How to specify timber frame wall ties
Ancon recommends that when tying a masonry outer leaf to a timber frame, in a UK town or city, wall ties should be specified to Types 5 to 7 in BS 5268-6.1: 1996.
Although BS 5268 was officially withdrawn in November 2009, it is Ancon’s expert opinion that timber frame wall ties should continue to be selected from the information given in Annex B, BS 5268 Part 6.1: 1996, until further industry guidance is made available to the industry.
The document, BS 5268-6.1: 1996 (Incorporating Amendments No. 1 and 2): Structural use of timber – Dwellings not exceeding seven storeys, provides recommendations for wall ties in timber framed buildings. Information is provided for the type of structure, location, embedment, density and positioning. Ties are classified as Types 5 to 7; minimum declared values in tension and compression are listed for Types 5 and 6.
Masonry-to-Timber Tie Types to BS 5268-6.1: 1996
|Type||Application||Density||Maximum Building Height||Geographical Location|
|Type 5||Timber frame tie suitable for domestic houses and industrial/commercial developments of up to three storeys||4.4 ties/m2
3-4 ties/m at unbonded edges
|15m||Suitable for flat sites in towns and cities where the basic wind speed does not exceed 25m/s and altitude is not more than 150m above sea level|
|Type 6||As Type 5 but suitable for developments of up to four storeys||As Type 5||15m||Suitable for flat sites in towns and cities where the basic wind speed does not exceed 25m/s and altitude is not more than 150m above sea level|
|Type 7||As Type 5 but suitable for developments of between five and seven storeys, being designed to accommodate the increased vertical differential movement||Calculated for actual performance required for each site location||18m||Calculated for actual performance required for each site location|
Note: Refer to BS 5268-6.1: 1996 and BS 6399-2: 1997 for complete information.
The geographical locations in BS 5268-6.1 are based on hourly return period wind speeds according to BS 6399-2: 1997.
Timber Frame Wall Tie Density
Timber frame ties should be installed at a minimum density of 4.4 ties/m2 where the wind speed does not exceed 25m/s and the site altitude does not exceed 150m; this density should be increased to 7 ties/m2 in more severe situations.
Why does Ancon recommend wall tie specification to a withdrawn code?
British Standards, BS 5628 (for masonry buildings) and BS 5268 (for timber frame buildings) allowed the specification of wall ties by the “Tie Type” method; types 1 to 4 relate to masonry-to-masonry ties and types 5-7 to masonry-to-timber ties.
When Eurocode 5 (Timber) and Eurocode 6 (Masonry) became the main design standards for the UK and the equivalent British Standards were withdrawn, the only method outlined in the codes for specifying wall tie spacing was by calculation (method outlined in Eurocode 6), which ran contrary to the industry norm in the UK. There is no information in Eurocode 5 regarding the design of wall ties for use with timber frames.
On withdrawal of the masonry code, the British Standards Institute issued a new NCCI (Non-Contradictory Complementary Information) Published Document, PD 6697:2010, for masonry buildings. This document covers information, including Tie Types 1 to 4, from the withdrawn masonry standard, which was not included within the Eurocodes.
However, there is currently no similar document analogous to BS 5268 for wall tie specification, so information on the wall tie Types 5 to 7 still only exists in this withdrawn standard.
Ancon, therefore, recommends that the specification methods outlined in BS 5268 should continue to be used for projects in UK towns and cities until this information is reproduced in a published document.
Why specify wall ties to Tie Types?
It is common knowledge that the calculation method outlined in Eurocode 6 generally creates much more conservative specifications and is less cost-effective.
The figures for the minimum capacity of masonry-to-masonry tie types (Table 12, PD 6697:2010) were agreed by the BSI committee responsible for PD6697, to try and ensure that the majority of new UK buildings didn’t require an unnecessary increase in the density of ties subsequent to the adoption of Eurocodes.
When should wall tie density be calculated?
Where local topography, surrounding buildings, shape of the building and distance to the coast etc. may have a greater impact on the final wind pressures of a building, Ancon recommends that wall tie density should be calculated, however this is ultimately at the discretion of the specifying engineer for an individual project.
If you are satisfied that your building fulfils the criteria noted in BS 5268-6.1, Table B.1, then a Type 6 wall tie specification should be satisfactory. That description is as follows:
- Type of structure: Suitable for tying masonry outer cladding on to softwood structural framework of residential/commercial buildings up to four storeys and not greater than 15m total height. Designed to accommodate the potential increased vertical differential movement likely to be encountered in the fourth storey of a building.
- Geographical location: Suitable for buildings on flat sites where the fundamental basic wind velocity is up to 25 m/s within towns and cities anywhere in the UK, except the north western fringes of Scotland and Ireland. Areas where the site is at an altitude of 150m or more above sea level are also exempt and tie density should be increased to 7 ties/m2.
Design Considerations – Additional Notes
Ancon provides the following notes on geographical location and wind load calculation:
- The above descriptions are written with wind loads to BS 6399 in mind. BS 6399 defines “town” in Clause 1.7.2, and also implies in Clause 188.8.131.52 that to be fully categorised as “within towns and cities” in terms of the terrain and building factor, the site must be in a location where the built up area extends at least 2km upwind - see Table 4. It also notes in Clause 184.108.40.206 that, for all sites inside towns (except exactly at the upwind edge or exactly 2km from the upwind edge), the simplifications of the standard method produce more conservative values for the building and terrain factor than the directional method.
- Is it necessary to design the wall ties for a peak pressure values which are only applied to a small area of the building? These areas are usually relatively small and close to corners where the masonry tends to be a lot stiffer. Again, the load used to design the cladding is down to the judgement of the designing engineer, and this does, in our experience, sometimes differ from the peak pressures for the project.
- Which code has been used to analyse the wind loads? The Eurocode 6 method is intended for use with wind loads to BS EN 1991. The same method of calculation could be used with BS 6399 wind loads, however the change in partial factor may make it more conservative to do so.
Contact Ancon for more information.
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