FRP Grating Thermal Expansion and Joints

A panel will not find its own room to move; the joint has to be designed

Peter Bian November 10, 2026 6 min read

FRP Grating Thermal Expansion and Joints

Why steel habits do not transfer

Steel grating is normally welded or bolted hard at both ends, relying on the ductility of the steel to absorb small movements. FRP has no such ductility: when thermal movement is restrained it does not yield, it cracks or pulls the fasteners out.

FRP is also anisotropic. Its coefficient of expansion along the fibre and across it can differ by two or three times, so a long panel does not simply get longer with temperature, it can also warp slightly.

Fixing should therefore follow a fixed end and a sliding end, with expansion joints between sections and fasteners that allow small movement. Typical platform practice is shown under FRP grating work platforms

Coefficients and direction

Steel expands at about twelve parts per million per degree Celsius and aluminium at about twenty-three; FRP generally falls between eight and twenty-five, depending on resin, glass content and direction.

Pultruded grating usually has a lower coefficient along the bearing bar, because continuous fibre restrains the resin; across the bar the resin dominates and the coefficient is clearly higher. Molded grating has random fibre, so the two directions are closer but both tend to be high.

These values have to come from the maker's data; no single figure covers every product. Change the resin or the glass content and the coefficient changes with it.

Calculating the movement

Movement is the coefficient times the length times the temperature change. A six-metre panel with a forty-degree swing at a coefficient of twelve parts per million moves about 2.9 millimetres; at a coefficient of twenty it moves about 4.8 millimetres.

One panel looks modest, but ten or more panels in a row over tens of metres accumulate a considerable total. A long walkway has to be calculated on its full length, with expansion joints at intervals.

The temperature difference is between the highest and lowest surface temperature, not the air temperature. A dark outdoor panel can run twenty to thirty degrees above air temperature in summer and has to be taken at the lowest surface temperature in winter, so the real swing is often wider than the weather data.

Sizing the gap

The joint width has to cover movement from the installation temperature up to the maximum and down to the minimum, taking the larger and adding a margin. If installation happens at the mid-point, half the movement goes each way; if it happens at an extreme, the gap on one side grows accordingly.

Too small a gap drives adjacent panels into each other, causing bowing and sheared bolts; too large a gap becomes a trip hazard and a water trap. Widths commonly sit between ten and twenty millimetres, subject to calculation and the maker's advice.

The usual cover detail is a bridge plate over the joint, fixed at one end and free to slide at the other. Fixing both ends of the cover will crack it under its own thermal stress. How span and joints interact is covered in support spacing and deflection for FRP grating

Fixed and floating ends

Every panel, or every continuous run, needs at least one fixed end and one floating end. The fixed end is bolted with standard hold-down clips; the floating end uses slotted holes or clips with clearance so the panel can slide along its length.

The fixed end is best placed on the side where it will not block drainage or traffic, usually against a wall or a structural beam. Where several panels join, the fixed ends face the same way so thermal movement does not push them into each other in the middle.

Corners, T-junctions and the perimeter of equipment foundations are the places that fail first. Restraint is high and movement has nowhere to go, so these locations need a sliding detail of their own or a deliberate gap.

Slotted holes and hold-down clips

The slotted hole is the usual sliding detail: an elongated hole along the direction of movement, with the bolt free to slide in it. Slot length is the calculated movement plus the bolt diameter plus about ten millimetres, and slot width is slightly over the bolt diameter.

A hold-down clip should hold the panel without clamping it dead. A common detail uses a clip with a disc spring or a rubber pad, tightened to the maker's torque, which resists wind uplift without fully restraining movement.

Slot edges need sealing as well, so water does not enter the cut. A slot removes more material than a drilled hole, so the radii at each end must be kept and the slot must not be cut square.

What happens when both ends are fixed

With both ends fixed, thermal movement turns into compressive stress. FRP is weaker in compression than in tension, so panel ends buckle, the grating at a joint is pushed apart, and star cracks appear around fastener holes.

This damage often appears in the first summer or the first winter, and after a few temperature cycles the cracks extend until the panel fails. Early cracking is not obvious, and by the time it is noticed a local repair is usually impossible.

There is a quieter failure as well: the grating does not crack, but the clips crush shallow depressions into it, the bolts loosen, the panel vibrates in wind, and the fixing points fail one by one. That kind of problem takes a long time to trace.

Choosing the installation temperature

Installation is best done near the middle of the annual temperature range, so movement to both the maximum and the minimum stays small. Avoid installing long walkways at midday in high summer or during a cold snap.

Record the temperature at installation as part of the handover file. Where the actual temperature differs greatly from the design assumption, the gap and slot length have to be recalculated rather than built to the drawing as issued.

Indoor and outdoor cases differ as well. A conditioned factory keeps the grating near room temperature, while an open platform has to be designed on surface temperature, and the two design swings can differ by a factor of two.

Specification and site briefing

The specification should state the design temperature range, the expansion coefficient used, the joint width, the position of fixed and floating ends, the slot dimensions and the fastener torque. A note to install to the maker's standard usually gets built the steel way.

The site briefing has to explain the sliding end and mark it on the panel. Many failures are not design errors at all; the installer simply did not know which end was meant to move. Full parameter wording is in the FRP grating specification guide