AD 286: Preloaded bolts

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Advisory Desk AD 286 Preloaded Bolts: The Net Effect of Applied Tension and Preload Although the fundamental issues relating to the All the components are in equilibrium and the effect of applied tension and preload on bolts load in the bolt is still P. The applied tension only were covered in AD 184, which deals with preload reduces the intensity of pressure on the faying due to tightening on ordinary (non-preloaded) surface from P to (P-T) and the equilibrium of the bolts, we continue to receive many questions ply is T + (P-T) = P. The bolt load remains P and is concerning this effect for preloaded bolts. This not the sum of P+T. Advisory Desk Note explains why the bolt load in From the equation of equilibrium for the ply, if preloaded bolted steelwork connections, when the applied tension equals the preload, P-T is zero subjected to applied tension, is not the sum of the and the plies are on the point of separation but preload (P) and the applied tension (T). the bolt load is still equal to P. When preloaded bolts are tightened or If T is greater than P the bolt will have torqued, considerable tension is induced into the stretched and the plates have separated; the shank of the bolt. This tension in the bolt is called bolt load is then equal to T. At this stage the the preload, P, and clamps the plies together in connection is behaving as a non-preloaded compression bolted joint. The bolt load cannot increase above as shown in the preload P unless the bolt stretches and the Figure 1. bolt cannot stretch until the plies separate. In In preloaded general, there is no increase in the bolt load bolts the unless the applied tension exceeds the preload. preload in A similar analysis can be carried out assuming the shank is the applied tension acts at the external face of balanced by the plies and that the plies are not rigid. In this compression case the plies and the bolt stretch slightly, due to under the the load applied at the outer face, but because head and nut the stiffness of the plies is much greater than that of the bolt, of the bolt, the strain and the increase in the bolt which induces load above the value of the preload P will be very Figure 1 – Preloaded bolt arrangement the required small. However, again, the main point to note is compression that the bolt load is not simply the sum of P+T. or pressure on the faying surfaces for non- Note that the value of T applied at the bolt slip joints. The equilibrium of the components position must include any value for prying action, (separated for clarity) before the application where it occurs. Prying is a complex subject and of the applied tension T is shown in Figure 2. depends on many variables including the bolt and Each component has a force P and an equal and ply dimensions as well as the layout of the joint. opposite reaction P. Therefore, everything is in If a designer engineer requires greater equilibrium. understanding of these issues they are referred When tension is applied to a connection in the first instance to the article by P. J. Gill component as a result of axial load or moment, – Notes on the Load Carrying Characteristics designers often refer to the calculated ‘bolt of Pre-Tensioned Bolts: Tensioned Joints – that load’ or ‘bolt tension’. However, in steelwork appeared in the Proceedings of the Jubilee connections the applied tension does not act Symposium on High Strength Bolts, The Institution directly on the bolt but instead acts on the plies or of Structural Engineers, 1959. This Advisory Desk plates in the joint. In non-preloaded bolted joints Note has drawn heavily from this article. the tension is then transferred to the bolt from the Contact: Thomas Cosgrove plies. In preloaded bolted joints the load path is Email: t.cosgrove@steel-sci.com different. Telephone: 01344 623345 Figure 2 - Equilibrium of components before Figure 3 – Applied tension acting at the faying Figure 4 - Equilibrium of components after application of applied tension, T surface of a preloaded joint application of applied tension, T 34 NSC May 2005


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