Fire tests of steel-to-timber connections with high fire resistance

This paper presents the results of twenty-one fire tests on unprotected steel-to-timber connections with dowel-type fasteners and one or two slotted-in steel plates. The experimental results of this type of connections available in existing studies have been mostly limited to a fire resistance duration of 60 min. The tests performed in this study target a significant increase of fire resistance to reach 90 and 120 min. Two configurations of connections with one and two slotted-in steel plates, i.e., two or four shear planes, were tested. The temperatures were measured at different locations in the wood members and along the steel plates. Two load levels for fire tests were determined on the basis of tests performed at ambient temperature. The experimental results show that the specimens proposed for steel-to-timber connections with dowel-type fasteners are suitable for achieving fire resistance of 90 and 120 min.     

Challenges with high aspect ratio nanoimprint

When nanoimprint is not used for lithography purposes (NIL), but for the direct patterning of polymeric layers, high aspect ratio patterns may be of interest for a number of applications. The definition of such patterns in a nanoimprint process deals with two aspects, a successful filling of the high aspect ratio cavities of the stamp used, followed by a successful separation of the high aspect ratio structures defined in the polymeric layer on the substrate, from the stamp. These two aspects are addressed by shedding light to the impact of capillary effects during the filling of high aspect ratio cavities, and to the deformation processes involved in the separation of the stamp from the polymeric structures, where adhesional energies have to be overcome without cohesional failure. Both aspects are discussed in terms of the geometries involved, the stamp geometries as well as the polymeric layer thickness, and correlations with thermally-assisted (T-NIL) and UV-assisted (UV-NIL) processing are deduced. The aspects discussed are typical of a nanoimprint situation with thin polymeric layers on hard substrates.     

Shear tests of glulam at elevated temperatures

Experimental investigations are conducted to characterize the evolution with temperature of the shear strength of glulam wood. To realize the tests, an original specimen with cylindrical shape has been developed and justified by a numerical study. The geometry allows obtaining thermal gradient within material to represent the real combustion of timber members, while keeping constant the temperature of sheared section. The experimental programs consider various parameters such as the presence or absence of moisture and the thermal gradient within the specimen. The experimental results are discussed and analyzed. They show the correlation between the density of the material and the reduction of its strength at high temperatures. The experimental failure loads are used to evaluate the reduction factors for wood strength depending on the temperature. These factors are compared with those given by EN1995‐1‐2 for the advanced calculations methods in fire situation. The comparisons show that the reduction factors given by EN1995‐1‐2 are conservative in comparison with the experimental results. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical investigations on the thermo-mechanical behavior of steel-to-timber joints exposed to fire

A three-dimensional finite element model is developed to predict the thermo-mechanical behavior of steel-to-timber doweled joints in tension parallel to grain exposed to fire. To manage the plastic yielding of the materials, the mechanical model is based on the von Mises criterion for steel and the Hill criterion for timber. In fire, the material characteristics depend on the temperature. Two different meshes are used for the thermal and the thermo-mechanical models. The thermal model is continuous, to take account of the thermal continuity between the joint components. The thermo-mechanical model is discontinuous, to consider the contact evolution between the joint components. The thermal model is used to predict the evolution of the temperature field inside the joint which depend on the gas temperature. It is validated on the basis of measured temperatures during fire tests. The complex transformations in wood during fire are represented by apparent values of thermo-physical characteristics proposed in the bibliography and calibrated on the basis of the experimental measurements. The mechanical model is validated by comparison with the experimental results of joints in normal conditions. The thermo-mechanical model is validated by considering the experimental failure times of some joints. The numerical models showed a good capacity to simulate the behavior of the timber joints in cold and in fire situations. These developed and tested models can be used as a general tool to analyze the behavior of a large variety of joint configurations to constitute a data base that can be used in safe and economic practice of fire engineering of wood joints.