Numerical Simulations of the Loading Process of Dowel-Type Timber Connections

Numerical simulations of the loading process of dowel-type timber connections

M. Dorn

, K. de Borst

, T. K. Bader

, and J. Eberhardsteiner

†

Vienna University of Technology, Institute for Mechanics of Materials and Structures Karlsplatz 13/e202, 1040 Vienna, Austria

michael.dorn@tuwien.ac.at

‡

University of Glasgow, School of Engineering Rankine Building, Glasgow G12 8QQ, Scotland

karin.deborst@glasgow.ac.uk

Key words: timber engineering, dowel-type connection, ductile and brittle failure modes ABSTRACT

In order to maximize utilization, numerical simulations are gaining importance in the design of timber structures. Dowel-type steel-to-timber connections are commonly used for a wide variety of loads.

The current design generation, namely the Eurocode (EC) 5 [1], is based on empirical tests, and lacks in many aspects a stringent mechanical foundation. The optimization of connections plays an important role for the competitiveness of timber structures, which is often restricted due to the inability of the design concepts to capture the large variety of fields of applications.

The mechanical behavior of these types of connections is mainly driven by stiffness differences between the steel dowel and the wood part, whereby the strength properties of the materials and the geometric layout of the connection considerable affect the connection’s stiffness and strength. In Figure 1, the typically observed load-displacement behavior is presented, and characteristic load stages are marked.

Figure 1: Typical geometry of a connection (left) with characteristic load-bearing behaviour (right) [3]

A numerical simulation tool is presented [2], which aims at the realistic representation of the

mechanical behavior of dowel-type timber connections at all load stages up to maximum load. In

order to model the different stages properly, a variety of features is included:

Contact interface: The interface between dowel and wood is modeled by a compliant, non- linear pressure-overclosure relation. The contact behavior is derived from experimental investigations and can handle non-reversible, plastic deformations, which occur because of the rough surface of the bore-hole. Due to the plastic deformations, hole-clearance may become significant when connections are unloaded and reloaded repeatedly.

Wood material: Wood is modeled with linear-elastic – perfect-plastic material behavior, which is assumed to be orthotropic in the principal material directions L, R, and T. Strength of the material is defined by a quadratic Tsai-Wu failure criterion, whereby perfect plasticity is applied once the stress state has surpassed the elastic domain. Although plasticity is not an appropriate failure mode for wood in tension and shear, the used criterion allows to identify critical states in multi-axial straining and gives indications of the behavior after the failure load has been reached.

Dowel: The dowel is typically made of construction steel S235 or S355, therefore a linear- elastic – perfect-plastic material model with von Mises-plasticity is used.

The current tool is able to reproduce the realistic connection behavior up to the serviceability limit state (Figure 2), which is verified by a comparison of simulation results with corresponding experimental data [3] for a large variety of connection characteristics, such as different wood qualities and densities, connection geometries, and dowel roughnesses [2]. Further improvements of the simulation model will aim at extending the predictive capability of the model to ultimate loads and assessing the length of the yield plateau in terms of displacements before final brittle failure occurs.

The field of application of the simulation model is on the one hand the design of dowel-type timber connections and the study of their behavior without the necessity for experiments. On the other hand, it constitutes a basis for the derivation of design guidelines in order to provide improved design rules for engineers.

Figure 2: Comparison of numerical simulations with experimental results [2]

References

[1] Eurocode 1995-1-1: Eurocode 5: Design of timber structures – Part 1-1: General – Common rules and rules for buildings (12 2010).

[2] Dorn, M.: Investigations on the serviceability limit state of dowel-type timber connections, PhD- Thesis, Vienna University of Technology, 2012.

[3] Dorn, M., De Borst, K., Eberhardsteiner, J.: Experiments on dowel-type timber connections,

Computational Mechanics, submitted for publication (2012).