How to Calculate the Load-Bearing Capacity of UHPC Panels Used as Floor Slabs? A Design Method Based on Composite Material Theory and Practical Verification

When UHPC panels are used as floor slabs (or horizontal components such as stair treads and balcony panels), their load-bearing capacity calculation must fully account for the uniqueness of UHPC as a fiber-reinforced cementitious composite material; the design codes for ordinary reinforced concrete cannot simply be applied. Based on extensive experimental data and engineering practice, Qinglong Group adopts calculation models founded on "fiber bridging theory" combined with elasticity or plasticity mechanics methods to ensure safe and reliable design.

The load-bearing capacity of UHPC floor slabs is mainly derived from their excellent tensile and flexural performance. The core of the calculation lies in determining the "tensile stress–crack opening width (σ-w)" relationship curve of UHPC, which must be obtained through standard four-point bending beam tests. Based on this curve, the cracking moment, ultimate moment, and corresponding deflections of UHPC components can be calculated. Typically, UHPC floor slabs are designed as "unreinforced" or "lightly reinforced" components, with steel fibers playing the primary tensile role. Two key points must be considered in the load-bearing capacity calculation: first, the orientation factor and length efficiency factor of the steel fibers, since not all fibers can perform the bridging function most effectively; second, the compression zone height of UHPC is very small, and its constitutive relationship (stress–strain curve) also exhibits a certain degree of plasticity and high toughness under compression.

To illustrate with a practical case: in an ultra-thin stair tread project for an innovative office space, we used 28mm thick UHPC panels with a cantilever length of 1.2 meters. The design load was 5.0 kN/m². We first obtained the flexural toughness index of this UHPC mix through testing, then used nonlinear finite element software for simulation analysis, with the model accurately incorporating the stress–strain relationship and crack opening model obtained from material tests. The calculation results showed that under the most unfavorable load combination, the maximum tensile stress at the bottom of the slab was lower than the ultimate tensile strength of UHPC, and the maximum mid-span deflection was far below the code limit of L/200 (where L is the span). For further verification, we fabricated 1:1 full-scale specimens for destructive load testing; the test results agreed well with the calculated results, with the ultimate load-bearing capacity exceeding 2.5 times the design value, providing ample safety reserve.

Beyond strength and deflection, shear and punching shear verification are equally important for floor slab applications. Due to UHPC's extremely high compressive and tensile strength, its shear capacity is also significantly enhanced. We refer to the French AFGC or Japanese JSCE design guidelines for UHPC, adopting shear formulas that account for fiber contribution in our calculations. In summary, the design of UHPC floor slabs is a rigorous process that runs from material performance testing to theoretical calculation and finally to physical verification, and its high-performance characteristics make thinner, lighter floor slabs with larger spans possible.