AD 324: Masking of faying surfaces for slip-resistant connections
The performance of slip-resistant bolted connections depends on the friction that can be developed between the faying surfaces and that in turn depends on the quality of the surface. The faying surfaces should be blast cleaned during fabrication but the question then arises about possible deterioration of the surface before assembly and tightening.
Section 6.5.1 of SCI publication P324 (Tension control bolts, grade S10T, in friction grip connections, 2004) says that the surfaces must be masked off after shot blasting. Clearly this is good practice, especially in situations where the steel would be exposed for some time and the surface would be adversely affected by rusting. Protection by masking tape is effective for short periods, although caution should be exercised in the choice of tape – see Guidance Note 7.05 in SCI publication P185 (Guidance notes on best practice in steel bridge construction, fourth issue, 2006). Current industry practice for building steelwork is for a “just in time” approach to fabrication and delivery of steelwork; in such circumstances there is time for exposed surfaces to develop only “light rusting”. For that condition the slip factor of 0.5 would still be appropriate.
Contact: A S Malik
Tel: 01344 636525