The fixing decides whether the panel stays put

FRP grating cannot be welded the way steel grating can. Welding heat destroys the resin and site welding is rarely available, so grating is held by clips, bolts and washers. Choose the fixing badly and the grating cracks, lifts, or the fastener rusts first.

A sound fixing does four things at once: it clamps the grating to the support, allows thermal movement, isolates dissimilar metals against galvanic corrosion, and survives the site environment.

The order of selection is grating type and support material first, fixing material and form second, then quantity and spacing. Reversing the order tends to buy parts that do not match.

The number, size and position of fixings belong on a layout drawing. Writing fix as the maker requires leaves site staff to work by habit, and edges and openings often end up under-fixed.

Common clip types

M-clips, L-clips, saddle clips and toothed clips are the usual types. An M-clip spans two adjacent load bars and spreads the clamping force; an L-clip suits edges and openings; a saddle clip suits tubular or profiled beams.

The clip must clamp firmly without crushing the grating. Clips with rubber or nylon pads spread the pressure, reduce stress concentration at holes and bar edges, and hold better under vibration.

Clip material must suit both the beam and the grating. A stainless clip on an aluminium beam needs an isolating pad, and on a galvanized beam the condition of the coating after drilling matters.

Set the number of clips by panel area and the maker's recommendation, usually at each support intersection or at a stated spacing. Too few clips leave the whole panel loose even when each part is good.

316 or 304

304 stainless suits ordinary atmosphere and costs less; 316 contains molybdenum, resists chloride pitting better, and is the first choice on the coast, in chemical plants and where washing is frequent.

Stainless is not maintenance-free. Where crevices, standing water, low oxygen and chloride occur together, even 304 pits. Pads, sealing and drainage slope that remove crevices and water do more than a higher grade alone.

Galvanized carbon steel costs less, but once the coating is damaged rust starts at the damage and stains the grating surface. In a corrosive site, the saving on fasteners is often less than the labour to replace them later. Installation detail is covered in the fastener installation practice

Ask for material certificates and the grade on the purchase order rather than the word stainless. 304 and 316 look alike and cannot be told apart on site, so the grade has to be on paper to be inspected.

Galvanic corrosion and isolation

Stainless in contact with aluminium or carbon steel forms a galvanic couple in wet conditions, and the less noble metal corrodes first. A stainless clip on an aluminium beam without isolation leaves white corrosion products around the bolt hole.

Isolation is simple: nylon washers, rubber pads, insulating sleeves, or proprietary clips with an insulating liner. A pad between fastener and grating is also worth fitting to limit stress concentration and wear.

Isolating materials must resist temperature and chemicals. Ordinary rubber swells and ages in solvents or at high temperature, so check compatibility rather than buying on price.

Bolts, torque and washers

Bolt diameter follows the load and the hole position. Do not use high-strength bolts to force FRP down; crushing the panel is the most common site failure, and FRP needs distributed pressure rather than higher preload.

Torque to the maker's figure. Without data, start low with a torque wrench and watch the washer compression and the grating surface, stopping at once if whitening or a crack appears.

Washers need enough area. An ordinary flat washer is often too small and concentrates pressure; a large-diameter washer, a square plate or a rubber-backed washer spreads the load over more material.

Spring washers do little to lock a joint on FRP. Nylon-insert nuts, thread adhesive or the maker's own locking arrangement are more reliable.

Slotted holes and thermal movement

FRP expands more than steel, so a long panel fixed at both ends will buckle or split as temperature rises. The effect is most visible on dark grating and outdoor platforms.

The usual detail is one end fixed and the other on slotted holes, or clearance at every hole with the clips providing location. Slots run along the panel length, with the hole two to four millimetres larger than the bolt, calculated from panel length and temperature range.

Holes and slots must avoid the critical section of a load bar. Edge distance follows the maker's requirement; if whitening or a crack appears at a hole, enlarge the washer or change the fixing method. The relation between grating specification and holes is set out in the grating specification page

Trim and seal the edge of every slot as well, because the inside of the hole also exposes fibre. Many crews seal the panel edge and forget the hole wall, and water enters from inside the hole.

Fixing to different support structures

On steel beams, clips can be fixed directly, provided welds and burrs do not bear on the grating; on aluminium beams, isolation is essential; on concrete, use cast-in items or chemical anchors, with the anchor itself protected against corrosion.

On timber beams, add washers to spread the pressure. The joint loosens as the timber dries, so re-tighten once early in service and check again at each inspection.

Increase fixing density at panel overlaps, edges, openings and stair mouths. These areas carry concentrated load and frequent traffic, and insufficient fixings show up there first. Typical walkway fixing layouts are shown in the platform walkway applications

Keep the contact face between fixing and beam clean. Paint, mortar residue and welding spatter make clips bear unevenly, and cleaning before installation is easier than rework afterwards.

Inspection and maintenance

Check periodically for loose clips, rusty bolts, cracks and whitening at holes, and lifted or shifted panels. Shorten the interval in corrosive sites.

Replace fasteners with the same material and size, and never mix carbon steel with stainless. Mixing accelerates galvanic corrosion and makes the next inspection harder to read.

Record fastener type, quantity, position and replacement date. Keep spares to the project's usual allowance, stored dry and apart from other metal parts.

Coastal projects also need attention to dissimilar metal contact. Beyond the isolating pad between a stainless fixing and a galvanized beam, arrange drainage so salty water does not sit on the contact face.

• Loose clip: re-tighten or replace; do not just add a washer.
• Crack at a hole: enlarge the washer or move the fixing, and replace the panel if needed.
• Rusty bolt: check the site chemistry, upgrade the grade or add isolation.
• Lifted panel: check fixing spacing and thermal gaps.