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•Fire tests were conducted to determine failure mechanism of composite bridge girders.•Effect of girder geometry on fire behaviour of composite girders is quantified.•Influence of fire and load scenario on progression of deflection is investigated.•Failure modes of girders with different structural characteristics are discussed.
This paper presents an experimental together with numerical study for investigating failure mechanism of typical steel–concrete composite bridge girders under localized fuel fire exposure. Three scaled bridge girders with different girder geometries were tested under combined effects of fuel (diesel oil and liquefied petroleum gas) fire exposure and structural load. Relevant thermal and structural responses in fire exposed composite bridge girders were measured, and then used to validate a numerical model developed to predict fire behaviour of composite bridge girders. The model is further utilized to perform parametric studies to determine effect of fire and load scenario, web slenderness, and height-span ratio on failure mechanism of fire exposed composite bridge girders. Results from fuel fire tests indicate that mid-span deflection in typical steel–concrete composite bridge girders increased rapidly from initial stage of fuel fire exposure. All these girders were observed to fail by large deflection and significant degradation in flexural capacity. The bridge girder with closed section offered an advantage limiting heating on one side, and thus has superior inherent fire resistance. Further, parametric studies demonstrate that failure state of composite bridge girders shifts from excessive deflection to strength limit with increase of fire severity and load level. The composite bridge girders with larger height-span ratio manifest a more rapid increase in mid-span deflection towards final stage of fuel fire exposure and thereafter could no longer sustain the applied load. When these bridge girders reach failure limit state, mid-span deflections are much smaller than L/20. Shear capacity degrades at a faster pace in composite bridge girders with higher web slenderness, and this can lead to significant web buckling at failure time.