Structural Fastener

What sets structural fasteners apart

A ship's structure is mainly steel plate and profiles, and almost everything on board ends up fixed to it: cable trays, pipe supports, ventilation ducts, electrical enclosures and gratings. Drilling and welding in an operating vessel are not always possible, particularly in tanks, near cargo or in finished accommodation, and any penetration or weld in a structural member has to be approved. Dedicated structural fasteners solve this with studs that are welded or shot into the steel, and with channel systems that spread loads and allow adjustment. Because they are fixed to the hull structure, they are treated as a structural detail and not as ordinary hardware.

Main designs

• Weld studs and threaded studs, fixed by stud welding to the steel surface to give a clean, threaded attachment point
• Powder-actuated or shot-in threaded fasteners, which are driven into the steel without welding or drilling
• Welding bushings and sockets that provide a threaded connection after being welded on
• Grating fasteners and clamps that hold floor gratings and walkways to their supports
• Channel and strut systems with brackets, connectors and accessories for building supports for cables and pipes
• Quick-fit mounting solutions such as bayonet or folding bases for fixtures that need to be taken off often

Main components

• The fastener body itself: stud, bolt, bushing, clamp or channel, in carbon steel or stainless steel
• Surface protection, such as galvanising or a coating, matched to the environment
• Nuts, washers and locking elements, which keep the connection tight under vibration
• Installation tools for stud welding or for driving fasteners, with the settings specified by the maker
• Insulating or isolating parts where the fastener must not make electrical contact with the structure

Selection and sizing

• Base material and thickness of the steel structure, which sets the type of fastener that can be used
• Load per fixing point, including dynamic loads from vibration and ship motion
• Corrosion environment: weather deck, tank space, machinery space or accommodation, and the matching surface protection
• Material pairing, since stainless steel against carbon steel can lead to galvanic corrosion
• Restrictions on hot work and on penetrating structural members or tank boundaries
• Spacing, edge distance and the positioning of fixings in relation to structural stiffeners and welds

Regulations and class

Fixings to hull structure are limited by class rules, particularly where they are placed on primary structural members, in tanks or near boundaries that must remain tight or fire-resistant. Many fastener systems hold type approval from a classification society for specific base materials, plate thicknesses and load ranges, and the approval can restrict where they may be used. Installation personnel normally need training on the tool and procedure, and the results are checked by sample tests such as torque or pull-out tests.

Typical faults

• Loosening of nuts and fasteners from vibration when no locking element was fitted
• Galvanic corrosion at stainless-to-carbon-steel contacts, especially in wet or salty areas
• Damage to the coating on the studs or channels at cut or drilled edges, which is where rust begins
• Fasteners driven or welded into steel that is too thin or too close to an edge, so the holding force is less than expected
• Fatigue cracking at highly loaded fixings, such as support brackets close to vibrating machinery
• Incorrect tool settings or poor stud welds, discovered only when the fixing is loaded

What to look for in a supplier

• A type-approval certificate that names the fastener type, the base material and the range of plate thickness
• Published load data for the fastener in the intended steel, not just for the fastener itself
• Matching installation tools and training for the crew or contractor
• Corrosion protection suited to the location, with documentation of the coating
• Complete systems with brackets, connectors and accessories that are tested together

Mark and record where fixings have been placed on structural members, and check that the approval allows that location. A fastener that is excellent in one place can be forbidden in another.