Load on a grating panel in service is not the uniform distributed load used to size the bearing bars. It is a wheel crossing a joint, which arrives as an impact at the edge of the panel and is transmitted through the frame and the seating rather than through the mesh.

That is why a panel with ample bar section can still fail: the bars are fine, but the frame has deflected, the seating has spalled the concrete under it, or the hook has worn until the panel rocks. Each of those is an edge detail, and each is usually specified in a sentence rather than a drawing.

Rocking is the visible symptom and the audible one. A cover that moves under traffic hammers its own seating, accelerating exactly the wear that caused the movement, and the noise is what the building owner reports long before anyone measures the deflection.

The preventive measures are unglamorous: make the frame with the panel, make the hook to the kerb that actually exists rather than to a standard detail, and bed the frame properly. None of them appear in a grating specification unless someone puts them there.