Structural Connections & Fastening
Three Ways to Hold a Panel Down
The connection between grating and support is where most on-site failures start — usually from a well-intentioned but over-tightened fastener. Pick the right connection type for the load, vibration and code requirement.
| Method | How It Works | Best Used For |
|---|---|---|
| Mechanical (clip / bolt) | Stainless G-clip or through-bolt locates the panel to the support flange | General industrial; disassembly required |
| Adhesive (bonded) | Panel edge or underside bonded to support with structural adhesive | Vibration, fatigue, no visible fasteners |
| Hybrid | Adhesive primary bond with mechanical clip as safety backup | Critical decks, fatigue-prone machinery |
G-Clip Selection and Spacing
The G-clip is a U-shaped stainless strip that hooks over the grating edge and bolts to the support flange. It transfers uplift and lateral movement while allowing the panel to slide longitudinally as temperature changes.
| Rule | Value |
|---|---|
| Clips per panel | Minimum 4 (one near each corner); more where spans are long or loads heavy |
| Clip spacing along support | No more than 1219 mm (48 in) between clips on any support line |
| Edge distance (clip to panel end) | At least 50 mm from the cut or molded edge of the panel |
| Material | SS304 indoor dry; SS316 in coastal, chemical or wash-down service |
| Function | Lateral location plus uplift restraint — not a structural clamp; allow sliding |
Through-Bolt Design Rules
Where panels are mechanically fixed through the support, hole size, edge distance and torque must all be controlled — FRP is far less forgiving than steel at the fastener hole.
| Parameter | Guidance | Reason |
|---|---|---|
| Hole diameter | 1.5x bolt diameter (e.g. 13 mm hole for M8) | Provides thermal sliding room |
| Minimum edge distance | At least 2.5x bolt diameter; keep at least 50 mm from panel edge | Prevents shear-out failure |
| Minimum pitch (bolt-to-bolt) | At least 3x bolt diameter | Avoids net-section tension failure |
| Washer OD | Minimum 25 mm (use 38 mm where bearing is critical) | Spreads load; prevents pull-through |
| FRP bearing strength | Typical 150-200 MPa (indicative) | Check bearing stress against this |
| Bolt | Torque (Nm) | Note |
|---|---|---|
| M8 | 15-20 | Snug-tight only; do not use impact wrench |
| M10 | 25-30 | Use a torque wrench |
| M12 | 40-50 | Upper limit — FRP cracks if exceeded |
Indicative torque values; always reference the clip manufacturer data sheet. Over-torquing crushes the resin-rich surface and starts a crack.
When to Bond Instead of Bolt
Surface Preparation
Sand the bond area to a clean, dull profile; wipe with acetone to release mold wax and oil. A glossy, unsanded surface gives near-zero bond strength.
Adhesive Choice
Epoxy for general structural bond; methyl methacrylate (MMA) for fast cure and good FRP-to-FRP; polyester for matching the resin system. Follow the adhesive datasheet for open time and cure.
| Parameter | Guidance |
|---|---|
| Bond line thickness | 0.5 to 1.0 mm (use glass-bead filler or shims to control) |
| Cure time | Handle at the manufacturer tack-free time (typically 2-24 h); full cure 7 days at 20 C |
| Preferred for | Repeated vibration, fatigue loading, where holes would weaken the section |
| Not preferred for | Field-critical disassembly or very high temperature excursions |
What the Panel Rests On
| Support Type | Notes | Bearing Length Min |
|---|---|---|
| Steel support | Hot-dip-galvanized or painted; inspect after 6 months for corrosion at the FRP/steel interface | 50 mm molded / 75 mm pultruded |
| FRP support | Pultruded beam (I/W section); matches CTE and eliminates differential expansion stress | 50 mm molded / 75 mm pultruded |
| Concrete support | Embed angle or anchor channel; keep a 6 mm shim gap for uneven concrete | 75 mm minimum |
Shimming
Use tapered or flat shims under panels where support is uneven; never force a panel flat by over-tightening clips.
Level Tolerance
Supports should be level within plus or minus 5 mm over 3 m; out-of-level framing creates eccentric point loading.
Corrosion at Interface
On steel supports, a nylon or EPDM pad between FRP and steel prevents galvanic contact and acts as a shim.
Leave Room to Move
Use Delta L = L x alpha x Delta T. For a pultruded panel with alpha near 12 x 10^-6 / C in the in-plane (transverse) direction and a 50 C seasonal change, a 6 m run moves 3.6 mm — enough to buckle if clamped rigidly.
| Step | Formula / Rule |
|---|---|
| 1 - Calculate movement | Delta L = L x alpha x Delta T (mm) |
| 2 - Choose slotted holes | Long axis along the movement direction; clearance at least Delta L + 2 mm |
| 3 - Anchor one end | Fixed bolts at one support; sliding clips at the other |
| 4 - Expansion clips | Use expansion-type G-clips where lateral restraint is still required |
| 5 - Long runs | Plan expansion joints every 6-9 m; never butt panels tight end-to-end |
How Connections Actually Fail
| Failure Mode | Symptom | Prevention |
|---|---|---|
| Bearing crushing | Cracks radiating from the hole | Washers, more bolts, check 150-200 MPa bearing strength |
| Net-section tension | Fracture across the net section through the hole line | Increase edge distance, reduce bolt pitch, stagger holes |
| Shear-out | Plug of material shears out ahead of the hole | Edge distance at least 2.5x bolt diameter |
| Cleavage | Splitting at the panel edge | Avoid single-row edge bolting; use multiple rows |
| Clip pull-out | Clip bends or the panel lifts off support | More clips, check clip rating for uplift |
Five Mistakes to Avoid
1 - Over-Torquing
Impact wrench on FRP causes crushed resin and cracks. Use a hand torque wrench at the values above.
2 - No Washers
Bolt head pulls through the thin top flange immediately. Always use at least 25 mm OD washers.
3 - Wrong Clip Material
Mild steel clips in coastal service rust in months and stain the FRP. Use SS316.
4 - Insufficient Edge Distance
Shear-out at the edge. Keep at least 50 mm from any cut or molded edge.
5 - No Thermal Allowance
Rigidly bolting every corner buckles panels in summer. Use slotted holes and sliding clips.
Need a Connection Detail?
Send your support section, panel size and material — ZeAllgrate returns a connection detail drawing.
Source: ZeAllgrate (Fiberglass Grating Manufacturers Council / ACMA) design guidelines; ASTM International; ISO 14122; OSHA 29 CFR 1910. Indicative data for specification guidance only — final design verified by ZeAllgrate engineering against certified load/span charts.