In this study, two numerical methods are used to predict the load-bearing capacity of multiple steel-timber dowel joints loaded parallel to the grain. The aim was to show the possibility and the advantages of using numerical methods when designing multiple dowel-type joints.

The results showed a good correlation to experimental results taken from the literature and to traditional joint calculations as given in Eurocode 5 (2004). The numerical methods are capable of predicting alternative dowel patterns that can be of great value, not only from a structural point of view, but also for aesthetical reasons. In addition, the numerical methods presented in this study can also be efficient tools for the structural engineer when considering complex loading situations in joints, i.e. in cases involving other loading situations than pure tension parallel to the grain.

The effect of friction between the dowel and the surrounding timber was studied for single dowel-type joints. The joints tested were divided into two groups of joints, where the surface of the dowels differed. For the first group, the dowels had a smooth surface and for the second group the dowels had a rough surface. A contactfree measurement technique was used in the experimental investigation. In addition to that, numerical simulations were carried out aimed at predicting the load-bearing capacity of the joints tested as well as estimating the coefficient of friction between the dowel and the surrounding timber.

Important conclusions from this study, which are supported by previous research, are that the load-bearing capacity of single dowel-type joints increases when the surface roughness of the dowel increases. A very small scatter in the results, in terms of the load-displacement behavior, was seen in the tests with rough surface dowels. For the joints with smooth surface dowels, the elastic response as well as the plastic response varied considerably between different tests. The current version of the European timber code EC5 does not explicitly take into account the effect of friction. In order to take the effect into account embedding tests have to be performed in order to be able to consider the benefits of using dowels with rough surface.

This experimental study examines the influence of moisture variations on the load-bearing capacity of steeltimber dowel joints. The glulam specimens used to manufacture the joints were first exposed to controlled climate changes. After being stored in the climate chambers, holes were drilled and dowels were inserted. Then, the joints were loaded to failure. The main aim of this study was to investigate how and to what extent moisture variations and gradients in the joint area affects the load-bearing capacity in tension parallel to the grain. The load-bearing capacity was found to be reduced under these conditions when compared to reference joints. Moreover, the brittleness of the joints increased with the time the joints had been exposed to drying. Moistureinduced stresses and cracks in the joint area were found to be a possible explanation of the results. A similar interpretation was given in other studies where other structural timber elements have been studied. This raises an important question of how such moisture effects should be considered in design codes. One possibility is to include the influence of moisture-induced stresses in the k_mod factor used in Eurocode 5. Another way could be to consider this type of moisture effect as an equivalent mechanical load case.

The paper gives an overview of ongoing research and research performed during recent years at Växjö University, Sweden, within a PhD study. Results from various experimental and numerical studies relating to the behaviour of dowel-type joints are presented.

The results are discussed in relation to the expected behaviour of the joints according to the current version of EC5. Possible implications of the results on future versions of EC5 will be discussed.

In addition to the numerical and experimental results, also the experimental methods used are discussed in some detail. Here, contact-free deformation measurements in combination with finite element analyses, has proven to be an effective mean to study the mechanical behaviour in detail, at a high spatial resolution (complete deformation- and strain fields).

Two different dowel-type joints of differing size were loaded in tension parallel to the grain. The strain distribution in the joint area was studied using a contact-free measurement system. The results were compared with those of numerical analysis. The measurement system was found to be a valuable complement to traditional measurement techniques and also to numerical analyses performed in parametric studies. The measurements obtained were shown to be particularly useful for studying the highly nonlinear behavior of timber that is of special importance in characterizing timber dowel joints.

Several observations of considerable interest were made on the basis of numerical results in combination with results of the contact-free measurement system. A non-uniform strain distribution in the joint area was detected, for example, and shear strains and tensile strains, both parallel and perpendicular to the grain were found to be concentrated in the areas most likely to influence the failure mode of the joint. A general observation made was that the larger size joint failed in brittle manner, an observation supported by previous research.

Joints are critical parts of timber structures, transmitting static and dynamic forces between structural members. The ultimate behavior of a loaded building depends strongly on the structural configuration and the capacity of the joints. The collapse of a whole building or less extensive accidents that may occur is usually starting as a local failure inside or in the vicinity of a joint. Such serious failures have recently occurred in our Nordic countries. Especially the collapse of two large glued laminated timber structures clearly indicates the need of an improved joint design. The trend toward larger and more complex structures even further increases the importance of a safer design of the joints.

An aim of this partly experimental and partly numerically based thesis has been to investigate if steel-to-timber dowel joints are affected by moisture-induced stresses. The experimental results showed that the load-bearing capacity of the joints is reduced by such a moisture influence. Most of the decrease in load-bearing capacity observed was found in joints initially exposed to restrained shrinkage deformations caused by the presence of dowel fasteners in the joint area. The load-bearing capacity was, however, also found to decrease in joints exposed to an initial decrease in moisture without any fasteners present in the specimens during storage before loading. An explanation of this unexpected behavior is that moisture gradients cause tensile stresses. It is shown by numerical simulations that the moisture-induced stresses are so large that they may have a considerable influence on the joint behavior.

Use of contact-free measurement methods, used in some of the experimental tests, was in many ways found to be superior to traditional measurement techniques, but was also found to be a valuable complement to the numerical analysis performed. From numerical results obtained in combination with results from contact-free measurements several observations of considerable interest were made. For dowel-type joints loaded in tension parallel to the grain a strongly non-uniform strain distribution was found in the joint area. It was further observed that the shear and tensile strains were concentrated close to the fasteners in the joint area. These concentrations will influence the failure mode of the joint. A general observation was that the larger sized joints failed in a brittle manner.

Keywords: constraint stresses, contact-free measurement, dowel-type joints, humidity variations, moisture-induced deformations, timber structures