A MODO DE CONCLUSIONES

As previously mentioned, the successful performance of mechanical connections is due to high ductility, lack of sensitivity to cyclic loads and their ability to dissipate energy.

In order to avoid splitting of wood and brittle failures, EC5 states the minimum end and side and spacing distances of the fasteners and these should be obeyed. They are given to ensure that the connection failure is not brittle.

Splitting may also be prevented by using reinforcing materials in the connection areas, which give higher tension strength in the direction perpendicular to the grain of the wood. Such reinforcing materials are, for example, plywood panels and densified veneers. In addition to preventing the wood from splitting, the reinforcement ensures the yielding of the fasteners and thus enhancing connection ductility.

To ensure the dissipation of energy, it is possible to take advantage of the slenderness of the fastener. The slenderness is defined as the ratio between the wood member thickness and the fastener diameter. Fasteners with high slenderness ratios dissipate more energy since the plastic yield points are, in this case, always formed in the fastener. Fasteners with low slenderness ratios perform more elastically and do not dissipate as much energy. In addition, the wood splitting may be prevented by increasing the member thickness in comparison to the fastener diameter.

To avoid an unacceptable strength loss in cyclic loading, three general principles should be followed. Details should be designed so that the elements cannot easily pull out, brittle material failures should be avoided and materials should be used which retain their mechanical properties during cyclic loading.

Dowel-type fasteners

Nails, staples and screws

Nails, staples and screws are usually made of hardened steel. In spite of this they perform plastically in a mechanical connection when designed appropriately. The nail length should be increased if there is a risk of pull-out. Smooth nails are not recommended for this reason. If the slenderness ratio of the nail is higher than 8, good ductility may be expected.

Fig. 5.8 Typical performance of a nailed connection under cyclic loading (nail slenderness 8.5).

(STEP C17)

A ductile connection between plywood and wood may be ensured if the slenderness of the nails is at least 4d. Experiments show that nailed shear walls with plywood sheathing possess high ductility and a high ability to dissipate energy as may be seen in Fig. 5.9.

Figure 5.9 presents a full-scale seismic experiment on a three-storey timber house, the experimental set-up, the loading scheme and the test results of the lateral displacement of the building top (Yasumura et al., 1988). The building is braced with shear walls with 9-mm plywood panels on the external walls and 12-mm gypsum boards on the internal walls. The result shows that these shear walls dissipate energy well and that the building as a whole has a high ability to deform without losing strength.

Fig. 5.9 Full-scale seismic experiment of a three-storey timber house braced with nailed shear walls (Yasumura et al., 1988).

Dowels

Slender dowels in connections may yield both from the steel and from the wood and these dissipate energy well. If the dowel length in the wood is higher than 8d, the connection performs in a ductile manner. If the connection is made of thick dowels and the fastener spacing complies with EC5, the plastic behaviour is only dependent on the performance of the embedment of the wood. Since the energy dissipation is low in this case, tests are usually recommended to evaluate the ductility property of such a connection.

Bolts

Bolted connections have oversized holes due to the production technique and this may result in an unequal distribution of forces under loading. This may cause the overstressing of certain fasteners and thus cause local splitting of the wood at these fasteners and prevent the redistribution of forces within the connection. For this reason, bolted connections for seismic zones should be precisely constructed and the bolts should have a high slenderness ratio.

Thick bolts (d > 16 mm) are usually not able to deform and do not dissipate energy. For this Test set-up

Lateral

deformation [mm]

Load

reason these should only be used in combination with other fastener types such as toothed ring connectors. (EC8 recommends the diameter of dowel type fasteners to be less than 16 mm).

Split ring and shear plate connectors

These are not recommended for use in energy dissipating parts of structures as they have low plasticity values.

Toothed plate connectors

With good design, toothed plate connectors may have good plastic performance. Spacing and end distances, particularly, should be considered in order to prevent the splitting of wood.

Nailplates

Although the force-deformation curves of nailplate connections typically show some plastic behaviour, it is recommended that prototype tests should be carried out if these connections are intended as energy dissipating connections. This is to avoid potential failure such as brittle metal plate failure and nail pull-out failure.

In Helsinki University of Technology, TKK (1995), experiments were carried out to test nailplates of the 'Träforband T150' type to determine their performance against seismic actions. The tests were done on cyclic loading using tension-compression, shear and bending loading modes. The nail length was 14.3 mm. The conclusion of this study was that these nailplates may be designed as energy dissipating connectors (class D, q=3), provided that the anchorage failure mode is dominant and that the anchorage strength is designed in accordance with a medium-term load duration class.