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NBS Shortcuts: fixings

Austin Williams looks at the types of screw, bolts and nails on the market and how to use them in this NBS Shortcut

SCREWS
The Construction Information Service suggests that BS 1210, (1963), ‘Specification for Wood Screws’ is ‘obsolescent but still relevant’, predominantly because many of the referenced British Standards are now obsolete. Even though the dimensions in the standard are written in inches, this is not enough to condemn it, being just one more of those British construction industry curiosities which says that it is still legitimate to measure certain things in imperial units. But even though the standard has now been in force for 45 years, there
is still an uncorrected howler in Annex G, where the final calculation factor has been written t1/t2 instead of t2/t1! Fortunately this error has little significance for architects and designers.

In order to comply with the standard, all screws must have a minimum tensile strength of 550N/mm² - although this regulation excludes coach screws - and must also have a shank diameter greater than 10mm. In order to address some of these issues, a new suite of British Standards for screws (BS 1580: 2007) has just been released… although there is still no European Standard.

NAILS
Wire nails date from the late 19th century. Before that, ‘cut nails’ were common, punched out or guillotined from a flat plate of rolled iron.
Nails, staples, wood screws, coach screws and bolts are all variants of dowel fasteners. The Timber Research and Development Association (TRADA) points out that all of them may be used for laterally loaded connections, but for axially loaded connections only nails, screws and bolts are normally to be used. As such, nails are normally required to resist just the shear forces at the interface (or interstice) between two or more joined materials.

Eurocode 5 ‘Design of Timber Structures’, which outlines the common rules for the use of structural timber in buildings, specifies a minimum tensile strength and provides design procedures for all of these fasteners. Eurocode 5 will replace BS 5268 in the next two-to-three years, but until then BS 5268-2 references compliance standards for fasteners that it covers.

When joining together two pieces of hardwood, at least 40 per cent of the nail length must extend into the lower portion; when joining softwoods together, that length should increase to at least 50 per cent. When hammering nails with a diameter of more than 5mm into hardwoods, pre-drilled holes should be used to avoid splitting the timber. Similarly, for nails more than 100mm long, pre-drilling is recommended. To ensure a good grip, the hole should be no more than 80 per cent of the nail diameter. Pre-drilled holes should be positioned at a distance at least 10 times their diameter from the end grain of hardwood; where there are no pre-drilled holes, nails should be positioned at least 20 times their diameter from the end grain.

When using a nail gun, be aware that firing the nails into unsupported 6mm plywood, say, or into any other thin material that is not directly located over a timber stud or a similar substrate, will result in the nail being shot straight through the material with the velocity of
a bullet.


BOLTS
For bolted connections, washers must have a minimum external diameter and thickness three times the diameter of the bolt (the hole itself may be up to 2mm wider than the bolt). TRADA recommends that the thickness of washer must be both 0.25 times the diameter (in ‘Wood Information, Section 2/3 Sheet 36: 2003) and 0.3 times the diameter (in ‘Wood Information, Section 2/3 Sheet 52: 2002)’.

PERFORMANCE
When comparing the jointing performance of fasteners of equivalent diameters it is key to consider that:
• nails generally have the advantage in terms of lateral load-carrying capacity;
• screws have better axial withdrawal resistance; and
• dowel joints provide higher load-carrying capacity.

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