Prying action

The concept of prying action can be most easily presented in terms of a T-stub loaded by a tensile force FT. The applied force is not concentric to the centre of the bolt and additional forces (Q) may be developed near the flange tip, increasing the bolt force itself. The tension induced in the bolts, from equilibrium, is thus FB= FT/2 + Q. This phenomenon is illustrated in Fig. 1.

Figure 1.

The development of the prying forces depends on the stiffness properties of both the T-stub and

the fasteners.

If the flange of the T-stub connection is relatively rigid, the flexural deformations of the flange will be small, when compared with the elongation of the bolts. The flange and the base would separate from each other over the entire area and very little prying force will be developed given that the flange would bend into a single curvature only. The fastener will behave much like a single bolt in tension. This is shown in Fig. 2.

Figure 2.

When a more flexible T-stub flange is utilised, the flexural deformation of the flange induces contact forces at the edges. These forces produce an extra bending moment in the flanges. With stronger bolts the flanges of the T-stub yield (double curvature), while the bolt elongation is reduced. This case is presented in Fig. 3.

Figure 3.

There is an intermediate third case between these two extreme conditions, where prying forces are present, but where the bolts rupture before the mechanism in the flanges with four yield lines has fully developed.

There are some important comments and notes regarding prying action:

• Prying action in high strength friction grip bolts is reduced only at a relatively low level of load and the behaviour of the bolts at ultimate load is not significantly affected.

• Even if prying action is low, the distortion of the connected parts results in significant bending of the bolt and local bending of the bolt nut or head.

• Prying action in Eurocode 1993-1-8:2005 is implicitly taken into account when determining the design tension resistance according to Table 6.2.

• Base plate connections usually have long anchor bolts and a thick base plate when compared to an

end plate. This results in the uplift of the T-stub from the concrete foundation and in these situations, prying of anchor bolts is not observed.