Effects of thermal strain on compressed reinforced concrete members considering the load bearing characteristics of the entire building structure

• Applicants: Dr.-Ing. Sascha Hothan; Professor Dr.-Ing. Jochen Zehfuß
• Subject Area: Structural Engineering, Building Informatics and Construction Operation ; Construction Material Sciences, Chemistry, Building Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 510524699

Thermal constraint in structural components exposed to fire can occur as internal as well as external constraint. While internal constraint results from temperature gradients in the cross-section, external constraint arises from hindered thermal expansion of the member and is mainly caused by the interaction of members exposed to fire with the surrounding (cold) structure. In case of fire, constraint can have both positive or negative effects on the fire resistance of structural members. In general, the interaction and the resulting constraint stresses between the fire exposed member and the surrounding structure are not considered in fire tests. Nevertheless, in reality thermal constraint occurs and its effects are examined in this research project. The column furnace at the Bundesanstalt für Materialforschung und -prüfung (BAM) allows the implementation of rigid as well as partially flexible boundary conditions that represent the supporting structure. For appropriate fire tests of real-scale reinforced concrete columns servo-hydraulic actuators are used. In this way, loads due to external constraint can be applied during the fire test depending on the deformation of the column and the entire structure. As a result, the member stresses can be applied more realistic than in standard fire tests. In addition to the experimental investigations, the fire behaviour of the reinforced concrete columns is simulated using finite element method. The numerical model of the columns is calibrated by means of the measured thermal and mechanical data gathered during the fire tests. To simulate a realistic concrete behaviour under constraint conditions thermomechanical material parameters are examined by means of cylinder tests at the Institute of Building Materials, Concrete Construction and Fire Safety at TU Braunschweig. These material parameters are utilized to develop material models to be implemented in the numerical simulations of the column fire tests. In this project numerous parameters influencing the behaviour of constrained members exposed to fire are investigated using fire tests and numerical simulations. These include variations of the eccentricity of the load, the stiffness of the column and surrounding structure as well as the load utilization. Based on the fire tests results and the validated numerical simulations, recommendations for fire tests are formulated regarding the constraint stresses resulting from the surrounding structure.

• DFG Programme: Research Grants