Weathering steel bridges

Weathering steel is a low-alloy structural steel with small additions of elements such as copper, chromium and nickel. In suitable climates it forms a dense, adherent oxide layer (patina) that slows further corrosion, so the steel can be left unpainted. This can reduce the need for coating maintenance over the life of the bridge.

When weathering steel is suitable

The patina needs alternating wet and dry periods. Weathering steel is generally not recommended where the steel stays wet for long periods, is immersed or buried, is exposed to marine salt spray or de-icing salt spray, or is in an atmosphere with high industrial pollution. National guidance documents, such as FHWA Technical Advisory T 5140.22 in the United States and CD 361 of the Design Manual for Roads and Bridges in the United Kingdom, describe these limits in detail. The suitability of a specific site is decided by the engineer of record, where necessary with a site corrosivity assessment.

Material standards by region

Each region specifies weathering steel in its own standards. The table lists common designations for bridge work. Grades from different standards are not automatically equivalent: chemistry, strength, toughness and testing requirements differ, and any substitution must be approved by the engineer of record.

Editions and grade lists change over time. The edition that applies is the one named in the project specification; it is confirmed before material is ordered.

Design and detailing

• Allow for the loss of section during patina formation where national guidance requires a corrosion allowance.
• Avoid details that trap water, debris or leaves; let all surfaces dry out.
• Keep water from deck joints away from the steel, or paint the steel locally near joints and bearings.
• Protect piers and abutments from rust staining while the patina forms, for example with drip details and drainage.
• Seal box sections or make them accessible for inspection.
• For pedestrian bridges, consider the surfaces people touch: railings and handrails are often stainless steel or galvanized.

Fabrication and connections

• Welding consumables are chosen to match the corrosion resistance and colour of the base metal where the weld remains exposed.
• Bolts, nuts and washers are of a matching weathering grade (for example ASTM F3125 Type 3 in North America) or as specified.
• Mill scale, markings, oil and grease are removed, usually by blast cleaning, so the patina forms evenly.
• Material certificates show the chemical composition and mechanical properties of each heat.

Inspection and maintenance

Weathering steel still needs inspection. The patina is checked for colour and texture: a fine, tight patina is expected, while flaking, laminar rust or areas that stay wet indicate a problem that must be corrected, for example by improving drainage or painting locally.

Information we need

• Project location and environment: distance to the sea, use of de-icing salts, humidity, industry nearby.
• The applicable design code and the material standard named in the specification.
• Required grade, toughness class and plate thickness range.
• Any parts that must be painted or galvanized, such as railings or areas near joints.

Final member sizes, details and specifications are determined by the engineer of record for each project. This page describes general practice and does not replace project-specific engineering.