A designer must understand the lead bearing capacity of that bridge, it is a fundamental requirement. The bridge must not be overloaded to a point that it collapses which might have a bearing on the overall success of an operation.
To provide assistance to bridge designers, tables of weights of commonly used equipment combinations have been compiled since the early days of military bridging. As far back as 1887, the ‘Instructions in Military Engineering’ manual listed likely weights of things as diverse as heavily laden elephants and cavalry in marching order. These allowed assumptions to be made about the safe operation of a bridge.
A follow on simplification was the concept of a bridge classification.
During WWI this was refined and loads were divided into 4 classes, light, medium, heavy and tank, with each one having a defining weight, spacing and where relevant, axle loads.
Despite this advance an improvement was sought and in 1928 the Royal Engineer Board started collating details on military loads and the increasing number and type of vehicles in service. The details were used to further categorise loads into Light (Brigade), Medium (Division), Heavy (Corps) and Super Heavy but the fundamental problem remained, matching bridge capacity to vehicle weight as they had to be looked up in tables before allowing to cross.
This was cumbersome and time consuming and relied on every type of vehicle appearing in the tables so in 1938 the system was looked at again by the Royal Engineers Board.
The elegant and simple solution they came up with was to invent a scale, or classification, related to weight but crucially, not only weight.
Each bridge type was allocated load class number and each vehicle was also given a load class number.
Instead of looking up and cross referencing a vehicle against a bridge classification a simple comparison of load class was performed, if the numbers matched or the vehicle was less than the bridge classification then it could pass.
A spacing of 80ft was assumed at the bridge classification took into account bending moment and other factors, it was not simply a weight (this is a key distinction)
Instead of weights of vehicles, each vehicle had a class, these starting at 3 and moving up to 24 in regular intervals.
If a vehicle’s load class was smaller than the bridges load class then it could cross and to assist with the rapid cross checking a standardised series of markings was designed, both bridge and vehicle had the marking in the same colours so a driver could simply compare the bridge sign with that painted on his vehicle and make the decision whether to cross without reference to bridge commanders or complex tables.
Civilian bridges were also classified using the new scheme and it ultimately developed into a NATO standard, the Military Load Class (MLC)
The NATO system uses 16 hypothetical classes; 4, 8, 12, 16, 20, 24, 30, 40, 50, 60, 70, 80, 90, 100, 120 and 150 as defined in STANAG 2021.
The number is merely an indicator although weight plays a significant part in its definition and a vehicles and bridge/ferry/raft MLC are calculated differently. Vehicle MLC’s use weight, wheelbase, axle loading, spacing and contact area
Because wheeled vehicles tend to be longer than tracked vehicles and therefore exert differing concentration of load and bending moment on bridges often have both a wheeled and tracked MLC.
Bridging equipment MLC calculations include weight, spacing of vehicles, safety factors and dynamic effects or impacts.
The NATO MLC system also defines normal crossing conditions but allows for a reduction of safety factors in emergency or tactical conditions. It also describes a method for defining a temporary vehicle classification if none exists.
The 16 classes are defined by 16 hypothetical wheeled and 16 hypothetical tracked vehicles of various weights, sizes and axles. It must be understood that MLC is not a vehicles weight but simply a reference number.
Each hypothetical vehicle has the bending moment and shear forces calculated and plotted on a graph at 1m intervals up to 100m, assuming a single span simply supported bridge. For other conditions the curves are re-plotted.
Classification of Vehicles
Vehicle and trailer combinations form part of the 32 definitions for tracked and wheeled vehicles. The standard defines treatments for exceptions such as towing and vehicles outside of the basic 16×2 classes.
At the end of these complex calculations the MLC number is derived and rounded up to provide a safety factor.
A temporary MLC can be calculated by multiplying the vehicles mass in metric tonnes by 1.20 for a tracked vehicle and 1.25 for a wheeled vehicle.
Classification of Bridges
Different methods are used for single and multiple span military bridges, civilian bridges, rafts, ferries and floating bridges that take into account a diverse range of factors.
Normal, caution and risk classes are defined for each bridge that recognises safety factors may be eroded in wartime but these may also need more restrictions placed on crossing speeds, for example.
If anyone actually fancies a look at STANAG 2021, click here
The document provides a couple of interesting examples; the Leyland DAF 4 tonner has an MLC of 11, the Leopard 2A5 an MLC of 66 and the M3 Rig vehicle an MLC of 26.
Using the expedient method of calculating MLC, the magic number for FRES SV Scout based on an initial in service weight of 34 tonnes is 41, too large for the in service Air Portable Ferry Bridge or Class 30 trackway for example, which have an MLC of 35 and 30 respectively.
There is also a joint US, UK and German design and test code for military bridging and gap crossing equipment, click here to read, at 116 pages it demonstrates the inherent complexity of getting stuff over a gap!
From the early work and subsequent refinement in the years prior to WWII by the Royal Engineers Board a simple and robust classification system for bridges, ferries and raft is now a NATO standard.
Yet another unsung achievement
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