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304 Barry’s Advanced Construction of Buildings
require any great degree of skill, can be quickly completed. The disadvantages of this connection
are that the bolts do not make a tight fit into the holes to which they are fitted and
there is the possibility of some slight movement in the connection. For this reason, black
bolts are presumed to have less strength than fitted bolts and their strength is taken as 80
N/mm 2 . These bolts are little used today for structural steel connections.
Turned and fitted bolts
To obtain more strength from a bolted connection, it may be economical to use steel bolts
that have been accurately turned. These bolts are fitted to holes of the same diameter as
their shank, and the bolt is driven home by hammering and then secured with a nut.
Because of their tight fit, the strength of these bolts is taken as 95 N/mm 2 . These bolts are
more expensive than black bolts and have largely been superseded by high-strength friction
grip (hsfg) bolts.
High-strength friction grip bolts
These bolts are made from high-strength steel, which enables them to suffer greater stress
due to tightening than ordinary bolts. The combined effect of the greater strength of the
bolt itself and the increased friction due to the firm clamping together of the plates being
joined makes these bolts capable of taking greater loads than ordinary bolts. Bolts are
tightened with a torque wrench, which measures the tightness of the bolt by reference to
the torque applied, which in turn gives an accurate indication of the strength of the connection.
Hand tightening would give no measure of strength. Though more expensive than
ordinary bolts, these bolts and their associated washers are commonly used.
Strength of bolted connections – single shear, double shear
Bolted connections may fail under load for one of two reasons. First they may fail by the
shearing of their shank. Shear is caused by the action of two opposite and equal forces
acting on a material. The simplest analogy is the action of the blades of a pair of scissors
or shears on a sheet of paper. As the blades close they exert equal and opposite forces which
tear through the fibres of the paper, forcing one part up and the other down. In the same
way, if the two plates joined by a bolt move with sufficient force in opposite directions, then
the bolt will fail in single shear, as illustrated in Figure 5.28. The strength of a bolt is determined
by its resistance to shear in accordance with the strengths previously noted. Where
a bolt joins three plates, it is liable to failure by the movement of adjacent plates in opposite
directions, as illustrated in Figure 5.28. It will be seen that the failure is caused by the shank
failing in shear at two points simultaneously, hence the term double shear. It is presumed
that a bolt is twice as strong in double as in single shear.
Bearing strength
A second type of failure that may occur at a connection is caused by the shank of a bolt
bearing so heavily on the metal of the member or members it is joining that the metal
becomes crushed, as illustrated in Figure 5.29. The strength of the mild steel used in the
majority of steel frames and the connections, in resisting crushing, is taken as 200 N/mm 2 .
The bearing area of a bolt on the mild steel of a connection is the product of the diameter
of the bolt and the thickness of the thinnest member of the joint. When selecting the
diameter and the number of bolts required for a connection, the shear resistance of the
bolts and the bearing area of the thinnest plate have to be taken into account.