Designing safe and efficient steel highway bridges demands rigorous adherence to national standards, particularly for defining the critical vehicle loads the structure must withstand. China and Australia employ distinct yet sophisticated frameworks for this purpose. This article compares the Chinese (JTG) and Australian (AS 5100) standards for vehicle loading on steel bridges, explores their integration into bridge design, highlights prefabrication approaches, and showcases an Australian example.
I. Design Philosophies and Core Standards
China: JTG Series (Ministry of Transport Standards)
Philosophy: Primarily uses a semi-probabilistic limit state design approach, similar in principle to international norms but calibrated specifically for Chinese traffic patterns, vehicle types, and environmental conditions. Safety is paramount, reflecting the high traffic volumes and diverse vehicle loads on Chinese highways.
Key Standard: JTG D60-2015 “General Specifications for Design of Highway Bridges and Culverts”. This is the cornerstone.
Vehicle Load Models:
Lane Load Model: The primary model for global analysis. Combines a uniformly distributed load (UDL) per lane (e.g., 10.5 kN/m for Design Load Class 1) with a concentrated load (CL) placed for maximum effect (e.g., 360 kN for Class 1). The intensity of UDL and CL decreases with increasing number of design lanes.
Vehicle Load Model: Used for local checks (e.g., deck slabs, pier caps). Consists of standard axle configurations representing heavy vehicles. The key model is often denoted as the “Standard Truck Load” (e.g., total weight 550 kN with specific axle weights and spacing).
Other Loads: Includes dynamic amplification factors, braking/centrifugal forces, crowd loading, wind, temperature, seismic (based on JTG/T B02-01), and accidental loads. Partial safety factors are applied to both loads and material resistances.
Steel Design: Governed by JTG D64-2015 “Specifications for Design of Steel Structures of Highway Bridges”. This standard provides rules for member design (tension, compression, bending, shear), stability (global and local buckling), fatigue assessment based on S-N curves and detail categories, connection design (welded, bolted), and corrosion protection.
Australia: AS 5100 Series (Bridge Design Standards)
Philosophy: Based on limit state design principles aligned with international best practice (similar to Eurocodes). Emphasizes probabilistic methods for determining load and resistance factors to achieve target reliability levels. Strong focus on site-specific conditions and durability in the Australian environment.
Key Standards:
AS 5100.1:2004 (Part 1: Scope and General Principles): Defines limit states (ULS: strength, stability; SLS: serviceability, durability), design working life (typically 100 years), and load combination principles.
AS 5100.2:2017 (Part 2: Design Loads): The critical standard for vehicle loading.
Vehicle Load Models:
SM1600 (Standard Traffic): The primary model for general bridge design. Consists of either:
A 360 kN static concentrated load on a 400×400 mm area (for local effects like deck punching shear).
A notional lane load combining a uniformly distributed load (UDL = 6.0 kN/m²) and a knife-edge load (KEL = 120 kN per lane). The UDL and KEL are applied per notional lane (3.0m wide).
S1600 (Special Vehicle): A moving load model defined by a series of axles representing a heavy vehicle configuration (e.g., 1x80kN steer axle + 3x160kN dual axles spaced at 1.2m intervals). Used for general ULS and fatigue checks. Dynamic amplification is included.
W80 Wheel Load: A single 80 kN wheel load on a 250×250 mm area for specific local checks.
Type T and Type C Traffic: For specific applications like rural/low-volume roads.
Other Loads: Includes dynamic allowance, braking/acceleration, centrifugal forces, collision loads, wind (AS/NZS 1170.2), temperature (AS 5100.2), earthquake (AS 5100.2), and erection loads. Load factors are applied within the combinations defined in AS 5100.1.
Steel Design: Governed by AS 5100.6:2017 (Part 6: Steel and Composite Construction). This comprehensive standard covers :
Material properties and selection (including toughness requirements).
Member design (tension, compression, bending, shear, torsion).
Stability (column buckling, lateral-torsional buckling, plate buckling – utilizing effective width concepts).
Fatigue: Uses a detailed modified Miner’s rule approach based on stress ranges and detail categories (similar to, but with specific Australian calibrations, Eurocode EN 1993-1-9).
Connection Design: Extensive rules for bolted (bearing type and friction grip) and welded connections.
Fabrication, Erection, and Corrosion Protection.
II. Key Differences in Vehicle Loading Approach
Primary Model Focus: China relies heavily on the combined Lane Load (UDL+CL) for global analysis, while Australia uses the moving S1600 axle train as its primary ULS/fatigue model, supplemented by the SM1600 lane load (UDL+KEL) which is simpler for some analyses.
Dynamic Effects: Both include dynamic amplification factors, but the calculation methods and factors differ. AS 5100.2 provides detailed formulas based on span length and surface roughness.
Load Factors and Combinations: The specific values for partial load factors and the rules for combining loads (permanent, traffic, wind, etc.) differ between JTG D60 and AS 5100.1. AS 5100.1 often employs more complex combination rules reflecting probabilistic calibration.
Fatigue Assessment: While both use S-N curves and detail categories, AS 5100.6 employs a specific modified Miner’s rule approach calibrated for Australian traffic spectra and bridge types, which can differ from the method outlined in JTG D64.
Specific Vehicle Types: The detailed axle configurations and weights in the “Vehicle Load Model” (China) and S1600/W80 (Australia) are different, reflecting regional vehicle fleets.
III. Prefabricated Steel Bridges under Australian Standards
Prefabrication (off-site manufacturing of major components) is highly compatible with Australian bridge design and construction, offering significant advantages in speed, quality, safety, and reduced traffic disruption. AS 5100 standards facilitate this:
Standardized Loading (AS 5100.2): Provides clear, consistent load models (S1600, SM1600) essential for designing repetitive, reliable prefabricated modules. Erection loads are explicitly covered.
Robust Connection Design (AS 5100.6): Detailed rules for bolted splices (especially high-strength friction grip bolts) and welded connections ensure the integrity of joints between prefabricated elements, critical for both ultimate strength and fatigue performance. The standard addresses the specific demands of site assembly.
Explicit Erection Considerations: AS 5100.1 and AS 5100.2 include specific provisions for temporary conditions during lifting, transportation, and assembly, which are paramount for handling large prefabricated steel units safely. Designers must verify stability and strength during all erection stages.
Fatigue Durability (AS 5100.6): The rigorous fatigue assessment methods ensure that connections, often potential fatigue hot-spots in prefabricated structures, are designed for the long-term cyclic demands of Australian traffic.
Corrosion Protection (AS 5100.6 & AS/NZS 2312): Standards mandate durable protection systems (painting, galvanizing) that can be optimally applied under controlled factory conditions before shipment.
IV. Example: Guildford Road Bridge Upgrade (Perth, Western Australia)
Project: A major infrastructure project involving the replacement of an existing bridge over the Swan River with a new dual carriageway bridge. Prefabricated steel was a key construction method.
Compliance with AS 5100:
Loading: Designed for heavy Perth traffic using AS 5100.2 models: S1600 axle train for global ULS and fatigue verification, SM1600 lane load (UDL+KEL) for simplified analysis and deck design, and W80 wheel load for local deck slab checks. Dynamic amplification and braking forces were critical inputs.
Steel Design & Prefabrication: The superstructure features large prefabricated steel girders and deck units. AS 5100.6 governed:
Member sizing and stability checks for the girders.
Critical Aspect: Design of high-strength bolted field splices (AS 5100.6 Section 12) connecting the massive prefabricated girder segments. These connections required meticulous design for ULS strength, fatigue resistance (using the modified Miner’s rule and S1600 traffic), and slip resistance during service.
Fatigue assessment of welded details within the factory-fabricated units.
Design for specific erection sequences and temporary supports.
Durability: High-performance paint systems were applied in the factory per AS/NZS 2312 and AS 5100.6 requirements for the marine-influenced environment.
Both Chinese (JTG D60/D64) and Australian (AS 5100 Series) standards provide robust frameworks for designing steel highway bridges, grounded in limit state principles but reflecting distinct regional traffic characteristics and design practices. China emphasizes a combined lane load model, while Australia utilizes a primary moving axle train (S1600) supplemented by a lane load model (SM1600). For steel design, AS 5100.6 offers particularly detailed rules for stability, fatigue (using a modified Miner’s rule), and connections, making it highly conducive to modern construction methods like prefabrication. The Guildford Road Bridge exemplifies how Australian standards are successfully applied to deliver complex, durable, and efficiently constructed prefabricated steel bridges designed to handle demanding vehicle loads safely throughout their design life. Understanding these nuanced differences is crucial for engineers working on projects in either jurisdiction or undertaking comparative studies.
Post time: Jun-27-2025