Department of Transport Standard BD 37/88 deals with loads for highway bridges. To allow for dynamic loading effects, British Standard BS4008 requires that load factors specified in BD 37/88 are multiplied by a further factor of 2 for highway designs and 1.5 for agricultural designs, as outlined above.

All cattle grids manufactured by Ian Harvey Fabrications Limited are designed to specified maximum BD 37/88 ratings. The limiting factor on all designs is the strength of the steel grid and is set by the elastic limit of steel to BS4/EN10210.

BD 37/88 describes two load situations, HA loading and HB loading:

HA loading:
HA loading refers to normal traffic loads and depending upon the grid design, can be taken to be 30, 80, 240 or 300 kN per metre of traffic lane (ie. measured along the length cattle grid), spread uniformly across the width of the grid.

The total HA load capacity is simply the HA load rating multiplied by the length of the grid. A 80kN/m, 2.6m long TrackWay grid, for example has a total HA load rating of 208kN. The short length of the grid in relation to HGV axle configurations mean that the grid cannot support the full weight of a heavy goods vehicle. Hence, the HA load figure is not the same as the maximum permitted vehicle load!

All cattle grids manufactured by Ian Harvey Fabrications Limited are rated at 30kN/m or greater. Maximum recommended gross vehicle weight limits for the different designs are outlined here: Recommended Load Limits.

HB loading:
HA loading refers to abnormal traffic load and for the purposes of design, refers to a 4 axle vehicle arranged as a pair of twin axles. The overall length of the HB vehicle is taken as 10, 15, 20, 25 or 30 m. In each case, the axle spacing of the front and rear pair is 2m and the overall vehicle width is taken as 3.5 m.

A single unit of HB loading corresponds to 10 kN per axle, or 40kN per HB vehicle.

The Stock-Stop HighWay grid and the Heavy Duty Driveway grid manufactured by Ian Harvey Fabrications Limited are rated to 30 Units of HB loading. This corresponds to a single axle load of 300kN and a total vehicle load of 1200kN. Maximum recommended gross vehicle weight limits are outlined here: Recommended Load Limits.