Certain aspects do require further consideration due to the nature of weathering steel and these are outlined here. Design considerations Although the corrosion rate of weathering steel is much lower than conventional carbon steel it cannot be totally discounted and allowance for some loss of section over the life of the bridge must be made. The thickness lost depends on the severity of the environment, and is defined for highway bridges in BD7/01 with a corresponding corrosion allowance as follows: Atmospheric Weathering steel Corrosion corrosion environmental allowance classification classification (mm/exposed face) (ISO 9223) C1, C2, C3 Mild 1.0 C4, C5 Severe 1.5 (none) Interior (Box girders) 0.5
• Bridges over roads subjected to de-icing salt to be classed as ‘severe’.
• No allowance required for interior faces of sealed box girders.
• Allowances apply to all fillet and partial penetration butt welds.
• No allowance is normally made to weathering steel HSFG bolts.
• Allowances apply to all structural elements, including stiffeners and bracing etc.
These additional thickness allowances should not be included in the effective sections for stress analysis. However, they may be included in the section properties for global analysis of the structure if so desired. Detailing considerations Formation of the protective rust ‘patina’ of weathering steel only occurs if the steel is subjected to wet /dry cycles. Hence, it is important that the bridge be detailed to ensure such cycles can occur.
Expansion joints Leaking expansion joints allowing salt-laden water to flow over the ends of beams has been identified as the main cause of defects in existing weathering steel bridges. Hence, the issue of joints requires careful consideration. Ideally, expansion joints should be avoided by the use of continuous and integral construction. However, if they are unavoidable they should be located away from the ends of the beams, and a positive non-metallic drainage system should be provided to convey any leaks away from the steelwork. It may also be prudent to locally paint the ends of beams directly beneath such deck joints, the colour of which should be selected to match the anticipated final colour of the mature steel.
The satisfactory performance of weathering steel
Carbon Steel, Corten Steel, Stainless steel
Rust, Bare, Powder Coated, Polished, Black Heat-resisting Paint, Iron Oxide
Diameter : 320mm--1200mm
Thickness : 1.2mm--3.0mm
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