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624 Wind loads acting on timber frame walls for designs to BS 5268-6

Report ID: 624

Published: Newsletter 54 - April 2019

Report Overview

A reporter says that the masonry shielding factors in BS 5268-6 (K100 and K200) were substantially reduced when PD 6693-1 for designs to EC5 was produced, apparently because the original BS method was not adequately vetted.

Report Content

A reporter says that the masonry shielding factors in BS 5268-6 (K100 and K200) [Ref. 1] allow substantial reductions to be taken on the wind loads acting on timber frame walls. When PD 6693-1 [Ref. 2] for designs to EC5 was produced, the masonry shielding effect was substantially reduced apparently because the original BS method was not adequately vetted. Although BS 5268-6 was withdrawn in 2010 without the K100 and K200 factors being amended, the standard continues to be used in the UK. There are two points the reporter makes about this:

  1. Where BS 5268-6 is used in new designs, the masonry shielding factors could result in a design that has a safety factor much lower than the designer actually thinks it is. Few engineers would be aware of the question mark over the validity of the K100 and K200 factors.
  2. When specifying remedial work; inadequate remedial work may be specified if the original K100 or K200 factors were taken into account again.

BS 5268-6.2 was first published in 2001 while the more widely used BS 5268-6.1 was published first in 1988 and a revision was published in 1996 with an amendment on 2007 making the standard over 10 years old. The use of withdrawn standards is never a good idea as they are not maintained and updated to correct mistakes and to take advantage of new knowledge and practices.

Ref. 1: BS 5268-6.1:1996 Structural use of timber - Part 6: Code of practice for timber frame walls - Section 6.1 Dwellings not exceeding seven stories; BS 5268-6.2:1996 Structural use of timber - Part 6: Code of practice for timber frame walls - Section 6.2 Buildings other than dwellings not exceeding four storeys

Ref. 2: PD 6693-1:2012 Recommendations for the design of timber structures to Eurocode 5: Design of timber structures Part 1: General - Common rules and rules for buildings

Comments

This report is a good example of the potential risks involved with designing to withdrawn codes. The interaction under wind load between the inner timber frame leaf and outer masonry leaf in timber frame walls is complex and may not be fully understood. Both BS5268-6 and PD6693-1 provide simplified procedures which attempt to quantify masonry shielding in a rational and safe manner, though the reporter is correct when stating that a lesser proportion of wind load is resisted by the masonry in the PD6693-1 procedure.

The background to the more recent PD6693-1 procedure is documented in BSI papers. To the best of the knowledge of CROSS, no derivation for the older BS6268-6 procedure, which has been applied for over 20 years, has been found. It would appear that masonry shielding calculations would be better done to the current procedure in PD6693-1 (EC5 support document) than to the procedure in the withdrawn BS 5268-6 standard.

In general, it is not good practice to use withdrawn standards and their application may prove difficult to justify to a client or court in the event of a problem arising.

 

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