Metallurgical Analysis of Failed Bridge Structural Steel Members in North West Nigeria: A Case Study Approach

This study presents metallurgical failure analyses of three selected failed structural steel bridge members from highway bridges in Katsina and Sokoto States, North West Nigeria. Bridge infrastructure failures in northern Nigeria have been attributed to diverse factors including material deficiencies, construction quality, overloading, and aggressive environmental exposure, yet formal metallurgical investigation of failed bridge components is rarely undertaken systematically. Failed member sections are obtained from bridge maintenance contractors and subjected to visual examination, dimensional survey, chemical composition analysis by OES, microstructural examination by optical microscopy and SEM, hardness mapping, and fracture surface analysis by SEM fractography. Finite element analysis of the stress distribution in each member at service load is performed to contextualise failure location. Findings indicate that two of three failed members exhibit fatigue crack propagation from surface weld defects, with crack fronts showing characteristic beach marks and final overload fracture zones. One member shows evidence of intergranular stress corrosion cracking attributed to sensitisation of the steel weld HAZ combined with moisture exposure from inadequate expansion joint drainage. Material composition in all specimens meets BS EN 10025 Grade S275 requirements. The study attributes failures primarily to weld quality deficiencies rather than substandard material supply and recommends post-weld inspection protocols for bridge fabrication in Nigeria. Keywords: failure analysis, structural steel, bridge, fatigue fracture, North West Nigeria.