What are the design points of a steel bridge?

2025-03-26 16:18:01

Driven by the strategy of a strong transportation country, steel bridges are reshaping the modern bridge construction pattern by virtue of the advantages of high strength, light weight and fast construction. Its design needs to integrate mechanical properties, manufacturing process, full life cycle management and other elements to form a systematic solution system.

I. Optimized design of structural system

Selection of cross-section form

The torsional stiffness of box section is 5-8 times that of plate girder, which is suitable for large-span bridges. The steel box girder of Hong Kong-Zhuhai-Macao Bridge (HZMB) adopts double-compartment trapezoidal cross-section with a width of 33.1m and a height of 4.5m, and the torsional stiffness coefficient reaches 3.2×10^6 kN-m²/rad, which can successfully cope with the load of typhoon of one in 50 years. The orthotropic anisotropic steel bridge panels are designed with U-rib encryption (300mm spacing) to reduce the wheel load stress to 120MPa.

Structural System Matching

Nanjing Yangtze River Bridge adopts double deck truss combination system, with a main span of 600m, truss height of 12m, and deck width of 40.5m. Through the design of variable truss height (12m in the middle of the span → 18m at the pivot point), the maximum tensile stress is controlled to be less than 235MPa, and the amount of steel used is reduced by 15% compared with that of the traditional scheme.

Second, fatigue and seismic synergistic design

Fatigue-sensitive node processing

Applying the hot spot stress method to optimize weld details, Hutong Yangtze River Bridge improves the slope of the S-N curve of the longitudinal rib butt weld from 3.0 to 5.0, and the design life from 100 years to 120 years. Dampers are set at key nodes so that the stress amplitude under 2 million cyclic loads is ≤69MPa.

Seismic toughness improvement

Adopting BRB anti-buckling bracing system, the displacement angle of the pier top of a sea-crossing bridge is controlled within 1/150 under E2 seismic action. Steel-hybrid combination pier body through the optimization of the arrangement of shear nails (spacing 150mm), interface shear bearing capacity increased to 12MPa.

Third, manufacturing and installation precision control

Digital pre-assembly

The steel box girder of Shenzhen-Zhongshan Corridor applies 3D laser scanning technology to control the segment matching error at ±2mm/30 m. The virtual preassembly guided by BIM model has discovered and corrected 156 structural conflicts in advance, saving rework costs of over 30 million RMB.

Intelligent Welding Process

Robot welding system realizes 99.6% passing rate of butt weld of 20mm thick plate in Hangzhou-Shauningbo Bridge. Laser-MIG composite welding increased the melting depth of U rib fillet weld to 8mm, and fatigue life was increased by 3 times.

Anti-corrosion and fire prevention system integration

Long-term protection system

Humen Second Bridge adopts the system of “epoxy zinc rich primer (80μm) + glass scale intermediate paint (200μm) + fluorocarbon top coat (80μm)”, with supporting electrochemical anticorrosion (protection potential -0.85V), and the design protection cycle reaches 30 years. A dehumidification system is set up inside the box girder, and the humidity is permanently controlled below 45%RH.

Improvement of fire resistance performance

The key supporting components are coated with intumescent fireproofing coating (2-hour fire resistance limit), and the temperature of steel is controlled within 350℃ under fire condition. A cross-river bridge was filled with cementitious composite materials in the steel tower columns, which raised the critical temperature to 650℃.

V. Dynamic performance optimization

Vortex vibration suppression design

Yangsigang Yangtze River Bridge adopts slotted steel box girder + deflector plate combination scheme, which raises the critical wind speed of vibration from 58m/s to 79m/s. The optimization of aerodynamic shape reduces the amplitude of vortex vibration to 1/3 of the normative permissible value.

Axle coupling control

TMD-tuned mass dampers (mass ratio 2%) are set up to control the vertical vibration acceleration within 0.35g when a heavy train passes. A railroad bridge was modeled by integrated rail-bridge modeling to reduce the derailment factor from 0.8 to 0.6.