Kurzbeschreibung
(Englisch)
|
Timber is known for its relatively low strength and stiffness perpendicular to grain. This often requires a reinforcement of zones that are exposed to tension or compression loads perpendicular to grain. There are different methods available to reinforce timber. One group of reinforcement methods is based on the use of pin shaped metal fasteners. These fasteners are either drilled-in (rods and screws) or bonded-in the timber. Their behavior under axial tension or pull-out loading and compression or push-in loading is regarded as being similar if buckling has not to be considered. Much more data on tension loaded rods and screws are available compared to data about compression loaded fasteners. Therefore the proposed research project mainly deals with this loading type of the fasteners. Based on research work that has already been published, an extensive series of mechanical test will be carried out in order to derive strength and stiffness data of bonded-in rods BIROD, drilled-in rods DIROD and fully threaded self-tapping screws STS. The analysis of the test data shall lead to the identification of important parameters that significantly influence compression and push-in strength and stiffness of the fasteners. With the help of the analyzed data numerical models will be created in order to describe and simulate the load bearing behavior of BIROD, DIROD and STS. The aim of the project is to provide strength and stiffness data as well as suitable numerical models regarding axial compression loading of pin shaped fasteners.
|
Abstract
(Englisch)
|
Timber is known for its strongly anisotropic behaviour. Where most of its properties paral-lel to the grain can be described as strong and stiff, this is not true for the properties perpen-dicular to the grain. This fact requires often that engineers who design a timber structure must foresee methods for strengthening areas in timber members or (re)design a structure in such way, that loading perpendicular to the grain is being avoided. There are several methods for strengthening timber for loadings perpendicular to the grain, one of which was being focused on within this research project. Besides being used as connectors in timber constructions, pin shaped fasteners like self-tapping fully threaded screws (STS) and threaded rods that are bonded-in (BIROD) or drilled-in (DIROD) timber are being used to strengthen timber. The application of such procedures leads to an enhancement of certain material properties of timber members, e.g. regarding the load bearing capacity for compression and tension perpendicular to the grain and shear loads. While BIRODS are being used in practice since about three decades, STS and DIRODS with steadily increasing diameters and lengths are newer to the market with about 15 years and about 5 years in service respectively. On all three techniques more or less intensive research work has been executed which lead to improved products with documented properties. Despite this, all three products are not covered by relevant European standards like Eurocode 5 and only a few national standards in Europe deal (partly) with these products, e.g. the National Annexes of DIN EN 1995-1-1 in Germany. Due to the specific properties of the relevant products made by different manufacturers the products often carry a national and/or European Technical Approval for the scope of their application. Although these specific approvals allow for designing structures that contain such products, a general or specific coverage of these very efficient and popular connectors in the relevant European and/or national standards is desirable. There are attempts to widen the coverage of BIROD, DIROD and STS by standards, e.g. a working group of CEN/TC 250/SC 5 “Eurocode 5: Design of timber structures” is working currently on the implementation of BIROD in EC 5. Within this project both, experimental and simulation works were carried out in order to assess the performance of fully threaded self-tapping screws when used for strengthening timber in compression perpendicular to the grain. The experiments were carried out on base of the results from previous works documented in literature. Several screw types and sizes as well as loading conditions were used to determine their respective influence on the load carrying behaviour. The screws were drilled in glulam made out of spruce with different densities and axially loaded in compression. It was found that the screws significantly increase the load carrying capacity of the timber in perpendicular to grain direction. For all screws buckling was the predominant failure mode. Another aspect that has been assessed was the influence of measuring the torsional moment while drilling – in the screws on the prediction of the load carrying capacity of the screws. It could be found that the recorded torque correlated well with the present density of the wood and like this is also usable for the prediction of the push-in/pull out strength of screws, even though the latter has not been verified within this project. To evaluate the effect of reinforcement and the influence of different parameters a 3D finite element model was generated that simulates the load transfer and the load carrying capacity of the reinforced timber element. The simulation of the reinforced timber construction was done using the finite element program ANSYS®. The model describes the behaviour of the screw, the timber and the interaction between the two. For the simulation of the screw and the timber cross-section, 3D elements are being used. The models provide a good description of the reinforcement effect by fully threated screws. Nevertheless, as the differences between the finite-element models and reality are still too big, further adaptation of material parameters need to be made. Apart from these changes, further tests needs to be done in order to use the models as an extension of tests.
|
