Effect of Construction Sequence on Stress in Steel Tube Concrete Arch Bridges

Literature Overview

This 2006 study published in the Journal of Chang'an University (Natural Science Edition) by Hu Qing'an, Ding Pengcheng, and Liu Jianxin from the Key Laboratory of Highway Engineering in Special Regions investigates how the sequence of concrete pouring in steel tube concrete (CFT) arch bridges affects axial stress and displacement in the arch ribs. The authors employed the ANSYS finite element program utilizing dead-and-alive element properties, modeling concrete as solid elements and steel tubes as shell elements, with the construction process divided into 23 discrete steps to simulate the actual pouring sequence. The research addresses a practical construction challenge that directly impacts the structural integrity and long-term serviceability of CFT arch bridges, which are increasingly adopted in mountainous and special terrain highway projects across China.

Core Technical Findings

The study establishes three critical quantitative relationships that have direct implications for construction methodology planning:

The 102% figure for one-time falsework removal indicates that the stress accumulated during staged removal is not merely additive but can be eliminated entirely by removing the falsework in a single operation, suggesting that the residual stress from sequential removal creates a significant permanent stress state in the arch rib steel tubes. The 9.8% difference between pouring sequences demonstrates that even when staged removal is unavoidable, the order of concrete placement provides a meaningful but secondary optimization lever.

Interpretation of Technical Points

From a structural mechanics perspective, the findings align with the principle of stress redistribution in composite arch structures. When concrete is poured into one chord tube before the other, the completed half-arch bears asymmetric loads during the construction phase, generating bending moments that the final symmetric design does not account for. The steel tube, acting as a shell element in the finite element model, experiences membrane stresses that are sensitive to boundary condition changes during construction. The arch foot, being the point of maximum axial force and minimum moment arm, is particularly vulnerable to construction-induced residual stresses.

The use of dead-and-alive element properties in ANSYS is a critical modeling technique here. By activating and deactivating concrete elements at each construction step, the authors captured the progressive change in stiffness and mass distribution throughout the 23-step process. This approach is superior to a single-step analysis because it accounts for the time-dependent stress history that governs whether residual stresses relax or accumulate permanently in the steel tube material.

Engineering Practice Implications

For bridge engineers involved in CFT arch construction, this study provides actionable guidance through a prioritized decision framework:

1. Primary recommendation: Design the falsework removal plan for one-time removal where structurally and logistically feasible. This requires careful coordination of crane capacity, temporary support design, and construction scheduling to ensure the arch can be self-supporting immediately after falsework removal.
2. Secondary recommendation: When one-time removal is impractical due to span length, terrain constraints, or equipment limitations, always prioritize pouring the lower chord tube before the upper chord tube. This reduces the unfavorable bending moment at the arch foot by approximately 10%.
3. Quality control consideration: During construction monitoring, strain gauges should be installed at the arch foot on both chord tubes to verify that measured stresses align with the predicted construction sequence effects. Any deviation exceeding 15% of predicted values should trigger a review of the actual pouring sequence.

Key Questions and Reflections

A question that arises from this study is whether the 23-step construction simulation adequately captures the actual construction timeline, which may involve concrete curing periods between steps that allow stress relaxation through viscoelastic behavior in the concrete. The study does not appear to incorporate time-dependent concrete properties such as creep and shrinkage, which could partially mitigate the construction-induced residual stresses over time. Additionally, the shell element modeling of steel tubes may not fully capture local buckling behavior at connection points where the chord tubes are joined to the arch foot supports.

Another consideration is the interaction between construction sequence effects and the long-term service loads. The study focuses on construction-phase stresses but does not explicitly analyze how these residual stresses combine with traffic loads, temperature effects, and live loads during the service life. For bridges in regions with significant temperature variation, such as those in northern China where this research group specializes, the superposition of construction residual stress and thermal stress could lead to accelerated fatigue at the arch foot.

Study Insights and Implications

This study represents a valuable contribution to construction methodology optimization for CFT arch bridges. The relatively modest 9.8% improvement from pouring sequence optimization suggests that construction sequence is a secondary design variable, while the dramatic 102% effect of one-time falsework removal elevates it to a primary design consideration. Engineers should incorporate construction sequence analysis into the preliminary design phase rather than treating it as a construction-phase afterthought. The finite element methodology demonstrated here, using dead-and-alive elements for construction simulation, is transferable to other composite steel-concrete structures where construction sequence affects final stress states, including steel tube concrete columns, composite bridges, and hybrid structural systems.