When openings are made in precast hollow-core slab floors, the cut elements lose their original supports and must therefore be supported by a header. There are various ways to achieve this, and one of them is by using a steel header beam (cradle). Traditional industry practice for designing these header beams often relies on the simplified assumption that the load from the concrete element is uniformly distributed across the lower flange of the beam. However, this assumption leads to an overestimation of the theoretical torsional moments in the mid-section of the header beam, which in turn results in a systematic over-dimensioning of the steel profiles.

The purpose of this study was to investigate how the actual transverse stiffness of the hollow-core slabs affects the load distribution on the lower flange of a simple header beam during the assembly phase. To answer this, a comparative study was conducted between two standard cross-sections of hollow-core slabs with different stiffnesses, HD20 and HD40.

Initially, analytical hand calculations were performed to establish a theoretical load distribution model. The calculations accounted for the reduced stiffness of the steel header beam due to local plate buckling (Cross-section class 4) as well as the internal shear deformations of the concrete element (Vierendeel action). The analytical model demonstrated that the stiffness of the concrete forces over 94 % of the load towards the stiff support points of the header beam, creating a loss of contact in the middle. Subsequently, this optimized load model was applied to a three-dimensional shell model in the finite element software FEM-Design, where the results were compared against the traditional uniform load scenario.

The results from the 3D analysis show that the assumption of a uniformly distributed load leads to local torsional deformations in the lower flange, measured at 21.8 mm for HD20 and 45.8 mm for HD40. When the stiffness-proportional load distribution was applied, the maximum torsional deformation was significantly reduced by approximately 66-68 % (to 7.27 mm for HD20 and 15.3 mm for HD40).

The results show that current simplified calculation methods do not fully describe the true behavior of the system. The transverse stiffness of the hollow-core slab acts to unload the weak mid-section of the steel header beam. By considering this interaction in the future, there is great potential to optimize steel consumption and improve the working environment during assembly.