AD 544: Washers for preloaded bolting assemblies

The SCI Advisory Desk sometimes receives questions about washers for preloaded bolting assemblies. When preloaded bolts are tightened and unhardened washers are used, if the contact pressure acting on the joint is too high, the washers may indent and deform when the bolt is fully tightened. If this occurs after the tightening is completed, the bolt preload will reduce, potentially affecting the integrity of the joint¹. Clause 8.2.4 of BS EN 1090-2:2018² and clause 6.3.1 of the NSSS³ require that hardened washers are to be used as follows:

• For property class 8.8 bolts a washer shall be used under the bolt head or the nut, whichever is to be rotated;
• For property class 10.9 bolts used with steel grade S235 washers shall be used under both the bolt head and the nut (it is noted that the use of steel grade S235 is highly unusual in the UK);
• For property class 10.9 bolts used with steel grades above S235 washers shall be used under the bolt head or the nut whichever is to be rotated, unless the use of washers under both the bolt head and the nut is specified.

Hardened plain chamfered washers according to BS EN 14399-6:2015⁴ shall be used under the heads of preloaded bolts and positioned with the chamfer towards the bolt head and towards the nut when fitted under the nut. Hardened plain (unchamfered) washers according to BS EN 14399-5:2015⁵ shall only be used under nuts (because of the corner radius between the bolt shank and head).

Both BS EN 14399-5:2015 and BS EN 14399-6:2015 note that washers according to those standards are not intended to be used in direct contact with oversized or slotted holes. As explained in AD 522, plate washers shall be used for connections with slotted and oversized holes. Hardened washers (as described above) are required on top of the plate washer.

1. Joint Bearing Pressure and the Use of Unsuitable Washers, Bolt Science, October 2024
2. BS EN 1090-2:2018 Execution of steel structures and aluminium structures, BSI
3. National Structural Steelwork Specification for Building Construction 7th Edition, BCSA
4. BS EN 14399-6 High-strength structural bolting assemblies for preloading – Plain chamfered washers, BSI
5. BS EN 14399-5 High-strength structural bolting assemblies for preloading – Plain washers, BSI

Contact: Liam Dougherty
Telephone: 01344 636555
Email: advisory@steel-sci.com