Fasteners under fire

• Applicant: Professor Dr.-Ing. Josko Ozbolt
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: from 2013 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 234406712

In the field of modern fastening technology, it is often necessary to effectively introduce high loads into reinforced concrete or steel-concrete composite structures or safely connect structural components together. Such connections are developed and investigated in the framework of fastening technology. For the usual environmental temperature condition the behavior of fasteners has already been studied in detail. However, for elevated temperatures, as they may arise for example in case of fire, the behavior of fasteners changes significantly. The decrease in the mechanical properties of concrete and steel and the thermal expansion with the associated restraint stresses lead to a degradation of load carrying capacity and significantly increases deformation. So far, the behavior of fasteners under fire exposure has not yet been adequately investigated.To better understand the behavior of fasteners at high temperatures under shear load, in the last two years fire tests on single-headed anchors were carried out in the framework of the DFG research project "Fasteners under Fire Exposure", for anchors close and far from edge of concrete slab. The tests were used for the calibration and verification of the existing thermo-mechanical model that was implemented into the 3D FE program MASA. The calibration was rather demanding. Namely, due to the high local thermal expansion together with existing mechanical constraints associated with the mechanical load, unrealistic damage of concrete are often generated. In order to avoid this, special procedures and element types had to be used. After verification of the model a number of numerical studies of single headed studs close and far from the edge were carried out. The numerical results showed good agreement with the experimental investigations and the experiments were correctly interpreted numerically. In the previous studies only the effect of high temperature on single anchors was investigated in detail. For fasteners with multiple anchor bolts in design practice it is normally assumed that the distribution of forces among anchors is the same as for the normal environmental temperature and it is based on the assumption that follows the concrete cone method (CC-method). For that reason in the subsequent application further numerical parameter studies should be carried out with a focus on group fixtures under fire exposure. Fasteners in the edge, in the corner and in the area away from the free edge should be investigated. For various embedment depths and bolt distances from the edge should be investigated whether the general assumptions of the CC method also apply for group of fasteners exposed to fire. A parametric study for group of fasteners under shear, tensile and combined loading will be carried out. The goal is to formulate safe and economic design code for fasteners exposed to elevated temperature that should be based on the CC-method.

• DFG Programme: Research Grants