UHPC-infilled hybrid top chords for precast truss deck plate systems: Composite-stage flexural performance

Precast truss deck plate systems have been widely used in floor construction for their ability to reduce formwork demand and accelerate construction. However, fabrication and welding constraints limit the chord diameter, restricting the applicable span up to approximately 4 m. To address this issue, this study investigates the composite-stage flexural performance of a precast truss deck plate system with UHPC-infilled hybrid top chords. The hybrid chord combines a lipped C-section steel with UHPC infill to enhance the compressive capacity of the top-chord region and improve the sectional flexural response. Six full-scale composite-stage specimens were tested under four-point bending, and nonlinear finite element analyses were conducted to clarify the load-resisting mechanism. Test results showed flexure-dominated behavior in all specimens, with the UHPC-infilled specimens achieving peak loads comparable to or slightly higher than that of the reference specimen. Strain distributions and numerical results indicated that the UHPC infill shifted the neutral axis upward by increasing the top-chord compressive capacity. A parametric study on additional tensile reinforcement further confirmed the structural advantage of the proposed system at the composite stage. These findings suggest that UHPC-infilled hybrid top chords offer an effective solution for improving the composite-stage flexural performance of precast truss deck plate systems and extending their applicable span beyond the current limit.