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832 Timber frame wall tie design

Report ID: 832

Published: Newsletter 57 - January 2020

Report Overview

The reporter believes that there is no clear way to demonstrate the adequacy of timber frame wall ties to current guidance in areas of moderate to high wind loads.

Report Content

Wall ties are generally set out based on standard geometries rather than explicit design, says a reporter. In the case of masonry clad timber frames, the specification normally takes the form of a density of 4.4 ties/m2, with 3 or 4 additional ties per metre provided at wall edges and openings. This specification is understood to have followed over from Annex B of BS 5268-6.1 (Structural use of timber - Code of practice for timber frame walls) and is intended for use in urban areas only. Using this guidance, the general specification for new builds outside the urban area shelter effects would be a density of 7 ties/m2.

In blockwork construction, a specification of ties at 450mm centres, or every second course, along studs, is common. This is largely due to the geometry of the block, with the prescribed densities being better suited to brickwork. For studs at 600mm centres, this equates to a density of 3.7 ties/m2, which corresponds to NHBC 2019 guidance (Section 6.2.14), but falls short of either of the above specifications. No adverse issues resulting from this lower tie density are known to the reporter.

A density of 3.7 ties/m2 is mentioned in reference to contribution to racking resistance in both BS 5268-6.1 and the IStructE Manual for the design of timber building structures to Eurocode 5, without reference to where such a specification would be acceptable.

Tie capacities are provided in Table B.3 of BS 5268-6.1. When the global safety factor of 4.2 in clause 4.13.5 is applied, the permissible tensile and compressive load capacity for a Type 5 wall tie at a density of 4.4 ties/m2 would equate to 0.63kN/m2 and 0.45kN/m2 respectively. The reporter says that the design wind loads for a significant proportion of buildings within the permitted geographical locations would be above these values.

Unlike other non-contradictory information in the British Standards, none of this wall tie information from BS 5268-6.1 has appeared in the Eurocodes, National Annex, or relevant Published Document to date, says the reporter. They add that correspondence with relevant specialists has suggested that as the Eurocode suite is more material specific, wall ties for connecting timber frame inner leafs to masonry outer leafs have not been adequately considered.

BS EN 845-1 (Specification for ancillary components for masonry. Ties, tension straps, hangers and brackets) does cover wall ties for timber frame construction, but directs the reader to BS EN 1996 (Eurocode 6 - Design of masonry structures) and PD 6697 (Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2). The National Annex to BS EN 845-1 states that the material factor for ties is to be taken from BS EN 1996, presumably overlooking, in the view of the reporter, the use of ties in arrangements other than traditional masonry
cavity construction.

In summary, the reporter believes that the combination of the large material factor of 4.2 combined with the low capacities of the ties, means that there is no clear way to demonstrate the adequacy of timber frame wall ties to current guidance in areas of moderate to high wind loads.

The reporter does however suggest a potential solution, which would be to use a material factor relevant to each specific failure mode; 3.0 for mortar failure (from BS EN 1996-1-1, Table NA.1), 1.15 for the tie in bending (from BS EN 1995-1-1, Table NA.3) and 1.3 for pull out of a fixing (from BS EN 1995-1-1, Table NA.3). They conclude by saying that generally all failures result from bending of the tie, and a material factor of 3.0 would therefore be conservative. If a future amendment to BS EN 845-1 were to require manufacturers to test a sample of the ties to ultimate failure in the various materials, the results would offer greater confidence to designers. Alternatively, a requirement for failure in mortar and timber to be 2.7 and 1.5 times greater than failure within the tie would remove the need for more complicated published data.

Comments

As in report 866 in this Newsletter, the report demonstrates that the viability of any structural system depends on the detailing/connections and design assumptions being realised. The importance of adequate ties is amply illustrated in the SCOSS Alert following the Inquiry into the construction of Edinburgh Schools in 2016.

As far as the prescriptive guidance is concerned, the reporter is correct. Note that specifying ties at every course of 225mm blockwork at 600mm centres gives a density of 7.4 ties/m², which is excess of the of 7.0 ties/m² given in BS 5268-6.1 for exposed locations. The guidance in BS 5268-6.1 has not been reviewed since 2007, and the standard is not maintained by the British Standards Institution (BSI). As the reporter notes, the guidance in PD 6697 is for masonry construction, and there is no equivalent guidance in PD 6693 for
timber frames.

Clause 5.4.2.3 in BS 8000-3:2001 (Workmanship on building sites - Code of practice for masonry) states that the maximum spacing of ties for masonry clad timber frames is 375mm vertical centres for studs at 600mm centres, and 525mm vertical centres for studs at 400mm centres, which gives equivalent densities of 4.4 ties/m2 and 4.7 ties/m2 respectively. This standard is currently being updated to be published in 2020, but CROSS understands that the guidance on wall ties for masonry clad timber frames will probably not change.

As far as the calculation route is concerned, the principles of the Eurocode design path are well established for using structural products. Characteristic values for the product are determined in accordance with a harmonised standard (hEN), if one exists for the product, and the designer applies the appropriate safety factors to reach a design value.

As the reporter indicates, EN 845-1 is the hEN for cavity wall ties, and it specifically considers asymmetric ties where one end is in masonry and the other is in timber. The Declaration of Performance / CE marking that a manufacturer is obliged to provide will state the lowest value for tension failure, whether the failure is in the timber fixing, the mortar bed or in the bending of the tie. Older certificates may not include the failure mode, but those that are compliant with the current 2013 version of the hEN will have it.

A partial material safety factor (gamma m) for wall ties of 3.0 is given in Table NA.1 of the NA to BS EN 1996-1-1 2005 +A1 2012. It may be halved “when considering the effects of misuse or accident”. As the reporter notes, this does not appear to reflect the possible variation in failure modes for timber frame wall ties, but given the equivalent factors for timber and steel are 1.3 and 1.1 respectively, a value of 3.0 or even 1.5 is conservative.

The reporter’s final paragraph provides a reasoned answer to their own concern. If the failure mode is known, the appropriate factor can be applied; if it is not known, the default factor is conservative. Many wall tie manufacturers are well versed on this issue and will often provide free design and assessment services.

 

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View other CROSS reports published in Newsletter 57


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