Abstract
Dowel-type fasteners are one of the most used type of connections in timber joints. Its design follows the equations included in the Eurocode 5. The problem with these equations is that they do not adequately contemplate the resistive capacity increase of these joints, when using configurations which provoke the so-called rope effect. This effect appears when using threaded surface dowels instead of flat surface dowels, expansion kits or nut-washer fixings at the end of the dowel. The standards consider this increase through a constant value, which is a poor approximation, because it is clearly variable, depending on the joint displacement and because is much bigger, especially when using nut-washer fixings. It is also very important because of the rope effect trigger interesting mechanisms that avoids fragile failures without warning of the joints. For these reasons, it is essential to know how these configurations work, how they help the joint to resist the external loads and how much is the increase resistance capacity in relationship with the joint displacement. The methods used to address these issues consisted of a campaign of experimental tests using actual size specimens with flat surface dowels, threaded surface dowels and dowels with washer-nut fixings at their ends. The resistance capacity results obtained in all the cases has been compared with the values that will come using the equations in the standards. After the tests the specimens were cut to analyze the timber crushings, their widths, the positions and level of plasticizations suffer in the steel dowels and in the washer-nut fixings and the angle formed in the dowel plastic hinges. With all this information the failure mode suffered by the joints has been identified and compared with the ones that the standards predict. The results for the size materials and types of joints studied shows that the crush width average values go from 20 mm with flat surface dowels, to 24 mm in threaded to 32 mm in threaded with washer-nut fixings. The rope effect force/displacement goes from 100 N/m in threaded surface dowels to 500 N/m in threaded with washer-nut fixings. Finally, the load capacities are on average 290% higher those indicated in the standard. The main conclusion is that the rope effect force should be considered in the standards in more detail as a function of multiple variables, especially the displacement of the joint.
Highlights
Timber structures are formed by pieces joined together in joints, which are of vital importance because they are responsible for transmitting the forces between the members of the structure, so that the whole remains stable [1,2,3]
Consider the effect of using nut-washer fixings at their ends which generates the so-called rope effect
The influence of the rope effect over the joint resistance is dependent on its contribution as a force vector and on the state of plasticization in the dowel, being necessary to know the relationship between displacement and rigidity of the assembly to assess the contribution of the rope effect Fax
Summary
Timber structures are formed by pieces joined together in joints, which are of vital importance because they are responsible for transmitting the forces between the members of the structure, so that the whole remains stable [1,2,3]. The studies with nut-washer fixings to investigate their contribution to the rope effect are more limited and are based on scale joint models, determining the contribution that this effect has in the different failure modes [15] With all this we must start from the basis that its design is based on the so-called performance theory, which was developed in Europe in the 1940s by Johansen [16], and provides equations for the load limit of connections with different mixed configurations, especially for timber-timber and timber-steel connections [17]. The value of Fax,Rk is calculated taking into account the pull out force using Equation (3) on nails whit treaded surfaces and using Equation (4) for nails with flat surfaces [7] Factors such as the threading of the dowel, the surface of the washer in contact with the timber and the possible pre-tensioning that increase the rope effect before the connection is loaded are not contemplated. This research analyzes the stress distributed towards the washer-timber contact zone at the ends of the dowel integrating the rope effect as a vector superposition to the resistant capacity of the connection as a whole [21]
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