Fases del Desarrollo Motor

As discussed in Section 2.3.2, the failure behaviour of timber is complex and dependent on grain

orientation and load direction. When dealing with a material prone to brittle failure, such as concrete or timber, it is imperative to provide ductility within the connection and maintain the predisposition of ductile failure modes. As the differential of strength between the orthogonal axes of timber is so large, the connector often has to transfer stresses generated by the stronger longitudinal axis, but must be transferred and withstood by one of the timber’s weaker axes. As a result, timber connections are often realised with multiple points of connection, with relatively large spacing between connection points. The high number of connection points is to reduce the stress of the timber surrounding each connector, and the subsequent risk of local timber failure, by distributing the load across more points. The spacing is to reduce the likelihood of group failure, which when combined with the large number of points needed, often becomes the key determinant of the overall dimensions of structural timber elements. Connection methods of this form include nails, screws, dowels, and glued-in-rods.

5.1.1 Connector Type: Dowels & Screws

Screws (and nails, which also fall into this group) are a long-established connection connecting method for timber construction. They provide fixity through a combination of resistance to withdrawal, tension, bending, and shear.

In timber design it is more important to refer to the behaviour of the joint rather than the connector alone, as it is the combined action that governs how well or poorly a connector works. Depending on the configuration of a connection, a connector may behave or fail in a different manner, and this is especially true for joints formed using screws or dowel type connectors. The spacing and embedment length determine how the connector interacts with the timber around it, and hence influences the stiffness and strength of the joint.

In their very nature, the connector points are locations where stress/load is concentrated to allow transmission across whatever interface. This can be problematic for timber especially compositely, because the magnitudes of loads that one would like to transmit or transfer may be very large. It becomes necessary to distribute the loads across many points of connection to prevent localised failure around the connectors. Conversely, if there are too many connection points and they are too closely spaced, group action can occur, where a failure happens through the surrounding material, taking a

series of connectors at the same time. Whilst there are established rules and guidance covering these phenomena for standard timber connections, this area is more complex for CLT and for creating a shear connection

Asiz & Smith[2] _{investigated the performance of simple screw connectors for joining steel and timber in} multi-storey buildings. Their study was not investigating composite action between the two materials, and hypothesised CLT panels resting on the top flange of a conventional steel beam, but they did find the screw connectors were suitable for use in the system and offered sufficient ductility for hybrid CLT- steel structures to work in seismic regions.

A further option is the HELIX type screw connectors. They are characterised as having a more loosely packed thread than conventional wood screws.

The helical connector was initially developed to create a more effective wall tie for securing masonry to timber frames and other remedial uses. Research performed by Coste[172,173] _{demonstrated using a} physical testing program that under lateral shear, timber connections using helical connectors show greater ductility than conventional screw connectors.

5.1.2 Connector Type: Nail Plate

Nail-plates (also known as “punched metal plate fasteners” or “truss plate connectors” - see Figure 5.1) are described in a technical declaration as “galvanised mild steel plates with rows of integral nails pressed out approximately at right-angles to one face of the plate”[174]_.

Nail-Plates are a timber connector typically used to connect, or reinforce the connections of, timber elements in trusses. Simply described, they are sheets of metal with spikes projecting from one face, with the spikes having been created by partial punching that leaves the nails connected to the parent plate. To ensure rigidity, the nails are shaped during the punching process. Connections are made by hammering the plate teeth into the timber across the join between the two elements – the nails are embedded into both elements whilst the plate itself provides tensile strength across the join. The load

path in nail-plate joints is from one timber element into the teeth, then through the teeth, the plate, and a second set of teeth (having crossed the joint interface) into the second timber element[175]_{. In}

conventional timber frames, the use of nail-plates is limited to the creation of butt-joints.

Nail plates have been investigated for suitability as a shear connector in the past, particularly for timber- concrete composite structures. In attempting to develop a prefabricated timber-concrete composite beam, they were proposed as a connecting device cast into the concrete slab, to be then pressed into an LVL beam to provide slip resistance. In the study of Blass & Schlager[91]_{, single shear tests were}

performed on the joint, finding that the connection typically exhibited a slip stiffness of 50kN/mm per connector and an average maximum load of 48kN per connector. The overall load-slip behaviour was observed as elastic-plastic. Failure was seen to occur via withdrawal of the nails from the timber, with some cases of tensile failure of the nails themselves.

In 2014, with support from the IStructE, testing was performed to determine the potential of Nail-Plate connectors as a shear interface connection, which is detailed in section 5.3. During the same time period, Jacquier & Girhammar were testing double-sided nailplates as a shear connection between CLT floor panels and GluLam beams[176]_.

5.2

Shear Connection - Background