Zhuojin Pipe Fitting Co., LtdEffect of Construction Sequence on Stress in Steel Tube Concrete Arch Bridges
Literature Overview
The paper by Hu Qing'an, Ding Pengcheng, and Liu Jianxin from Chang'an University addresses a critical practical issue in the construction of steel tube reinforced concrete (CFST) arch bridges: how the sequence of concrete placement in the upper and lower chord tubes, combined with the method of falsework removal (one-time versus staged), influences the axial stress and displacement of the arch ribs. Published in the Journal of Chang'an University in 2006, this study employs ANSYS finite element software with birth-and-death element techniques to simulate a 23-step construction process, providing engineers with quantitative guidance on optimizing construction sequences to maximize structural safety margins.
Core Technical Approach and Methodology
The researchers adopted a hybrid finite element modeling strategy in which the concrete within the arch ribs was discretized using solid elements and the steel tubes were modeled using shell elements. This approach effectively captures the composite action between the steel tube and the infilled concrete while maintaining computational efficiency. The birth-and-death element feature in ANSYS was leveraged to activate concrete elements sequentially, mimicking the progressive concrete placement process. The entire construction was decomposed into 23 discrete steps, a level of granularity that allows for the tracking of stress redistribution at each stage.
The key modeling parameters and assumptions are summarized below:
| Parameter | Description |
|---|---|
| Concrete modeling | Solid elements (birth-and-death activation) |
| Steel tube modeling | Shell elements |
| Construction steps | 23 discrete stages |
| Analysis type | 3D nonlinear finite element simulation |
| Software | ANSYS |
| Key output variables | Axial stress, displacement at arch feet and crown |
The division of construction into 23 steps is particularly noteworthy. In practice, the concrete placement in a large-span CFST arch bridge is not instantaneous; it involves pumping, vibration, and setting phases that each introduce different loading conditions. By resolving the process at this level of detail, the study captures transient stress states that simplified models would overlook.
Key Findings and Their Engineering Implications
The study produced three principal quantitative results that carry significant implications for construction planning:
- One-time falsework removal versus staged removal: When the falsework is removed in a single operation rather than progressively, the axial stress at the arch feet is reduced by 102% compared to staged removal. This dramatic difference arises because staged removal creates a series of partial load releases that generate cumulative bending moments and secondary stresses in the arch ribs. One-time removal, by contrast, allows the structure to transition directly from the supported to the self-supporting state, minimizing the number of stress redistributions.
- Lower chord first versus upper chord first: When concrete is placed in the lower chord tube before the upper chord tube, the axial stress at the arch feet is reduced by 9.8% compared to the reverse sequence. This is physically intuitive: placing concrete in the lower chord first adds weight at the bottom of the arch, which tends to increase the compressive axial force in the arch ring in a manner that is more favorable to the structural action of the arch. The upper chord, being at the top, contributes more to bending when placed first.
- Recommended construction strategy: The authors recommend one-time falsework removal as the preferred approach whenever practical constraints allow. If one-time removal is not feasible due to span length, site constraints, or equipment limitations, the secondary recommendation is to place concrete in the lower chord tube first and the upper chord tube second.
| Construction Scenario | Axial Stress at Arch Feet (Relative) | Safety Margin Assessment |
|---|---|---|
| One-time removal, lower chord first | Lowest (baseline) | Highest safety reserve |
| One-time removal, upper chord first | Moderate | Good safety reserve |
| Staged removal, lower chord first | Higher | Reduced safety reserve |
| Staged removal, upper chord first | Highest | Lowest safety reserve |
Interpretation from a Structural and Welding Perspective
From the perspective of a practitioner who works with steel pipe structures, the findings have direct relevance to the welding and fabrication phase of CFST arch bridge construction. The arch ribs in such bridges are typically fabricated from large-diameter steel tubes, often in the range of 600 mm to 1200 mm in diameter, welded from plate or rolled tube segments. The welding sequence and the residual stress state introduced during fabrication interact with the construction sequence stresses identified in this study.
The 102% stress reduction from one-time removal is a striking figure that should prompt engineers to carefully evaluate the risks associated with one-time removal. In practice, one-time removal of falsework for long-span arches introduces dynamic effects that must be controlled. The weld joints in the arch ribs, particularly butt-welded splices between prefabricated segments, are potential weak points during the sudden load transfer that accompanies one-time removal. The welding engineer must ensure that all welds have been properly inspected (typically by radiographic testing or ultrasonic testing) and that any weld repairs have been completed before the removal operation.
The 9.8% difference between lower-chord-first and upper-chord-first placement, while smaller in magnitude, is still significant for the long-term fatigue performance of the structure. Repeated stress cycles during construction can initiate microcracks in the heat-affected zone (HAZ) of weld joints, particularly if the weld metal has lower toughness than the parent steel. The choice of welding consumables and preheat parameters should account for the additional thermal and mechanical cycling that will occur during the concrete placement phases.
Integration with Engineering Practice
In my experience with steel tube structure projects, the construction sequence is often dictated by site logistics, crane availability, and weather windows rather than by structural optimization. This study provides the quantitative justification needed to advocate for structurally optimal sequences even when they impose additional logistical challenges. The key recommendation for project teams is to incorporate construction sequence analysis into the early design phase, not as an afterthought during construction planning.
A practical FMEA (Failure Mode and Effects Analysis) approach can be applied to the construction sequence decision. The primary failure mode to consider is excessive axial stress at the arch feet leading to local buckling of the steel tube or yielding of the weld joints. The severity of this failure mode is high, as it could lead to partial collapse of the arch during construction. The occurrence is moderate, depending on the construction sequence chosen. The detection difficulty is relatively high, as stress states during construction are not easily measured in real time. The recommended preventive action is to follow the sequence recommended by this study, and the recommended detection action is to install temporary strain gauges at critical weld locations during the construction phases.
Key Questions and Reflections
Several questions arise from this study that merit further investigation:
- The study focuses on axial stress at the arch feet, but what about the stress state at the crown of the arch, where the bending moment is typically maximum? The interaction between axial stress and bending stress at the crown may be equally critical for weld integrity.
- The 23-step simulation assumes idealized concrete placement conditions. In reality, concrete placement rates vary, and the rheological properties of the concrete change over time. A time-dependent analysis incorporating concrete setting kinetics would provide more realistic predictions.
- The study does not address the effect of welding residual stresses on the construction sequence stress analysis. In welded steel tube structures, residual stresses can be as high as the yield strength of the material, and their interaction with construction-induced stresses may be significant.
- The study is based on a specific bridge geometry. The sensitivity of the construction sequence recommendations to span length, rise-to-span ratio, and tube diameter should be investigated systematically.
Study Insights and Implications for Practice
This study demonstrates that construction sequence is not merely a logistical consideration but a structural design parameter that directly affects the stress state and safety margin of CFST arch bridges. The quantitative results provide clear guidance: one-time falsework removal is strongly preferred, and lower-chord-first concrete placement is the recommended secondary strategy. These findings should be incorporated into construction method statements and risk assessments for all CFST arch bridge projects. The use of birth-and-death element techniques in finite element analysis represents a powerful tool for construction sequence optimization, and this methodology can be extended to other steel tube composite structures where construction-induced stress is a concern.