What a cutout changes

On a complete panel, load travels from the surface into the bearing bars and along them to the supports at each end. A cutout removes a section of that path: the severed bearing bars can no longer carry load continuously, and the load on either side has to find another route.

A cutout also redistributes stiffness. Bars beside the opening take extra concentrated force, and the governing action changes with position: bending at midspan, shear near the supports. Several holes in a row are worse than the same holes spread out, because one stretch of bar is weakened repeatedly.

Some openings are required by equipment, some are pipe penetrations, and some are cut on site to clear an obstruction. The last kind is most often underestimated, because there is usually no drawing and nobody has checked the capacity afterwards.

How load gets around an opening

Load has two routes around a hole: sideways through the cross bars into adjacent bearing bars, or through a frame or secondary beam added at the edge that carries it directly to a support. Both routes need a real member; the grating cannot be expected to carry the force on its own.

Cross bars are formed integrally in molded grating and transfer load reasonably well; in pultruded grating they are fixed with pins or clips, mainly to hold spacing and take small shear, and cannot be treated as a primary load path.

For a small opening the uncut bars next to it can share the load, but how much they share depends on cross bar stiffness and on the distance to the hole. Once the opening exceeds one bar space, that sharing falls away quickly.

Size and position limits

A common rule is to keep openings within the middle third of the span, away from the supports where shear peaks, to leave at least one full bearing bar between the edge of the hole and the support, and at least one between adjacent holes.

Corners should be rounded. A square notch in a brittle material such as FRP creates a stress concentration, and cracks usually start at the corner and run along the fibre direction. A radius of two to three times the panel depth is typical; the maker's recommendation governs.

There is no universal definition of a large opening; the test is whether bearing bars are severed and whether a frame is needed. Any opening that people can walk through needs edge trim and guarding, not just a hole cut in the panel. The relationship between openings and supports is covered in support spacing and deflection for FRP grating

Basic reinforcement methods

Edge trim is the simplest reinforcement: a ring of FRP profile around the opening, fixed with bolts or stainless clips, tying the cut ends of the bearing bars together. The trim section is normally at least as deep as the bar it replaces.

A header beam is the second method: a cross member on each side of the opening, spanning to the supports and carrying the edge load down directly. Pipe and equipment openings with heavy concentrated loads usually need this.

A third method is local doubling: adding a layer of grating around the opening, or a matching panel bolted into a sandwich. It is quick to install, but the bolt holes are themselves stress raisers, so their number and position need control.

How molded and pultruded cut differently

Molded grating can be cut in either direction, and the grid opens at the cut, so an edge trim is needed to close it. The two directions carry load similarly, so it matters little which side is cut, but the cut should stay away from the stress concentration near the supports.

Pultruded grating cut along the bearing bar only severs cross bars and loses little; cutting across a bearing bar severs continuous fibre and is the case to avoid. Where a cross cut is unavoidable, put a header on each side so the load leaves the cut bar ends into structure.

The two processes have different opening limits, so opening positions should be fixed at the selection stage rather than cut on site. The process differences and cutting rules are compared in molded versus pultruded grating

Sealing cut edges

Cutting and drilling expose glass fibre at the surface. The fibre itself has limited resistance, and water or the process medium creeps along the fibre-resin interface, showing as white marks, fuzzing and delamination as attack spreads inward from the cut.

The treatment is to coat the cut with the specified edge resin, two or three coats with each allowed to surface-dry before the next, so the fibre ends are fully encapsulated. On chemical, coastal and food projects this step is mandatory.

The edge resin must be compatible with the parent resin and cannot be replaced with ordinary paint. Temperature and humidity during application matter too; cold or damp conditions leave the resin under-cured and the seal ineffective. The full fabrication sequence is described in FRP grating fabrication, cutting and sealing

Inspection after field cutting

After cutting, check the position and size of the opening against the drawing, then look for chipped edges, delamination and white cracks. Chipping deeper than a set limit has to be repaired and cannot simply be covered by the seal coat.

Check the distance from the opening to the support, the number and condition of the remaining bearing bars, and that every bolt in the trim or header is present. Bolts must not be over-tightened, because FRP crushes easily around a bolt hole.

Once the seal has cured, check coverage, especially at corners, bolt holes and panel edges, which are the places most often missed. A thickness gauge or a visual comparison with a reference sample can be used where the specification requires it.

Mistakes that crack panels

The first mistake is putting the opening too close to a support. Shear peaks there, the stress concentration at the edge adds to it, and cracks appear quickly. The second is grouping holes too tightly, so the bar between them carries load from both sides and reaches its limit first.

The third mistake is leaving corners square, and the fourth is skipping the seal. The first can crack within weeks; the second shows up only after months of moisture and corrosion, and is the more insidious of the two.

Another common error is fastening FRP with ordinary steel bolts. The stiffness difference is large, tightening concentrates local compression, and the edge of the hole crushes; bolts with large washers and the maker's torque setting are the correct detail.

Drawings and inquiries

The drawing should show the position, size, corner radius, reinforcement method and edge sealing requirement for every opening. A rectangle marked only as an opening leaves the supplier unable to tell whether a header is needed, and both the price and the finished detail drift.

Attach the grating layout to the inquiry, with the bearing bar direction and support positions marked, so the supplier can see whether the opening lands on a bar. Ask for a check calculation where the opening is significant. Full parameter wording is in the FRP grating specification guide

• Keep openings away from supports and inside the mid-span zone.
• Round the corners and leave intact bearing bars between holes.
• Add a header or edge trim whenever a bearing bar is cut.
• Seal every cut edge and inspect it item by item.