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4.2.1 Dowel action

The fastener is mounted in an angle to the force direction, most often perpendicular to the same. When loaded, the dowel will press against the surrounding timber members, creating embedding pressure against the dowel, see Figure 4.6.

Figure 4.6: Stocky and slender dowel.

The dowel will act as a beam with a distributed load from the embedding pressure. If the dowel is stocky it will not bend, but if it is more slender it will deform in bending ultimately creating one or more plastic hinges in the dowel. Once deformed, the shear action can be complemented with a tensile action in the dowel. The tensile action can be increased by using bolts with a head and/or a nut to achieve anchorage, by mounting screws in an angle to the force direction to carry loads on the threads or by using screws or other dowels with a rough surface to increase withdrawal strength.

4.2.2 Material parameters

The load carrying capacity of a dowel-type connection in shear is determined by three parameters; the embedding strength of the timber fh, the dowel strength represented by the yield moment My and the anchorage capacity enabling tensile action in the dowel Fax.

The embedding strength itself is affected by several parameters:

• The density of timber: a higher density gives a higher embedding strength

• The fastener diameter d: small diameters yields higher embedding strength than larger diameters. The hole can be allowed to exceed the fastener diameter by 2 mm without any effect on the embedding strength, Blass (2003).

• The angle α between grain and load direction: the highest embedding strength is obtained in compression parallel to the grain and the lowest perpendicular to the grain. • The friction between dowel and timber: dowels with a rough surface obtain higher

embedding strength than those with a smooth surface.

• The moisture content in wood: as for all other strength properties, the moisture content affects the embedding strength negatively if it is high and positively if it is low • Any reinforcement of timber in tension perpendicular to the grain: the failure in

embedding is initiated by a crack along the grain caused by the dowel expanding the hole when pressing against the wood. It is thus the strength in tension perpendicular to the grain that limits the embedding strength. Any reinforcement in tension

perpendicular to the grain will therefore increase the embedding strength.

• If the hole is pre-drilled or not: if the hole is pre-drilled, most of the load parallel to the grain will be carried through compression parallel to the grain, while holes that are

Figure 4.7: Embedding strength. a) Test set-up, b) typical test result.

Fmax 0 u F Displacement Fo rc e F a) b) 4.2.2.1 Embedding strength

The embedding strength is the pressure the wood around the dowel can sustain. It is determined through a test made according to SS-EN 383, see Figure 4.7 a), where a stocky dowel is pressed against a wooden hole. The embedding strength is determined as the maximum force applied divided by the projected area of the dowel, A = d × t and is measured in MPa. A typical test result is shown in Figure 4.7 b).

Bent to an angle of 45˚

Empirical expressions for the determination of characteristic embedding strength for softwood capture characteristic density and the fastener diameter:

where fh,0,k is the characteristic embedding strength under loading parallel to the grain. Note that the density ρk should be inserted in kg/m3 _{and the diameter d in mm in the Equations}

4.1 – 4.2.

For nails the embedding strength does not vary to any larger extent with the angle between load and grain direction. For bolts the effect is larger and a reduction is made using

Hankinson’s formula if loading takes place at an angle α to the grain:

where fh,α,k is the embedding strength under load directed in an angle α to the grain direction and d is the diameter of the fastener in mm.

k90 is here given for softwood. The embedding strength fh,0,k is calculated according to

Equation 4.2. For other materials than softwood, such as particleboard, plywood and fibre- board, refer to Volume 2, Chapter 10, alternatively to Eurocode 5, Sections 8.3 and 8.5.

4.2.2.2 Yield moment

The yield moment is referred to as the plastic moment in steel design. It is the moment needed to produce a plastic hinge in a dowel and is determined through a test for nails according to SS-EN 408, see Figure 4.9, or by calculating the plastic moment for larger dowel diameters.

Eq. 4.1 Eq. 4.2

Eq. 4.3 Figure 4.8: Load situation in a pre-drilled and not pre-drilled hole. The fibres are cut off upon predrilling while they are bent when the hole is not pre-drilled.

without pre-drilling, that is d < 8 mm with pre-drilling, all diameters

t1 t2

t1 t2 t1

a (I) b (I) c (I) d (II) e (II) f (III)

The characteristic value of the yield moment My,Rk of a dowel-type fastener is dependent on the fastener diameter d in mm and the material of the dowel represented by fu the ultimate steel strength in N/mm2_{. Empirical expressions for the determination of yield moment are}

given for nails and larger diameter dowels:

For all other dowel-type fasteners testing must be performed according to SS-EN 14592, to determine My,Rk, alternatively acquired from the supplier.