Welded and bolted connections

The two types of connection

Welding

The material becomes continuous: the connection is rigid and transfers the forces well.

Its conditions, though, are strict:

• a qualified welder and a qualified procedure,
• inspection: the quality of the weld can be checked by non-destructive testing,
• conditions: cleanliness, temperature, shelter,
• managing distortion: the heat input causes warping.

Bolting

The connection consists of components that can be put together and taken apart.

Its advantages:

• checkable: you can see how many bolts there are and how,
• demountable: an advantage at alteration and demolition,
• reliable under site conditions,
• no heat input, so the surface protection is not damaged.

The usual division of work

Welding in the workshop, bolting on site.

In the workshop the conditions and the inspection are there; on site the bolted connection is faster, safer and better documented.

That logic also arose from corrosion protection: welding a hot-dip galvanised member means the coating is locally destroyed and has to be repaired. Hot-dip galvanising.

The thin-walled side

In thin sheet welding is problematic for several reasons: it burns through, the coating vaporises and the quality is hard to check. So the self-drilling screw is the established solution there. Clinching, riveting and welding thin sheet.

Where the two technologies meet, the welded preparation is generally made on the thicker side and the connection itself is bolted.

Verification

Execution requirements apply to both types of connection: qualification and testing for welding, tightening and recording for bolting. EN 1090 and execution classes.

The point

Welding is the workshop’s tool, the bolt the site’s. That is not tradition but a consequence of the logic of quality control and surface protection.