Steel Tube Concrete Arch with Steel Web PC Box Girder Composite Bridge

Literature Overview

This paper by Sun Chao, Zhang Fan, Chen Baochun, and Wu Qingxiong, published in the Journal of Guangxi University (Natural Science Edition, 2012, Vol. 37, No. 4, pp. 737-743), presents an innovative structural system for arch-bridge construction that replaces the conventional reinforced concrete web of a prestressed concrete (PC) box girder tie beam with steel web plates. Funded by the Ministry of Transport Western Transportation Construction Science and Technology Project (2008-318-000-13), the research is conducted at Fuzhou University's School of Civil Engineering. The study uses an existing steel tube concrete arch-bridge as the baseline and performs a trial design with steel web PC box girders to evaluate static and seismic performance improvements.

Core Technical Content and Design Rationale

The fundamental motivation for this research is self-weight reduction. In traditional SRC arch-bridge systems, the PC box girder tie beam contains substantial reinforced concrete webs that contribute significantly to dead load without providing proportional structural benefit. By replacing these concrete webs with steel flat plates or corrugated steel plates, the overall structural weight is reduced, leading to decreased internal forces under gravity loading.

The study compares two steel web configurations: flat steel web plates and corrugated (wave-form) steel web plates. Both configurations maintain the box girder's torsional rigidity and shear capacity while dramatically reducing the dead load. The corrugated steel web provides additional bending stiffness in the vertical direction through its geometric form, while the flat steel web relies on transverse stiffeners for local buckling resistance.

Comparison of Steel Web Configurations

Interpretation of Technical Points

The reduction in dead load produces a cascading effect on structural performance. Lower internal forces under gravity loading translate to reduced prestress requirements in the PC tendons, potentially allowing for fewer or smaller diameter tendons. This in turn reduces the number of anchorages and associated construction complexity. The decreased mass also lowers the fundamental natural frequency of the bridge system, which has implications for seismic design.

The paper reports that the trial design bridge exhibits improved static and seismic performance compared to the original bridge. The lower self-weight reduces the seismic inertial forces (F = ma), which is particularly beneficial for bridges in seismic zones. However, the reduction in natural frequency must be carefully evaluated to ensure that the structure does not approach resonance frequencies associated with traffic loading or wind-induced vibrations.

Effects of Self-Weight Reduction on Structural Response

Integration with Engineering Practice

From a steel pipe and welding fabrication standpoint, this bridge system introduces specific requirements for the steel web plates and their connections to the steel tube concrete arch ribs and diaphragms. The steel web plates are typically fabricated from structural steel grades Q345q or Q370q per GB/T 1591, with thicknesses ranging from 8 mm to 16 mm depending on span and loading conditions.

The welding connections between steel web plates and the arch rib steel tubes are critical structural elements. These connections must transfer both shear forces and bending moments, requiring full-penetration groove welds with complete fusion. According to JTG/T F80/1 and ISO 15614-1, the welding procedure specification (WPS) must be qualified for the specific steel grade, thickness range, and joint configuration. Preheat temperatures of 100-150 degrees Celsius are recommended for Q345q steels with thicknesses exceeding 20 mm to control hydrogen-induced cracking.

Welding Requirements for Steel Web Connections

Key Questions and Reflections

A significant practical concern is the long-term durability of steel web plates exposed to the interior environment of a box girder. Unlike reinforced concrete webs, steel webs are susceptible to corrosion if not properly protected. The design must incorporate a robust corrosion protection system, such as hot-dip galvanizing per ISO 1461, epoxy coating systems, or sacrificial anode protection. The fire resistance of steel webs also requires attention, as steel loses significant strength above 550 degrees Celsius. Fire-rated coating systems or fire-resistant board enclosures may be necessary to maintain structural integrity during fire events.

Another reflection concerns the fatigue performance of steel web connections under repeated traffic loading. The welded joints between steel webs and diaphragms experience cyclic shear stresses that may lead to fatigue cracking over the design life. Fatigue detail categories per JTG/T F80/1-2017 should be carefully evaluated, and detail optimization (such as using through-plates or load-spreading plates) may be necessary for critical connections.

The economic analysis presented in the paper suggests cost savings of 10-20%, but this must be evaluated in the context of the entire bridge lifecycle, including maintenance costs for corrosion protection of steel webs. A proper life-cycle cost analysis would provide a more complete economic picture.

Study Insights and Implications

This research demonstrates that steel web PC box girders are a viable and beneficial alternative to conventional reinforced concrete webs in SRC arch-bridge systems. The self-weight reduction translates directly into improved structural performance, reduced prestress requirements, and lower construction costs. For steel pipe and welding fabrication engineers, the key implication is that this bridge system creates demand for high-quality steel plate fabrication with stringent welding quality requirements. The success of the system depends critically on the integrity of welded connections, making welding procedure qualification, welder certification, and rigorous NDT protocols essential elements of the quality assurance program.