Johan Sjödin

Contact-free measurements and numerical analyses of the strain distribution in the joint area of steel-to-timber dowel joints

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. ZusammenfassungZwei verschiedene Stabdübelverbindungen unterschiedlicher Grösse wurden auf Zug in Faserrichtung belastet. Die Dehnungsverteilung im Bereich der Verbindung wurde mittels eines berührungslosen Messsystems untersucht und mit den Ergebnissen aus einer numerischen Analyse verglichen. Das Messsystem erwies sich als wertvolle Ergänzung zu herkömmlichen Messtechniken sowie zu den Ergebnissen von Parameterstudien. Die Messungen erwiesen sich als besonders hilfreich zur Untersuchung des stark nicht-linearen Verhaltens von Holz, das für die Charakterisierung von Holzdübelverbindungen grosse Bedeutung hat.Die numerischen Ergebnisse führten zusammen mit den berührungslosen Messungen zu einigen interessanten Erkenntnissen. So wurde zum Beispiel eine ungleichmässige Dehnungsverteilung im Verbindungsbereich festgestellt. Darüber hinaus fand man heraus, dass Schubdehnungen und Zugdehnungen parallel und senkrecht zur Faser konzentriert in den für den Bruch massgebenden Bereichen vorkamen. Generell wurde in Übereinstimmung mit früheren Untersuchungen festgestellt, dass bei den grösseren Verbindungen ein Sprödbruch auftrat.

An experimental study of the effects of moisture variations and gradients in the joint area in steel-timber dowel joints

Abstract This experimental study examines the influence of moisture variations on the load-bearing capacity of steel-timber 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. Moisture-induced 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.

A numerical study of the effects of stresses induced by moisture gradients in steel-timber dowel joints

This experimental study examines the influence of moisture variations on the load-bearing capacity of steel-timber 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. Moisture-induced 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. (Less)