The potential advantage of partially threaded bolts is that they obviously have a slightly higher shear resistance if the shear plane is in the unthreaded length. The disadvantages of calculating precise unthreaded lengths, which must be neither too long nor too short, and relating each bolt length to specific connections, far outweigh the increased resistance. On site, multitudinous bags of different bolt lengths give ample opportunity to install the wrong bolts. In contrast, a standard M20 x 60 mm fully threaded bolt may be used in the vast majority of site connections.
The use of fully threaded bolts was recommended in the first “Green Book” of 2002¹ and the Eurocode version of 2014².
Concerns with fully threaded bolts may relate to the supposed increased in bearing deformation, if the threads engage with the steel rather than the unthreaded shank. Investigations of the behaviour of fully threaded bolts were reported by Graham Owens in 1992³. Although fully threaded bolts in bearing show a lower initial stiffness, the bearing strength actually increases slightly, due to the constraint offered when the threads dig into the plate material. The deformation in bearing of a fully threaded bolt is slightly more than that of a plain shank, but the increase is not relevant when bolts are already in 2 mm oversize holes.
If designers are concerned about deformation in a joint, the issue does not concern whether fully threaded or unthreaded bolts are specified – the difference in performance is insignificant. If deformation in the joint must be avoided, preloaded assemblies must be specified.
It should be noted that shear and tension resistances quoted (in the Blue Book, for example) always use the cross section in the threaded length as the basis of the resistance calculations – and are therefore safe.
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1. Joints in steel construction. Simple connections (P212), SCI and BCSA, 2002
2. Joints in steel construction. Simple joints to Eurocode 3 (P358), SCI and BCSA, 2014
3. Owens, G, W., The use of fully threaded bolts for connections in structural steelwork for buildings. The Structural Engineer, Volume 70, September 1992