A span is not one fixed number

How far FRP grating can span is not a question with a single number for an answer. It depends on the grating type, thickness, load-bar depth, load type, permitted deflection and safety factor. Tighten the deflection limit on the same panel and the permitted span shrinks.

The common mistake is to ask only how far it spans, without stating load and deflection. The right order is to fix the design load and deflection limit first, then read the load table, then set the support beam spacing. Working in the reverse order usually means rework.

This article covers the general method for moulded and pultruded grating. Actual values must come from the product load table and the project calculation; nothing here replaces a structural check.

How to read a load table

A load table usually gives uniform and concentrated loads at different spans. Uniform load appears in kg/m2 or kN/m2, concentrated load in kN or kg, with the loaded area and load position stated.

Table values normally include a safety factor, but the factor differs between makers, and the conditions for walkways, access platforms and industrial floors differ too. Before reading a table, confirm the safety factor, load combination and intended duty, and never compare tables from different makers directly.

Table values also assume a particular support condition: simply supported at both ends, with load at midspan. Cantilevers, continuous multi-span runs and local supports cannot use the table directly and need a separate calculation for the actual support layout. For the basis behind table values, see the grating load capacity tables

Deflection limits

Deflection is usually the governing criterion in span selection. Walkways commonly use L/100, or a stricter limit set by the project; precision equipment, vehicle traffic and platforms with drainage requirements may demand less movement.

Be clear which load the limit applies to. Some projects control live-load deflection, some control total deflection, and some call for L/200. At the same span, controlling total deflection gives a smaller allowance and needs closer beams.

Less deflection is not automatically better. Closer beams add cost and installation work and reduce headroom; the sensible approach is to meet the service requirement first, then compare the cost of the options.

Deflection can be checked roughly on site with a level or a taut line, but acceptance should rest on the calculation and a measured load. If deflection clearly exceeds the calculated value, check the supports and fixings before blaming the grating.

Concentrated loads and wheel loads

A concentrated load often governs the span more than a uniform load. A modest force over a small area can produce local stress and local deflection beyond that of the same total uniform load, so check the span for the concentrated case.

Wheel loads must use the actual wheel pressure and contact area. Small hard wheels are more severe than large pneumatic ones, and the wheel pressures of a hand cart, a forklift and equipment castors differ widely; do not substitute one for another.

The load position matters as well. A concentrated load near a support produces little bending moment, while midspan is the worst position. Design checks the worst position, and in service heavy items should not be stored at midspan for long periods.

Setting the beam spacing

In practice, work back from the load table: assume a beam spacing, read the permitted load at that span and compare it with the design load; if it is short, reduce the spacing or move to a thicker grating with deeper load bars.

Beam spacing must also suit the load-bar direction. Load bars should run perpendicular to the support beams, or as the maker instructs; reverse the direction and the span capacity drops noticeably, especially with moulded grating.

Grating over several beams could in theory be treated as continuous, but site joints between panels, clips and thermal gaps weaken the continuity. Check conservatively as simply supported, and add a continuous-beam check only for important platforms.

Once the spacing is fixed, put the span, load, deflection limit and grating type into the drawing notes, so purchasing does not order by type number alone with no support condition. The link between grating type and load bars is set out in the grating specification notes

Cantilevers and end support

The permitted length of a cantilever is usually much less than a midspan span, with the ratio taken from the maker's data. A cantilever has no support at the far end, so moment and deflection concentrate there and it cannot be estimated as half a standard span.

End support needs enough bearing length. If the grating sits on too little beam width, fasteners end up over air and the edge lifts or cracks under load.

Cantilevers at platform edges, stair openings, equipment openings and inspection hatches need a separate check. These positions often carry concentrated loads and frequent foot traffic, so the margin should be larger than in ordinary areas.

Installation direction and support detail

Support beam tops must be level; differences in height introduce extra stress into the grating. Level them with shims or adjusting bolts before fixing, and do not use fasteners to pull a panel flat.

Fix the grating to the beams with clips at the maker's recommended spacing, closer where wind, seismic or vibration loads apply. Too few fixings let the panel move and chatter, and the holes wear larger.

Long panels expand and contract with temperature. The fixing must allow longitudinal movement, with one end fixed and the other on slotted holes, or a gap calculated for the range, so panels do not jam and buckle. Typical walkway layouts are shown in the work platforms

• Load bars perpendicular to the support beams; never reverse the direction.
• Level the beam tops before fixing; do not pull the grating into place.
• Fix one end of a long panel and slot the other.
• Increase fixing density at edges, openings and cantilevers.

Checking and site verification

After the calculation, run an independent check on four points: whether the design load is complete, whether the span matches the beam spacing, whether the deflection limit uses the right load combination, and whether the fixing allows thermal movement.

Where the project requires it, carry out a load test and watch midspan deflection and any unusual noise; in service, check periodically for sag, distortion and loose fixings, and assess anything unusual promptly.

Re-check the capacity before changing the use. Adding equipment, storing materials or allowing vehicles all change the load case, and the original design should not be assumed to have spare capacity.

File the calculation, the load table and the site inspection records together. Before altering a platform, adding load or replacing panels, read the original records and decide whether a new calculation is needed rather than judging from memory.