What a load table answers

The load table a manufacturer publishes cross-references three things: span, load and deflection. It answers one specific question: at a given support span, how much load can this panel carry while deflection stays within the stated limit.

It is not the upper limit of the product, and it is not a general table for comparing one maker with another. Every table rests on a particular support condition, load type and deflection limit, and once the premise changes the numbers no longer transfer. Molded and pultruded tables differ especially in basis, as covered in molded versus pultruded grating

The three axes of the table

The first axis is span. Tabulated spans commonly run from 300 millimetres to 1.5 metres and beyond, stepping by 50 or 100 millimetres. That span is the clear distance between supports, not the panel length and not the centre-to-centre spacing, so confirm the basis before reading anything else.

The second axis is load type. Uniform load is given in kilopascals or kilograms per square metre, concentrated load in kilonewtons or kilograms with a stated contact area, and line load in kilonewtons per metre with a stated position. The three do not convert into one another.

The third axis is deflection. Some tables carry deflection as a separate column; others compute an allowable load from one fixed deflection limit and publish only the result. The second style is quicker to read, but a different deflection requirement means asking for the data again.

Working with uniform load

Uniform load is the most used column. People, equipment and small material stocks on walkways and platforms are normally converted into a uniform figure first. Maintenance surcharges, temporary storage and snow belong in that conversion; sizing on normal foot traffic alone usually comes out low.

A tabulated uniform load assumes the load covers the whole span. If the same total weight sits in a band at midspan, the bending moment is higher, so the case has to be checked as a concentrated or line load instead of being waved through because the total is under the limit.

Tabulated values also assume the full panel width, with several bearing bars acting together. On a narrow strip only two or three bars remain, each carries more, and the full-width figure stops applying. Typical platform load combinations are set out under FRP grating work platforms

Concentrated and line load

Concentrated load is normally tabulated over a square contact area, commonly 100 millimetres square. A smaller footprint is more severe for the same load, because shear concentrates on a few bearing bars and the local limit is reached first.

Check the real contact area before using the column. Equipment feet, scaffold standards and jacks often bear on less than the assumed patch, so either ask the maker for data at a smaller area or add a spreader plate to distribute the load.

Line load covers loads distributed along a line, such as railing base plates, pipe supports and temporary guardrail fixings. Direction matters: running along a bearing bar and running across it are two completely different structural cases, and a good table lists them separately.

Where the safety factor sits

The allowable loads in a table usually include a safety factor, although the table does not always say so. A common approach divides the short-term failure load by two to three, with higher values for heavy, impact or crowded duty; the exact multiple follows the maker's standard and the project specification.

That factor covers short-term static load only. Sustained load, repeated load, elevated temperature and chemical attack all consume the remaining margin, and none of them normally appear in the table, so each has to be applied as a separate reduction.

Deflection usually governs first

A common walkway limit is span divided by 160, or 6.4 millimetres, whichever is smaller. The limit comes from how the panel feels underfoot and how it drains, not from the point of material failure, so a panel that passes on strength can still fail on deflection.

Deflection is extremely sensitive to span. For a simply supported span under uniform load it rises roughly with the fourth power of the span, so going from one metre to 1.2 metres can more than double it rather than adding twenty percent.

That is why the same panel feels generous on a short span and needs more depth or closer supports on a long one. How support spacing drives deflection is covered in support spacing and deflection for FRP grating

Interpolation and units

Tables step the span at fixed intervals, so a real span between two rows needs interpolation. Deflection is not linear in span, and straight-line interpolation between rows tends to be unconservative, most visibly towards the long-span end.

The safe approach is to take the more conservative of the two adjacent rows, that is, read the table at the longer span, or use the maker's calculation tool. If you interpolate by hand, check at least three points so the direction of the curve is known.

Unit conversion has three usual traps: kilopascals against kilograms per square metre, kilonewtons against kilograms, and inches against millimetres. One kilopascal is about 102 kilograms per square metre and one inch is 25.4 millimetres; after converting, substitute back and check that the magnitude still makes sense.

What the table leaves out

The first omission is support width. Grating needs a sufficient bearing length on the beam; too little crushes or slips locally, and the value is normally absent from the table and has to come from the installation manual. The second is load duration: tabulated figures come from short-term tests, and deflection keeps growing under sustained load.

The third is temperature and medium. Heat reduces the load capacity of the resin and corrosion thins the section, and neither is part of a standard load table, so both need a reduction for the actual duty. The fourth is cutouts and notches, which sever bearing bars and force the load to divert around the opening, so the tabulated figures stop applying.

Fixing detail changes the result as well. Too few hold-down clips, bolts over- or under-tightened, or an untrimmed edge all push actual behaviour away from the table, and installation quality has to be checked separately at handover.

Writing it into the specification

The specification should define the span basis, the load type and magnitude, the deflection limit, the required safety factor, and the support width and fixing method. A requirement that says only that capacity must not fall below a figure leaves the supplier unable to choose a basis, and the quotation defaults to a standard product.

Where concentrated or line loads exist, state the contact area, the position and whether they combine with other loads. Those details decide between a deeper panel and closer supports, and they drive cost directly. Full parameter wording is in the FRP grating specification guide

• Fix the span basis and deflection limit before reading a table.
• Check uniform, concentrated and line load separately.
• Take the conservative row when interpolating, and back-check units.
• Confirm support width, sustained load, temperature and cutouts separately.