Any military commander will always strive for freedom of movement and the opposing commander will equally strive to deny that freedom.
Obstacles might be natural or man-made.
Mobility and counter mobility is the core role of military engineers.
Breaching those gaps, providing the force with that essential mobility and freedom of movement is as old as warfare but as speeds of movement increased, the geographic span of operations equally enlarged and the increasing weight of the machinery of war since the turn of the century military bridging has had to rapidly evolve; faster to deploy, longer and greater load bearing are constant drivers for innovation and ingenuity.
The British Corps of Royal Engineers and associated civilian engineers have been at the forefront, the iconic Bailey Bridge is of course a symbol of this but before and since have been a number of equally revolutionary equipment’s and techniques that have advanced the art, and it is an art.
We might argue the same for many areas of military endeavour but bridging or gap crossing is without a true stand out and yet little exists online to celebrate this.
In understanding the background we need to look at a few underlying issues that govern the subject.
What is different between a military and civilian bridge?
The main difference is the speed of construction, commanders do not want to wait for bridges that will last a century to be built; time is always precious. So it speed that fundamentally governs the design and operation of military bridging. Detailed site preparations and surveys are unlikely to be possible; the military engineer must carry out a short survey and simply get on with it.
There are always exceptions of course; a number of WWII Bailey bridges are still in regular use today and the military bridges constructed by Roman engineers would hardly be characterised as flimsy rush jobs!
Although they go hand in hand and must be mastered by military engineers, mobility and counter mobility are fundamentally different. For mobility to be maintained; roads, bridges and railways must be constructed or those that are already there and damaged by deliberate actions or natural events must be repaired. Counter mobility seeks to deny mobility to enemy forces by destroying that infrastructure, laying mines or demolishing bridges for example.
So it is mobility that the military bridge will improve or enable.
Military bridging is a broad term and can be broken down into a number of equipment sub categories;
Construction Bridging; sometimes called expedient bridging or non equipment bridging will use locally available materials such as timber, stone or other engineering stores such as rope, cable or a block and tackle to create the bridge. Despite the full range of equipment and assault bridging equipment now available this is a core Sapper skill and still taught today.
Equipment Bridging; this uses specially designed and manufactured equipment, the Bailey Bridge is a good example of equipment bridging. The category can be subdivided into floating equipment and dry equipment. Recent terminology describes these as wet or dry support bridges.
Assault Bridging; a relatively new term that describes specialist equipment bridging equipment used in the direct fire zone, more often than not supporting armoured vehicles in the assault phase
These are joined by three terms that define how and where they are used;
Close support Bridging (CSB) assets are mechanically launched bridges that enable gap crossing in the combat zone, with crews protected by armour, assault bridging in the categories above.
General Support Bridging (GSB) and Logistic Support Bridging (LSB) are a mix of mechanically and hand launched equipment with no armour protection and generally used in different locations or phases of an operation. Logistic Support Bridging usually has greater spans and load bearing capacity, used on lines of communication to provide new, replacement or greater capacity bridges.
Infantry bridging are somewhat the odd one out but no less important.
Factors influencing the design of military bridging, especially the equipment and assault type include;
Cost and Ease of Production; if a bridge is to be available at the point of demand it must first be manufactured and purchased from the manufacturer. If it is so complex and difficult to produce it is quite simply, unlikely to be obtained and therefore available. Ease of production and its likely big brother, cost, is therefore a fundamental consideration.
Reliability and Maintainability; bridging equipment will need to be used and reused several times over several years. It must be easily maintained or have minimal maintenance requirements both when it is installed and when it is not being used. It must also be reliable once installed, carrying the desired load until it is no longer needed.
Flexibility; if a bridge can be used for differing gaps then this flexibility will provide many advantages, the Bailey Bridge was so flexible and adaptable it was used for floating bridges, suspension bridges and a full range of spans. The bridge must also be installed in a wide range of locations, bank conditions such as load bearing and slopes will vary, for example.
Speed of Installation; as soon a bridge is built it will be used, armoured vehicles or men and material queuing up waiting for a bridge to be opened is never a good thing so the ability to build a bridge at speed and recover it with equal rapidity is a major factor.
Resistance to Environmental Factors; military bridges will be used in all climates and must be able to work in ice, rain or desert conditions and resistance to NBC agents is a recent addition to the list.
Transportability; equipment bridges need to be transported to site and therefore use the transport equipment available to the force using it. Transportability over long distances is also an increasingly important consideration given the general expeditionary nature of recent and likely future operations.
Load Bearing; finally, a bridge must be able to traffic whatever equipment is held by the force using it, whether that is a Land Rover or heavy Equipment Transporter carrying a Challenger 2 main battle tank. Load bearing will also include related factors such as load distribution, crossing speeds, wind loading and allowable deflection.