Grate Loading and Capacity selection chart
When selecting an access cover or grate for trafficable areas, consideration should be given to the maximum load that the cover will be subjected to in everyday use. Access covers and grates are given load classifications designated by the classes A, B, C, D, E, F and G. Load classifications indicate how much force a particular cover can withstand.
The table below shows load classifications as set out in Australian Standard 3996 (Access Covers and Grates).
Selecting an appropriate cover or grate also depends on the where it will be installed and the various types of traffic that it may receive. Whilst load classifications provide a guide, nominal loads are based on stationary loads, which poorly simulate real world traffic conditions.
Areas subject to moving traffic will exert a dynamic load which will try to twist a cover out of position. If traffic moves quickly or turns on a cover, a higher load class should be selected and extra design features should be sought such as slotted drainage, locking grates down with bolts or steel reinforcement.
Small or Solid Wheels
Small or solid wheels concentrate a heavy load to a very small point where it is transferred to the cover. This can be the case with forklifts and rubbish skips which often feature solid wheels and cause massive damage to underspecified covers. A grate or cover with a higher load class will be required in these situations.
Large or Pneumatic Wheels
Large or pneumatic wheels will spread a load over a larger area and exert a lower stress on the grate or cover. This is why a heavy tractor can be classified in load class B with smaller vehicles.
Areas at the base of a steep decline
Covers at the base of a decline will experience short periods of increased load as a large portion of the vehicles total weight can be transferred to the front wheels. The cover will be under high stress and a higher load class will be required in these situations.