Axial Loading Of Metal Dowel Connection Systems

The strength equations given in the following sub-sections assume that the tensile strength of fasteners will always exceed their withdrawal capacity from the connection. If, however, there is a need to evaluate the tensile strength of the fastener, it should be carried out in accordance with the requirements of EN 1993-1-1.

10.8.1 Axially loaded nails

The withdrawal capacity of nails loaded axially is dependent on the type of nail being used. Smooth round wire nails give the poorest result and with threaded nails the

Perpendicular Grain

Perpendicular to grain Slant nailing

Fig. 10.19. Nailing in tension.

Perpendicular to grain Slant nailing

Fig. 10.19. Nailing in tension.

capacity is greatly increased. However, no matter the type, nails are not considered capable of sustaining axial load in end grain.

Also, EC5 does not permit axially loaded smooth nails to be used in situations involving permanent or long-term loading, and where threaded nails are used, only the threaded part of the nail is to be taken as relevant for determining the nail strength.

Ignoring tension failure of the nail, there are two possible failure modes when subjected to axial loading:

• pointside withdrawal of the nail;

• pull-through of the nail head.

Nails can be driven perpendicular to the grain and/or at a slant, as shown in Figure 10.19, and will fail in the mode with the lesser capacity. For nails compliant with EN 14592, only the threaded part of the nail is considered to be capable of transmitting axial load, consequently the headside capacity can only utilise the head pull-through resistance. With smooth nails, there will be withdrawal resistance for the pointside penetration and the headside resistance takes both the headside pull-through strength and the shank friction resistance on the headside of the nail into account.

When a nail is subjected to an axial force, Fd, the following condition has to be satisfied:

where Fax Rd is the design withdrawal capacity of the nail.

The design withdrawal capacity of the nail is obtained from the characteristic withdrawal capacity, Fax Rk, as follows:

Km where the functions are as previously defined and Fax Rk is the characteristic withdrawal capacity of a nail derived as follows:

For nails, other than smooth wire nails, as defined in EN 14592:

Fax,Rk = min pen fax,kdt fhead,kdh2

For smooth wire nails:

Fax,Rk = min fax,kdt pen fax,kdt + fhead,kdh where:

/ax k is the characteristic pointside withdrawal strength;

/head,k is the characteristic headside pull-through strength;

d is the nail diameter, and for square or grooved nails d is the side dimension;

tpen is the pointside penetration or the length of the threaded part in the pointside member;

tis the thickness of the headside member;

dn is the diameter of the nail head. For smooth wire nails this is 2.25d for nails of 2.65 mm up to 3.75 mm diameter and 2d for all nail sizes greater than 3.75 mm diameter.

Values for /ax k and /head,k can be determined by testing, and EC5 gives the following values for smooth nails with a pointside penetration of at least 12d:

/ax>k = 20 x 10-6pk N/mm2 /head,k = 70 x 10-6Pk2 N/mm2

(EC5, equation (8.25)) (10.58) (EC5, equation (8.26)) (10.59)

where pk is the characteristic timber density in kg/m3.

If the pointside nail penetration is less than 12d the withdrawal capacity of the nail has to be linearly reduced by multiplying by the factor ((tpen/4d) - 2). When the minimum nail penetration of 8d for smooth nails is used, the factor will be zero and there will be no axial withdrawal strength. This procedure must also be applied to the headside penetration of the nail to determine the strength contribution from shank friction towards the headside strength.

When threaded nails are used, the threaded or deformed length of the shank should be at least 6d and the pointside penetration must also be at least 6d. For a threaded pointside nail penetration of 8d the full value of the characteristic withdrawal strength can be used and for values less than this the strength is reduced linearly by multiplying by ((tpen/2d) - 3). Although no characteristic withdrawal strength is given in EC5, in the draft amendment in Appendix C, to be able to be classified as a threaded nail in accordance with EN 14592, a minimum value of characteristic withdrawal strength will be defined.

Where structural timber has been designed to function under service class 1 or 2 conditions but will possibly be installed at or near the fibre saturation point, the values of /ax,k and /head,k must be multiplied by 2/3 to take account of the reduction in the respective strengths when drying out.

The spacings, end and edge distances for axially loaded nails are the same as those given in EC5 for laterally loaded nails.

See Example 10.13.5.

Table 10.12 Minimum spacings and edge distances as illustrated in Figure 10.20, when using axially loaded screws*

Direction in which the screw

Minimum spacing between

Minimum edge distance to

has been driven

adjacent screws a\

the screw a2

At right angles to the grain

4d

4d

In the end grain

4d

2.5d

* Based on Table 8.6 in EC5 (NB: In the proposed draft amendment to EC5, summarised in Appendix C, 8.7.2, this table will be revised).

d is the screw diameter and the minimum pointside penetration of the threaded part of the screw must be 6d.

* Based on Table 8.6 in EC5 (NB: In the proposed draft amendment to EC5, summarised in Appendix C, 8.7.2, this table will be revised).

d is the screw diameter and the minimum pointside penetration of the threaded part of the screw must be 6d.

10.8.2 Axially loaded bolts

With axially loaded bolts, the strength of the connection is dependent on the tensile strength of the bolt and the bearing strength of the material onto which the bolt washer beds. The tensile strength of the bolt is derived using the strength equations in EN 1993-1-1.

When bearing onto timber or wood products, the bearing capacity below the washer should be calculated assuming a 300% increase in the characteristic strength of the timber perpendicular to the grain over the contact area, i.e. fcjk = 3.0 x /c,9o,k.

When using a steel plate, the bearing capacity per bolt should not exceed that of a circular washer with a diameter that is the lesser of 12t (where t is the plate thickness) or 4d (where d is the bolt diameter).

10.8.3 Axially loaded dowels

Dowel fasteners cannot be used to take tensile loading.

10.8.4 Axially loaded screws

With screws it is stated in EC5 that there are five possible failure modes:

- withdrawal of the threaded part of the screw;

- when used with steel plates, there is the risk of tearing off the screw head;

- failure by the screw head pulling through the timber or wood product;

- the screw failing in tension;

- when used in conjunction with steel plates there is the risk of a block shear or plug shear failure.

Failure modes in the steel or in the timber around the screw are brittle-type modes and the significance of this should be taken into account in the design of a connection subjected to axial loading.

To control block failures and ensure that the minimum withdrawal resistance is achieved, minimum spacing and penetration requirements are specified for axially loaded screws. The minimum pointside penetration of the threaded length of the screw must be 6d, where d is the outer diameter measured on the threaded part, and the a2

ai v a2

Fig. 10.20. Spacing for axially loaded screws at right angles to the grain.

spacing criteria are as given in Table 10.12 and shown in Figure 10.20 for screws driven at right angles to the grain. In the proposed draft amendment to EC5, summarised in Appendix C, 8.7.2, this table will be revised and also a minimum timber thickness criterion will be introduced.

When a screw is subjected to an axial design force Fd, the following condition has to be satisfied:

where Fax a Rd is the design withdrawal capacity of the screw when loaded axially at an angle a to the grain.

The design withdrawal capacity of the screw is obtained from the characteristic withdrawal capacity, Fax a Rk, as follows:

where the functions are as previously defined and F 1ax,a>Rk is the characteristic withdrawal capacity of a single screw at an angle a to the grain and is:

where:

• d is the outer diameter of the screw measured on the threaded part.

• £e{ is the pointside penetration of the threaded part of the screw minus one screw diameter (to take account of the pointed end of the screw) and must be at least 6d.

• /ax,a,k is the characteristic withdrawal strength at an angle a to the grain and is defined as:

sin2 a + 1.5 cos2 a where /ax,k is the characteristic withdrawal strength perpendicular to the grain, and equals 3.6 x 10-3pk'5 N/mm2, where pk is the characteristic density of the timber or wood product.

In the proposed draft amendment to EC5, summarised in Appendix C, 8.7.2, a revised expression is given for the evaluation of the characteristic withdrawal strength of an axially loaded screw, which will result in a change to equation (10.62).

Equation (10.62) only relates to the axial withdrawal strength of the threaded part of the screw in the pointside. The axial withdrawal strength of the headside of the screw will be obtained from equation (10.56) using the headside pull-through strength equation and adopting the characteristic pull-through strength given in equation (10.59). If the screw diameter is greater than 6 mm, the headside pull-through strength must comply with the rules for calculating the axial withdrawal capacity of a bolt.

If the pointside penetration of the screw is less than 10d the tensile strength of the screw is unlikely to dictate the failure strength of normal joints. When greater than 10 d the tensile strength based on the threaded area of the screw should be checked using BS EN 1993-1-8.

Where a connection is formed using a group of screws loaded by a force component acting along the axis of the shank, the effective number of screws in the group will be:

where nef is the effective number of screws, and n is the actual number of screws acting together in the connection.

Where the strength of each screw in a connection loaded axially in tension is determined by the strength of the pointside penetration length, and each screw shank is at an angle a to the grain, when n screws are loaded axially, then:

Fax,a,Rk = nef(^d4f)0'8fax,a,k (EC5, equation (8.38)) (10.65)

Km where the functions are as previously defined.

Where the screws are fixed perpendicular to the grain and loaded axially, equation (10.65) reduces to:

As previously stated, in the proposed draft amendment to EC5, summarised in Appendix C, 8.7.2, the equation used to evaluate the characteristic withdrawal strength of axially loaded screws is to be amended, which will result in a change to equations (10.65) and (10.67).

See Example 10.13.6.

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