Bridge Plates... interesting topic! From our research, we come across several forum post over the internet, and here is a summary...
In 1909 it seemed to be the responsibility of a general staff officer at the approach to a bridge to give instructions to commanders of units about the suitability of the crossing. By 1911 the responsibility of giving the bridge a load classification passed to the engineers in the unit constructing the bridge. They were required to indicate at each end of the bridge the greatest permissible load. The examples below take little account of motor transport:
“Bridge to carry infantry in fours”
“Bridge to carry infantry in file”
“Bridge to carry guns not heavier than 18-pr”
“Not for animals”
“Bridge for all arms. No road engines”
By 1924 a simpler classification was in use:
“Foot-bridges”- Infantry in single file.
“Pack-bridges” - Infantry in single file, cavalry in single file, pack animal transport.
“Medium bridges” - Infantry in fours, cavalry in half-sections, horsed transport, cars, armoured cars, light & med
artillery, 3-ton lorries & all ordinary MT up to 5½-ton axle load.
“Heavy bridges” – Heavy artillery, tractors & MT up to 16-ton axle load, tanks up to 18 tons.
“Super-heavy bridges” – Axle loads & tanks in excess of above loads.
By 1935 the classification in use was refined further:
“Assault”- Infantry in single file.
“Pack” - Infantry in single file, cavalry in single file, pack animal transport.
“Light” - Infantry in fours, cavalry in half-sections, vehicles & weapons with forward units,
horsed transport, horse & field artillery, cars, ambulances, lorries & AFVs max 4½-ton weight.
“Medium”-All loads normally with a division, 4-whld MT up to axle load of 5.7 tons, 6-whld vehicles max
axle load 3.6 tons or a rear bogie of 7.2 tons.
“Heavy” - All loads normally with an army in the field. All 4-whld MT, 6-whld MT up to axle load of 8.25 tons or
bogie load of 16.5 tons, tracked vehicles with sprung tracks up to 19.25 tons, provided track bearing length is not less than 13 feet.
“Super-heavy”- Vehicles whose weights exceed “Heavy”
By 1939 all bridges where classed according to the maximum weight of a vehicle in tons giving the following Classes 5, 9, 12, 18 or 24. It should be noted that bridges classes apply not just to fixed bridges but to pontoons bridges as well.
Notes on M.T. Administration W.O. Code No.11060 issued in November 1954 suspended Bridge Classification Signs in the U.K. In overseas theatres this sign could still be used at the discretion of the C-in-C and G.O.C-in-C. where considered necessary. In fact such classification was only ordered in Bridge Classification Signs (Overseas Only) W.O.Code No.8229. Although vehicles at that time were classed in multiples of 1 ton, it was not just the laden weight. They were calculated from data which included the load capacity, tyre sizes etc. Bridges classes were extended as follows:
5, 9, 12, 18, 24, 48, 50, 60, 70.
Bridge Load Classification (STANAG 2021) Introduced 1960-63
The bridge load classification was said to originally have represented by the rounded up weight of the vehicle in tons. "The new system relates to the characteristics of the vehicle which includes overall weight, number of axles, distance between axles, axle load (unladen & laden), tyre size, ground clearance etc and bears no direct relationship to the former system."
It must be remembered that is a STANAG, which is a NATO Standard applicable to many countries. The UK had already taken some account of factors other than weight. What changed in the U.K. was that the mechanism for calculating a figure was more comprehensive and now standardised with other NATO countries.
The classification of bridges was also changed to take account of the effects that particular vehicles would have on the bridge. However the application of the system is the same as before, in that only a vehicle load class less or equal to the bridge load class number may cross the bridge. The load class number is based on vehicles travelling at normal convoy speeds at a spacing of 100 feet. The classification also relates to ferries, and takes into account an assessment of the state of the approaches to the crossing to produce an overall classification.
There was no point in UK based vehicles having a Bridge Class displayed, as the Joint Service Road Transport Regulations JSP 341 states that: The UK will not prepare classification signs for civilian bridges and ferries in UK territory in peacetime.
