Timber frame racking panel design (BS5268): Additional contribution value of secondary panel

When the secondary panel is Category 3 or 4 the additional contribution value appears to be incorrect and the value used is greater then that stated in table 2 of BS56268-6.1

Solution

In this instance the calculation is following the guidance from note 9 in table 2:

"NOTE 9, For both category 3 & 4 board materials, if the fixing specification for each plasterboard layer is changed to 2.65 mm diameter plasterboard nails at 150 mm spacing, then their basic racking resistances, both as primary board or as secondary board, may be increased by 50%"

Please be aware that this is not advised within the calculation itself as it is assumed the engineer has a working understanding of the code.