Point Loads on Sandwichelements Determination of the Effective Width and Cross-Section Activation

• Applicant: Professor Dr.-Ing. Bernd Naujoks
• Subject Area: Structural Engineering, Building Informatics and Construction Operation ; Applied Mechanics, Statics and Dynamics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 534883913

In principle, many millions of square meters are currently available for the installation of photovoltaics on top of large halls with sandwich roof elements. Thus, the State of Baden-Württemberg was first to pass legislation requiring the mandatory installation of such systems on non-residential buildings as early as mid-2022 - other German States are currently discussing such measures as well.However, the main obstacle to a simple and widespread installation, which requires the corresponding static verifications, is a lack of a generally valid mechanical model for the distribution of the point loads of these systems on the sandwich elements (especially the influence of local stresses and deformations on global failure of the thin-walled face layer due to wrinkling). Here the cross-sectional activation under point loads, i.e. the stress distribution within the sandwich panel, has not been sufficiently clarified on a theoretical level. The individual solutions implemented so far in new buildings are regulated by building authority approvals and are therefore not generally transferable. In the existing building inventory currently approval in individual cases is mandatory - and associated with complex test series as well as time-consuming and cost-intensive expert opinions.The final objective of the project is to demonstrate that a design based on the principle of the effective width captures the complex stability failure with sufficient accuracy. For this purpose, extensive test series will be conducted: using tried-and-tested measurement technology, but for the first time specially adapted to this problem, the stress distribution and deformation of two-span sandwich panels under point-loads will be clearly recorded using a large number of strain gauges and displacement transducers. With this database as a fundament, validation of numerical models is possible. Based on this, the validated numerical models can be used to determine the stress or distortion states of the sandwich panel which can only be inadequately measured (e.g. shear stresses within the core material). In addition to the tests, the numerical models will be used to perform a wide-spread parameter-study to cover the amount of possible configurations.This knowledge gain allows, for the first time, the verification of derived theoretical models for stress distribution or cross-section activation. Ultimately, the transfer of the complex stability problem of elastically bedded thin-walled metallic cover layers with local stress peaks into a simple and firm design approach based on the effective width is possible.This will enable the generally valid design of sandwich elements even on existing buildings - and thus opens the way for the use of large roof areas by photovoltaics. The research project thus indirectly will offer a useful contribution to the German and European "Energiewende".

• DFG Programme: Research Grants