Structural Timber Connections with Dowel-Type Fasteners and Nut-Washer Fixings: Mechanical Characterization and Contribution to the Rope Effect

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.

Parametric Study of Three Types of Timber Connections with Metal Fasteners Using Eurocode 5

“…But as the dowel diameter increases, the hole connection also increases, which weakens the resistance of the cross-section and allows different failure modes to occur, as reported by [12].…”

“…These connections normally support a structural member and are connected to another member by a combination of dowel and bearing action [1,11]. Chybinski [12] presented an experimental investigation to study the load carrying capacity, stiffness, load-slip, failure modes and ductility of aluminum-timber screwed connections.…”

Load-Carrying Capacity of Double-Shear Bolted Connections with Slotted-In Steel Plates in Squared and Round Timber Based on the Experimental Testing, European Yield Model, and Linear Elastic Fracture Mechanics

“…The slenderness ratio between the thickness of the embedded timber element and the diameter of the fastener t 1 /d has a huge influence on the failure mode (see Figure 3). The second term on the right-hand side in the above expressions represents the contribution from the rope effect [42], which should be limited to 25% of the Johansen part for bolts. If the contribution from the rope effect is not known, then it should be taken as zero.…”

“…For this reason, the bolts were also tightened by hand, using a spanner without a specific tightening force, to eliminate the rope effect [42]. The tensile load was generated by electrohydraulic cylinders with a capacity of 1200 kN.…”

“…The aim was not to simulate ideal conditions, but to verify the behavior of connection under realistic conditions. For this reason, the bolts were also tightened by hand, using a spanner withou specific tightening force, to eliminate the rope effect [42]. The tensile load was genera by electrohydraulic cylinders with a capacity of 1200 kN.…”