Finger Plate Expansion Joint Steel Grade and Surface Treatment Selection

Finger plate expansion joints are fabricated from structural steel and must be protected against corrosion to achieve their design service life. The choice of steel grade and surface treatment system depends on the bridge environment, the required service life, and the maintenance program. Steel grade selection for finger plates follows the same principles as other structural steel components. S355 per EN 10025 or ASTM A572 Grade 50 is the standard choice for most applications, providing adequate strength and weldability. For bridges in cold climates, the sub-grade designation must be specified to ensure adequate impact toughness at the design minimum temperature. High-strength steel grades such as S460 or S690 can be used to reduce the finger plate thickness and weight, which may be advantageous for large-span bridges where the finger plate weight is significant. However, high-strength steels require more careful welding procedures and have lower fatigue resistance at the same stress level, which must be considered in the fatigue design. Hot-dip galvanizing per EN ISO 1461 is the standard surface treatment for finger plate expansion joints in most environments. The zinc coating provides cathodic protection to the steel, preventing corrosion even if the coating is damaged. The minimum coating thickness for structural steel is 85 micrometers, providing a design life of 25-30 years in a C4 (high corrosivity) environment. Thermal spray zinc (TSZ) is an alternative to hot-dip galvanizing that has no size limitations and can be applied to assembled components. TSZ coating thickness is typically 100-200 micrometers, providing a longer design life than HDG. TSZ must be sealed with a penetrating sealer to achieve its full corrosion protection potential. Duplex coating systems combining zinc (HDG or TSZ) with an organic topcoat provide the best long-term corrosion protection. A typical duplex system consists of hot-dip galvanizing plus a two-coat epoxy system, providing a design life of 40-50 years in C5 (very high corrosivity) environments. Duplex systems are recommended for coastal bridges, industrial zone bridges, and other aggressive environments.