Bolting and riveting

Bolts are available in standard sizes and a range of lengths.

Bolts can act in two ways, namely in tension and shear. The individual parts of a bolt comprise a hexagonal head and a circular section shank, which may be fully or partly threaded. They come in shank diameters from very small (6mm) up to 36mm. Larger diameters may be available to special order.

Two standard bolt strengths are commonly used.

Two grades of bolt are commonly used. These are grade 4.6 and 8.8. The first digit relates to the ultimate strength of the material, whilst the second is the ratio of yield stress to ultimate strength. Thus grade 4.6 bolts have an ultimate material strength of 400 N/mm2, and the yield (or proof) stress is 60% of the ultimate strength. Similarly grade 8.8 bolts have an ultimate strength of 800 N/mm2 and a ratio of yield/proof stress to ultimate strength of 80%. Simple grade 4.6 bolts without corrosion protection are commonly called 'black bolts'.

Other bolt types may be used in special circumstances.

Turned and fitted bolts are similar to 'ordinary' grade 4.6 and 8.8 bolts, but are machined on their shanks and installed in reamed holes. This creates a very tightly fitting bolt, slip is negligible, and higher stresses can be used. Due to the high cost of the bolts themselves and the costs of installation, these bolts are not often used.

High Strength Friction Grip bolts (HSFG bolts) work in a different way. They are made from a material very similar to grade 8.8 bolts, but have different shaped heads and nuts. The nut is tightened in a carefully controlled manner to put a large pretension into the bolt shank. This has the effect of clamping together the plates between the bolt head and the nut. The mating surfaces are called 'faying' surfaces, and they must be flat and unpainted. Because the tightening requires careful supervision, this connection is relatively expensive and generally used only where slip of a connection must be avoided, for example to avoid a connection working loose due to vibration, or where the direction of loading can be reversed due to normal imposed loads. They are not normally essential for reversal due to wind loads only.

Single sided bolting systems such as Flowdrill and Hollobolt are used where it is necessary to make a connection directly to the face of a hollow section.

Rivets are no longer a practical method of forming structural connections.

Rivets used to be a very common method of forming steelwork connections, but they are almost never used now. Before fitting, rivets are rather like unthreaded bolts. They are installed hot in pre-drilled holes, and then the portion of shank protruding from the hole is hammered down to form a second 'head' to the rivet.


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