Stiffness analysis of partially-filled narrow steel box-UHPC composite beams under negative bending moment

ObjectivesThis study investigates the flexural performance of partially-filled narrow steel box-UHPC （ultra-high performance concrete） composite beams under negative bending moment， static load tests were conducted on nine composite beams with varying steel fiber contents， reinforcement ratios， and relative filling heights under reverse concentrated force.MethodsThe contributions of different parameters to the enhancement of the beams’ mechanical properties were quantitatively assessed. Additionally， theoretical deflection calculations， which take into account interface slip and flange cracking， were compared and validated against experimental results.ResultsThe stiffness of the partially-filled narrow steel box-UHPC composite beams reached its peak in the overall elastic stage. After flange cracking， the stiffness slightly decreased. When the lower flange of the steel box locally yielded under compression， multiple through-cracks formed at mid-span. At the ultimate load stage， there was no significant reduction in bearing capacity. Compared with partially-filled narrow steel box-Normal Concrete （NC） composite beams and narrow steel box-UHPC composite beams， as the parameters increased， the ultimate bearing capacity， stiffness at the serviceability limit state， and stiffness at the ultimate limit state of the partially-filled narrow steel box-UHPC composite beam increased by up to 57%， 25%， and 21%， respectively. The loads corresponding to crack widths of 0.05 mm， 0.1 mm， and 0.2 mm increased by up to 155%， 266%， and 100%， respectively.ConclusionsThe partially-filled narrow steel box-UHPC composite beam exhibits high stiffness， excellent crack resistance， and superior bearing capacity. The current codes for calculating the deflection of composite beams under negative moments are overly conservative. In contrast， the effective moment of inertia method， which considers the influence of interface slip in composite beams under negative moments， provides more accurate calculations of the deflection of the test beams.