Strength behaviour

Ring and shear-plate connectors are circular in shape and manufactured from aluminium alloy, hot rolled or temper rolled steel strip, hot rolled steel alloy strip, grey cast iron or cast metal, in accordance with the requirements of BS EN 912. They fit into preformed grooves in the timber members that accurately profile the connector and are capable of taking much greater loads than are achievable with toothed-plate connectors.

The strength equations for these types of connectors are given in EC5, 8.9, and are only applicable to connectors with a diameter no larger than 200 mm. With the exception of three type A5 split-ring connectors in BS EN 912 where dc is 216 mm, 236 mm and 260 mm, all of the others in the standard comply with this limit.

Ring connectors are of solid cross-section and where they are formed with a cut across the section they are referred to as split-ring connectors. The split allows the connector to be easier to fit and also to be relatively flexible to accommodate distortion that may arise in the joint after assembly due to changes in moisture content. The design rules in EC5 are the same for the ring and the split-ring forms.

These connectors are only suitable for timber-to-timber connections and are shown in Figure 11.5. They are referenced as type A1 to A6 in BS EN912 and those to which the EC5 strength equations will apply range from 60 mm to a maximum of 200 mm in diameter. The connectors are held in place by bolts and washers, but unlike toothed-plate connectors, the bolts and washers will not contribute to the lateral strength of the connection.

In a connection formed using a ring connector, the load is passed from one member onto the ring by embedment stresses, and after shear transfer to the part of the ring in the second member, passes into that member by embedment stresses. The function of the connector bolt is to ensure that the ring remains fully embedded in the members. An example of this type of connector is shown in Figure 11.5.

Shear-plate connectors are used where there is a requirement for a timber-to-steel (or concrete) connection, and where the joint is to be demountable or where connections are to be formed on site.

In BS EN 912 shear-plate connectors are referenced as type B1 to B6, range in diameter from 65 to 190 mm, and are held in place by bolts and washers. With these

The profile of the connector is dependent on the type being used

Fig. 11.5. Split-ring connector.

types of connectors the shear strength of the bolt is a key element in the transfer of lateral load across the connection. However, because the shear strength of the bolt specified for use with these connectors will always exceed the lateral strength of the connector, EC5 does not require the bolt shear strength to be checked. Also, as with a ring connector, the connector bolt is not considered to contribute to the lateral strength of the connector.

In a shear-plate connection, the load passes from one member into the shear plate by embedment stresses and the bolt is then loaded through bearing stresses between the shear plate and the bolt. From there it is transferred through the bolt by shear stresses to the second shear plate in the case of a timber-to-timber joint, or, in the case of a timber-to-steel (or concrete) joint, directly into the steel (or concrete) member. An example of the use of this type of connector is shown in Figure 11.6.

The design requirements for split-ring and shear-plate connectors are covered in EC5, 8.9. For a joint using these connectors the strength model adopted assumes that when the connector is loaded parallel to the grain the joint strength will be the lesser of the strength due to embedment failure of the timber at the connector or shear block failure of the timber at the loaded end of the connection.

The timber within the area bounded by the connector is assumed to have sheared off before the failure load of the connection is achieved and does not contribute to the strength. Assuming that a shear-plate connection is being used, the design conditions to be satisfied for member B in a two-member connection are shown in Figure 11.7.

Two modes of failure are possible. These are embedment failure and shear block failure as shown on the loaded face of the connector at its loaded end. If the embedment resistance of the timber exceeds the resistance offered by the shear block, shear block failure will dictate the strength, and if it is smaller, embedment failure will occur. As shear block failure is a brittle failure mechanism, failure by the ductile embedment failure mode is the preferred design condition. Where shear block failure will not occur,

as in the case of member A in Figure 11.7, only embedment failure will be relevant when determining the strength of that member.

To prevent other forms of brittle failure in this type of connection, EC5 specifies minimum spacings, end and edge distances as well as minimum member thicknesses, and the respective criteria are the same for shear-plate and ring connectors. The requirements for spacings and distances are as given in Table 11.2.

Minimum member thicknesses are given in EC5,8.9(2), and the criteria to be met are the same as for toothed-plate connectors. The requirements are summarised as follows:

where the thicknesses are as shown in Figure 11.8 and t1 is the thickness of the outer timber member(s) in the connection (in mm), t2 is the thickness of the inner timber member in the connection (in mm), and he is the embedment depth of the ring or shear plate (in mm).

Member A Member B

Design force on connector

Member A Member B

Design force on connector

Embedment stresses on loaded face c>

connector

Shear block on loaded face of connector

Shear block failure plane

Shear failure plane of timber bounded by the connector

Fig. 11.7. Resistance of connection at the loaded end.

Shear failure plane of timber bounded by the connector

Embedment stresses on loaded face c>

Design force on connector

Shear block on loaded face of connector

Shear block failure plane

Fig. 11.7. Resistance of connection at the loaded end.

Connector

Connector

Fig. 11.8. Relevant dimensions used for connections fitted with split-ring and shear-plate connectors (based on EC5, Figure 8.12).

The same equations are used to determine the characteristic strength of a split-ring and a shear-plate connector, based on a connection formed with timber having a characteristic density of 350 kg/m3, a loaded end distance of 2dc, a side member thickness of 3he, and a central member thickness of 5he:

• From equation 8.61(a) in EC5, the characteristic strength of the shear block (in N) at the loaded end when loaded in tension parallel to the grain is:

where dc is the connector diameter (in mm).

• From equation (8.61b) in EC5, the characteristic embedment strength of the timber (in N) at the loaded face of the connector is:

where he is the embedment depth (in mm) of the connector type being used.

The connector strength will be the minimum value obtained from equations (11.14) and (11.15). For a connection in which the connector is loaded in an 'unloaded' end condition as shown in Figure 11.9 (i.e. 150° < a < 210°), shear block failure will not arise and the strength will only be based on equation (11.15).

To take account of the effect of variation in member thickness, loaded end distance and characteristic density as well as the increase in shear block strength when there is a steel member in the joint, modification factors must be applied to equations (11.14) and (11.15). The characteristic load-carrying capacity parallel to the grain (in N), Fv 0 Rk, per connector, per shear plane, incorporating these factors is:

Fv,0 Rk = minjk1k3(31.5dchc) (b) (embedment) (EC5, equation (8.61»

timber bounded by the connector

Fig. 11.9. Connection using split-ring connectors loaded in an 'unloaded' end condition.

timber bounded by the connector

Fig. 11.9. Connection using split-ring connectors loaded in an 'unloaded' end condition.

where:

dc is the connector diameter (in mm).

k1 is a modification factor taking into account the effect of member thickness 1

t2/5he where t1 and t2 are as defined in Figure 11.8 and he is the embedment depth (in mm). The strength of the connection will be reduced by 25% if the minimum permitted values of t1 and t2 are used. Also, when t1 and t2 exceed 3he and 5he, respectively, there will be no increase in joint strength.

k2 only applies to the block shear strength at a loaded end and is 1.0 unless the angle of load is between -30° < a < 30°, in which case:

[a3it/2dc where a3 t, the loaded end distance, is obtained from Table 11.2, or, for members with a sloping end, measured as shown in Figure 11.3; dc is as previously defined and ka = 1.25 for connections with one connector per shear plane. For connections with more than one connector per shear plane, ka = 1.0. k3 is a modification factor for timber density and:

[Pk/350

where pk is the characteristic density of the timber in the joint (in kg/m3).

As previously stated, the reference strength equations (11.16) are based on a connection in which the timber has a characteristic density of 350 kg/m3 (i.e. strength class C14 as specified in EN 338:2003). As in the case of connections using toothed-plate connectors, for connections using ring or shear-plate connectors, factor k3 will increase the strength of the connector when higher strength class timber is used. With these types of connectors a maximum increase of 75% will be achieved when the characteristic density is at least 613 kg/m3. This will cover the requirements for all strength classes of softwood in BS EN 338 as well as hardwood strength classes D30-D40, but there will be no increase beyond this limit.

k4 is a modification factor that depends on the materials connected and only applies to the shear block failure equation (11.16, equation a). It is obtained from

,1.0 for timber-to-timber connections .

■1.1 for steel-to-timber connections

0 0

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