How to Calculate the Load-Bearing Capacity of UHPC Panels Used as Floor Slabs

When UHPC panels are used as floor slabs (or horizontal members such as stair landings and balcony slabs), their load-bearing capacity calculations must move beyond the design paradigm of ordinary reinforced concrete, fully accounting for UHPC's unique material constitutive relationship of "ultra-high compressive strength, high tensile strength, a remarkable tensile-to-compressive ratio, and fiber-enhanced toughness". The core of the design rests on the principle that "fibers carry the tensile force after cracking, while the plastic load-bearing capacity of the section is verified at the ultimate limit state", with cracks and deflection under the serviceability limit state subject to strict control. In a cantilevered UHPC stair platform project for an innovation showroom in Shenzhen, Qinglong Group validated the safety and accuracy of its calculation method by combining nonlinear finite element analysis with full-scale loading tests.

Material Constitutive Models Are the Foundation of Calculation

The tensile strength of ordinary concrete is negligible, whereas UHPC retains a relatively high residual tensile strength after cracking (a stress-crack width relationship curve). Load-bearing capacity calculations must adopt experimentally validated multilinear or curved constitutive models for UHPC, including the compressive stress-strain curve and the tensile stress-crack width (or strain) curve. These key parameters (such as compressive strength fc, ultimate compressive strain εcu, cracking strength ft, and post-peak softening curves) must be obtained through material property tests. Qinglong conducts material property testing on every engineering batch of UHPC, providing accurate input for design.

Flexural Capacity Calculation of Normal Sections

For UHPC slabs reinforced only with fibers and without steel reinforcement, the flexural capacity calculation is based on "section equilibrium" and "material nonlinearity". Through sectional strain analysis, the depth x of the compression zone is determined, and then the stress diagrams of the compression and tension zones are integrated to calculate the section's moment of resistance Mu. Since the contribution of UHPC's tension zone cannot be ignored, the calculation is more complex than for ordinary concrete, and the layered (strip) method or purpose-built calculation programs are usually employed. For UHPC slabs with steel reinforcement, the synergistic action between the steel bars and the UHPC must be considered, with the tensile constitutive relationship being a superposition of the two.

Shear Capacity and Punching Shear Verification

Owing to its high tensile strength and the bridging effect of its fibers, UHPC possesses outstanding shear capacity. Its shear capacity Vc usually consists of three components: the matrix contribution, the fiber contribution, and the axial force contribution (if any). The calculation formulas are mostly based on experimental regression; national codes have not yet been fully unified, and the softened truss model or fracture-mechanics-based formulas are commonly used. For floor slabs, the punching shear capacity near columns and near points of concentrated loads must also be verified with particular attention to prevent brittle punching failure. The punching cone angle and failure mode of UHPC differ from those of ordinary concrete and require dedicated research.

Serviceability Limit State: Crack and Deflection Control

This is another key focus in UHPC floor slab design. Although UHPC cracks are extremely fine (<0.05mm) and its self-healing capacity is strong, they still need to be controlled. The maximum crack width under the quasi-permanent load combination must be calculated to ensure that durability and appearance requirements are met. Deflection calculations must account for the post-cracking stiffness reduction of UHPC, and its long-term deflection must also factor in the effects of shrinkage and creep. Since UHPC has a high elastic modulus (approximately 45-55GPa), its short-term deflection is usually smaller than that of ordinary concrete slabs under the same load and thickness, but its long-term performance needs careful evaluation.

Therefore, load-bearing capacity calculations for UHPC floor slabs are highly specialized work that must be performed by structural engineers familiar with UHPC material properties. Qinglong Group not only supplies the material but also provides calculation support based on test data and joint design recommendations, ensuring structural safety without fail.