In practice, site connections are bolted, whilst attachments during fabrication are welded.
The relative cost of welding and bolting depends on the individual steelwork contractor's particular facilities and organisation. However, it is generally cheaper to make a bolted joint than a welded one, particularly on site and if holes can be punched rather than drilled. Welding is either labour intensive (in the case of manual metal arc welding), or capital intensive (in the case of the automatic welding processes). Making holes, using banks of drills on an automatic machine, is relatively quick and cheap. The cost of weld inspection by ultrasonic or radiographic or dye penetrant testing is another extra (and source of delay if the weld fails). In practice most steelwork contractors have suitable welding facilities, and attachments effected during fabrication are typically welded, with site connections bolted.
Site welding may be used for special circumstances.
Site welding is used where the full strength of a member must be used at a connection, and where tolerance, geometry, or appearance requires. In exposed steelwork, welding may also be preferable to splice plates, which may allow water to penetrate and cause corrosion.
Where a completely non-slip joint is required, welding may also be the most suitable solution.