivalent to shop-fabricated welds. Pre-qualification of welding procedures for site conditions, including wind shields, pre-heat requirements for low-temperature welding, and post-weld inspection protocols, is essential.

The connection between the outer arch ribs and the central web or cross-beam system requires careful attention to weld design. These connections must transmit both axial and transverse forces, and the weld details should be designed to ensure ductile failure mode rather than brittle fracture. The use of partial-penetration groove welds with appropriate reinforcement at the weld toes can help reduce stress concentrations.

Engineering Practice Integration

The findings of this study have direct implications for construction planning and execution:

1. Construction sequencing: The optimized sequence should be incorporated into the construction method statement and approved by the project engineer before work begins. Any deviation from the optimized sequence should be reviewed for structural safety implications.
2. Quality control: The concrete pumping operation should be monitored for temperature, slump, and placement rate to ensure that the specified pumping sequence is followed and that concrete quality is maintained throughout the pumping process.
3. Structural monitoring: Strain gauges, displacement sensors, and inclinometers should be installed at critical locations (arch foot, crown, quarter points) to monitor the structural response during construction and verify that the actual behavior matches the finite element predictions.
4. Weld inspection: All on-site welds should be inspected using ultrasonic testing (UT) and magnetic particle testing (MT) before the concrete pumping operation begins, to ensure that the steel tube is structurally sound and leak-free.

Study Insights and Implications

This study provides practical guidance for the construction of complex CFST arch bridge systems, particularly those with dumbbell-shaped configurations. The optimization of cable-sling construction sequences, concrete pumping order, and concrete age requirements offers a systematic approach to ensuring structural safety and serviceability during construction. The finite element analysis methodology, incorporating time-dependent material behaviors and steel-concrete interaction, provides a reliable tool for predicting structural response and identifying critical construction stages. Engineers should adopt these optimization measures as standard practice for similar projects, while also conducting project-specific analysis to address unique geometric and loading conditions. The emphasis on weld quality and structural monitoring throughout the construction process underscores the importance of integrating fabrication quality control with construction management to achieve the desired structural performance.