In order to study the flexural performance of UHPC-partially filled composite beam with narrow steel box girder under hogging moment,six test specimens were fabricated to explore the influence of reinforcement ratio,UHPC steel fiber content and flange thickness on sectional strain distribution,failure mode and ultimate bearing capacity of composite beams.The experimental results showed that the cracking load of UHPC flange with 3%steel fiber content was 63.1%,29.8%,7.7%higher than that with 0,1%,2%steel fiber content,respectively.It shows that the addition of steel fiber can control the development of cracks,improve the plasticity of flange,and effectively improve the brittleness phenomenon.The bending capacity was increased by 5.3%with reinforcement ratio of flange increased by 1%.When UHPC was used to replace the ordinary concrete with 1/2 thickness of flange,the cracking ability was increased by 56.2%and the ultimate load increased by 7%.A formula for calculating the flexural capacity of composite beams was proposed based on the simplified plasticity theory.The calculated values are in good agreement with the experimental ones.
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