The Inplane Racking Resistance Of Timber Walls Under Horizontal And Vertical Loading

The stud walls associated with timber-framed buildings are usually sheathed on one or both faces with the sheathing securely fixed to the studs, enabling the wall to act as a rigid diaphragm. The fixings (e.g. nails) provide the bulk of the racking resistance through timber bearing and nail deformation when the lateral external force is applied as shown in Figure 9.8b. Horizontal sliding of the wall is resisted by anchorages such as nails or bolts along the sole plate sufficient to resist the applied forces. Overturning about the bottom corner may also occur (particularly in walls with high aspect ratios) if adequate/appropriate holding-down fixings are not provided, as shown in Figure 9.8c. In most timber-framed buildings, beams and floors are designed as simply supported elements on pin jointed walls and the lateral strength and stability of the structure is provided by the use of such diaphragms.

In timber design, the in-plane lateral resistance of a wall diaphragm is referred to as the racking resistance of the wall and examples of some diaphragm walls are shown in Figure 9.9. In Figure 9.9c a typical failure resulting from the lack of adequate racking strength is illustrated.

From the initial conceptual analysis, the load paths of the forces through the structure to the foundations will be determined and the diaphragms to be designed to provide lateral stability and overturning resistance can be identified.

The racking resistance of a wall can be obtained either by tests or by calculation and in EC5, 9.2.4, two simplified calculation methods, Method A (9.2.4.2) and Method B (9.2.4.3), are given. Method A has been developed in Europe and Method B is a soft conversion of the procedure developed in the United Kingdom for racking strength and given in BS 5268 [6]. Although EC5 recommends the use of Method A, NA.2.9 of the UKNA to EC5 states that Method B should be used.

The method used in BS 5268 to determine the racking resistance of wall diaphragms is based on apermissible stress approach and has been developed taking into account the results of racking tests on walls. Unfortunately, in the conversion process to limit state methodology, the EC5 codifiers have incorrectly interpreted some important factors in

(a) Timber wall during assembly and erect

(b) Holding-down strap to prevent overturning (c) Lack of adequate racking strength results in failure (photo courtesy of APA, The Engineered Wood Association)

(b) Holding-down strap to prevent overturning (c) Lack of adequate racking strength results in failure (photo courtesy of APA, The Engineered Wood Association)

Fig. 9.9. Examples of timber shear walls.

the UK procedure [7] and the method will not give an accurate result. Recognising the deficiencies in the methodology and also that neither Method A nor Method B (even when correctly converted from the UK methodology) fully covers all design issues, these methods are to be replaced in EC5 by a unified method. Among other things, the proposed unified method will effectively include for the effect of openings and also take into account the contribution from the use of plasterboard when determining the racking strength.

In the following section, Method B as defined in EC5 has been used to calculate the racking resistance of the wall. For the reason given above, the value obtained will not be an accurate assessment of the racking resistance, however, as the unified method is likely to adopt a similar approach to that used in Method B, it has been given to show how the method is applied.

9.5.1 The in-plane racking resistance of timber walls using Method B in EC5

As defined in EC5, a wall assembly can comprise one or more walls with each wall able to be formed from one or more panels as shown in Figure 9.10. The panels must,

Key:

(1) Wall panel 1; (2) Wall panel 2; (3) Wall panel 3; (4) Wall 1;

(5) Wall 2 (6) Wall 3; (7) Wall assembly; (8) Sheet;

Fig. 9.10. A wall assembly comprising several wall panels.

however, be made from sheets of wood-based panel products compliant with EN 13986 [8] and, in the case of LVL panels, with EN 14279 [9]. Softboards complying with the requirements of EN 622-4 [10] can be used for wind bracing but, as required by EC5, 3.5, the strength must be verified by testing. Although plasterboard can be used to provide racking resistance in walls designed in accordance with BS 5268, in EC5, Method B, it is not acceptable to include any contribution from the use of this material.

A panel can only contribute to the in-plane strength of a wall when its width is at least the panel height divided by 4. For example, in the case of wall panel 1 in Figure 9.10, h/b must be < 4. The fasteners between the sheathing and the timber frame must be either nails or screws and should be equally spaced around the sheet perimeter. The spacing of internal fastenings should not exceed twice the perimeter fastener spacing.

When an opening is formed in a panel, the lengths of panel on each side of the opening should be considered as separate panels. And where panels are combined to form a wall:

(a) the tops of the panels must be linked across the joint by a member or by the structure,

(b) the vertical connection strength to be provided between adjacent panels should be evaluated and be at least 2.5 kN/m, and

(c) the wall must be able to resist overturning and sliding forces by either anchorage to the supporting structure or from permanent actions applied to the wall or from a combination of both.

The design procedure is outlined below.

Consider a typical layout for wall i as shown in Figure 9.11.

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  • katja
    Can plasterboard be used for racking resistance in EC5?
    9 years ago

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