Definition and calculation

Open area is the proportion of the panel plane that is void, expressed as a percentage. It follows from four dimensions: bearing bar width, cross bar width, bearing bar spacing and cross bar spacing.

The approximation is to take the area of one repeating cell, subtract the solid area within it, and divide the two. A molded panel has rectangular cells and is simple to work out; pultruded grating also needs the actual cross bar width.

The published figure is usually nominal, calculated from the standard panel and spacing. Real product shifts a little with mould wear, cutting and edge trim, so acceptance need not argue over decimals, but the calculation basis should be confirmed.

A simple example: a 50 millimetre square cell with bearing bars and cross bars 8 millimetres wide has about 736 square millimetres of solid area in a cell of 2,500 square millimetres, giving an open area near 70 percent. Real product also loses a little to thickened nodes and surface treatment, so published data usually sits slightly below the geometric figure.

Molded grating geometry

Molded grating is formed in a mould, with common openings of 38 millimetres square, 38 by 102 millimetres and 50 by 50 millimetres. Smaller openings mean more solid material per square metre and a lower open area.

A fine mesh is steady underfoot, holds small objects and stops heels entering, but it drains slowly, traps mud, is harder to clean and weighs more. A coarse mesh does the opposite: fast drainage and low weight, but poor fall protection.

Molded open area commonly falls between about 40 and 80 percent. Do not choose from the percentage alone; look at the shape and size of a single opening, because fall protection and heel entrapment depend on the individual cell, not the total ratio.

Pultruded grating geometry

Pultruded grating is built from bearing bars and cross bars, with bearing bar spacing commonly 20, 30 or 40 millimetres and cross bar spacing commonly 100 or 150 millimetres. Its openings are long slots, running along the cross bar direction.

Pultruded open area is usually higher than molded, commonly 60 to 85 percent. Slots drain very fast, which suits storm rain, washdown and heavy debris, but small tools drop straight through.

Bearing bar spacing is the critical dimension. Closer bars give steadier walking and better fall protection, but add weight and cost and reduce open area. The choice depends on who walks there, what they carry and what sits below.

Drainage: more than a hole size

Drainage depends not only on open area but on opening shape, orientation and panel slope. At the same open area, slots aligned with the water flow drain faster because the water does not have to divert around cross bars.

Standing water on a platform is usually not a shortage of open area but a support beam standing above the panel underside, an edge board blocking the outlet, or a membrane below the grating with no drainage path left.

Storm duty has to be checked against local rainfall intensity and catchment area, not judged from the open area figure. Roofs, open platforms and wash bays need an overflow path so short-term water cannot exceed the design level.

For platform drainage and support detailing, see FRP grating work platform solutions

Ventilation and heat

Open area sets the air exchange between the space above and below the panel. In enclosed buildings, pump pits, tank tops and service ducts, poor ventilation lets flammable gas, heat and odour accumulate.

High open area helps natural convection, but it is not a substitute for mechanical ventilation. Gas density, leak rate and room volume drive the ventilation design; the grating only reduces airflow resistance.

Grating around hot equipment also has to handle radiant heat and rising hot air. Too little open area creates a local hot zone that affects the equipment below and the people walking past.

Open area also affects drying. After washdown or rain, water under the panel evaporates through air movement, and a finer mesh with more cross bars dries more slowly; long wet periods speed fastener corrosion and biological growth.

Debris passage and blockage

Debris passage cuts both ways. Large openings let mud, ice and packaging fragments fall through and keep the platform clean, but the same openings let wrenches, bolts and parts drop onto people or equipment below.

Debris in chemical and mining duty is often sticky and cakes on the cross bars. However high the open area, tight cross bar spacing still blocks it, so the number of cross bars matters as much as the open area.

Where rotating equipment, piping or a walkway lies below, choose a finer mesh or fit baffles and catch trays under the grating. High open area and fall protection are conflicting aims and one has to give.

Cleaning has to match the mesh. Mud trapped in a fine grid is hard to move with a broom and usually needs a pressure washer, which in turn drives debris behind the cross bars where it sets harder, so the routine cleaning method belongs in the selection.

Heels, small objects and walking comfort

Heel safety depends on the size of a single opening, not on open area. A stiletto heel can be around ten millimetres across, so any opening wider than that carries an entrapment risk.

Industry normally means safety footwear with a broad sole and low entrapment risk, but visitor routes, connecting walkways and public platforms see every kind of shoe, so selection should consider the least favourable user.

Small objects behave the same way. Screws, washers, gauges and phones can all fall through, so where work is done above precision equipment, specify a fine mesh or lay temporary boards over the work area.

Open area also changes how the floor feels. The larger the opening, the smaller the sole contact area, and long walks become more tiring; trolley and ladder wheels also drop in or jam more easily.

Writing it into the specification

The specification should state open area, opening size, bearing bar spacing and cross bar spacing. Open area alone is not enough, because the same percentage can describe completely different geometry.

State the service scene as well: who walks there, whether trolleys are used, what sits below, and whether storm rain or washdown occurs. That information decides the trade-off in opening size.

A change in open area alters self-weight and bearing bar section, so span checks follow FRP grating support spacing

For mining and other duties with heavy debris and wash water, see FRP grating in mining

Panel type, mesh size and edge trim are all set out in the FRP grating specification