When Is Peak-Load Agreement Enough? Element Formulation and Tension Input in Finite Element Models of UHPC Deep Beams

Finite element models of ultra-high-performance concrete (UHPC) deep beams are usually judged by peak load, yet a correct capacity can come with a wrong mechanism. Our earlier shell-element study of these beams reported 104–107% of the measured capacity for normal- and high-strength concrete (NSC, HSC) and 63% for UHPC, ratios that would be 0.32–0.54 if its predictions are applied loads, with flexural failure where the tests failed in shear and a code-derived UHPC tensile strength 38% below the measured value. A 2 × 2 design—layered shell versus solid elements, each with a code-derived and a measured tensile strength—separates the two causes in four beams. The shell model failed in flexure and over-predicted capacity 1.25–2.86 times; its element effect (with elastic transverse shear), −0.61 to −2.06, far exceeded the tension effect, −0.05 to +0.09. The measured tensile strength raised the solid UHPC predictions by 11–25%, yet left them at 0.72–0.73, consistent with missing fibre bridging but not separable from the mesh dependence of the solid model. The solid model came within 5–21% of the capacity of the beams without fibres, yet failed by diagonal cracking where the tests failed in diagonal compression. Peak-load agreement does not validate a deep-beam model.