I considered with interest a golden rule within Robert Tucker's letter in the November issue, reported to have been previously published in The Structural Engineer. The golden rule read, 'If more than two-thirds of a primary structural wall covering the width of a building is to be removed, then some form of sway frame needs to be introduced.' This is the first time I have come across this rule. The deemed-to-satisfy guidance within the Building Regulations 2010 Approved Document A - 'Structure' (applicable to England) does not stipulate that a sway frame needs to be introduced if in excess of two-thirds of a wall is to contain openings. I would like to share an example relevant to assessing the need for provision of a sway frame, based on a cavity masonry wall with 100mm inner and outer leaves and a 100mm cavity width: 1) Imagine an external wall of 4.8m overall length with a central 3.0m opening leaving 0.9m length of wall on each side; this may be seen to satisfy Approved Document A Diagram 14, so could be considered and approved by building control without referring to a structural engineer. 2) Then imagine the same wall but 5.3m overall length with a 3.5m central opening, again leaving 0.9m length of wall at each end; this is outside the scope of Approved Document A, but does the wall panel have any less resistance to sway compared with the original 4.8m long wall design? To me there is no less sway resistance; in fact, sway stability is probably enhanced slightly due to the greater overall width.
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