Military Bridging - Encyclopedia


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"MILITARY BRIDGING (see under Pontoon, 22.69). - At the beginning of the 10th century all the armies of the civilized Powers were equipped with pontoon trains of various forms. The European continental nations all had steel boat-shaped pontoons varying in size from the large German bipartite pontoon, which had about 8 tons effective buoyancy, to the Italian high-prowed pontoon specially suited for the swift current of the rivers in that country and capable of carrying lorries when two pontoons were placed stern to stern, and the French and Belgian pontoons, which were somewhat smaller than the British. The British army adhered to the bipartite wooden boat-shaped pontoon, 21 ft. over all in length, 5 ft. 3 in. beam, and 2 ft. 5 in. in depth, with a maximum effective buoyancy, when immersed to within 6 in. of the gunwale, of about 41 tons. The advantages of the wooden pontoon with waterproof canvas skin, as proved by the South African War, were lightness, quietness for night work, and the ease with which bullet holes could be plugged, or holes caused by shell splinters repaired. On the other hand, the steel pontoons undoubtedly stood the rough handling of active service better, and did not suffer like the wooden pontoons when they had to be stored in the open under a hot sun. They can also be more readily manufactured in large quantities in war-time, whilst the difficulty of obtaining a sufficient supply of thoroughly seasoned material greatly hampered the rapid expansion of the British bridging trains. Taking all considerations into account it seems probable that the next pontoons designed for the British army will be of galvanized steel, somewhat larger and appreciably deeper than the present pattern.

The British pontoons (as shown in fig. II) were made in two sections, the bow section having its gunwale rising towards the bow, and the body curved and tapered forward, so as to reduce the force of the current against the bridge. The stern section was rectangular in form, so that two pontoons could be coupled together stern to stern, or any number of sections could be coupled together to form rafts capable of bearing the weight of the heaviest gun carried in the field. Figure 12 shows the various uses to which the pontoon sections are put in forming light, medium, or heavy bridge. Normally when packed for travelling (as in fig. i 1) and when used in the normal form of light bridge designed to take a column of infantry in fours, field guns, and horse transport, the bow and stern sections were coupled together as one pontoon, which could be lifted off its carriage and launched by sixteen men gripping the handles at each side. The wagons carried also the superstructure of timber roadbearers (or " baulks "), which fit on the saddles of the pontoons to form the bridge, "chesses " or planks forming the roadway, and " ribands " or wheel-guides which hold the ends of the " chesses " secure and form the curb of the roadway. In addition to the pontoon wagons a bridging unit always included wagons carrying adjustable timber trestles known as " Weldon trestles." These were an important part of the equipment, being used to form the piers of the bridge in shallow water near the bank where the pontoons could not float, or to make a landing-stage when the pontoons were used as rafts on a wide river, or without the pontoons to bridge the narrow streams or dry gaps.

Light Bridge.

?

. .

'Medium Bridge. - 1 Saddle beams I Pontoons Lashed ¦ A Pontoon or '[[Longitudinal Section. Fig]]. 12.

In the organization of a British division of 1910-14 were included two, and in the division of 1915 three, " Field Companies Royal Engineers," each of which, besides its other military engineering equipment, included two pontoons and one trestle wagon, the latter carrying two trestles; the three wagons among them carried also five bays of superstructure for light bridge, using five baulks to a bay.' This gave every division the means of crossing a river independently, the engineers being able rapidly to form three bridges up to about 75 ft. in length, or one bridge of about 200 ft.; if used to form bridge of half-pontoons capable 1 The length of bridge section between two points of support technically called a " bay " is normally 15 ft.; thus a bridge supported on the two shore transoms, with piers formed of two pontoons and two trestles, would consist of five bays equal to a span of 75 ft. The width of the roadway of the bridge as normally formed is 9 ft. clear between ribands.

of carrying infantry in file and pack animals, the equipment could be extended to bridge about double this width.

Bridging trains moving in rear of the army carried each 42 pontoons and 16 trestles with superstructure, as a reserve for the crossing of wide rivers, and these were later supplemented with a superstructure of heavy steel joists, so that the pontoon equipment could be used to form medium and heavy bridges to carry mechanical transport and the heaviest guns and tractors on the road. The pontoon trains were originally drawn by horses, but to save the great number of horses a pontoon train requires, and to give greater mobility, some were adapted for mechanical transport. These consisted of " four-wheel-drive " lorries, each trailing two pontoon or trestle wagons, and were able on good roads to cover much greater distances in less time than the horse-drawn bridging trains.

The " Field Squadrons Royal Engineers " attached to cavalry divisions were equipped with a lighter form of collapsible boat, and each cavalry regiment was provided with an air-raft equipment. A special cavalry bridging train equipped with small steel pontoons was provided for use in Egypt and Palestine. These forms of bridging equipment could take the lighter natures of transport accompanying a cavalry brigade, including horse artillery guns.

On the other hand, the British army when it took the field in 1 9 14 had no reserve of heavy bridge equipment, nor any of the portable steel-girder bridges which were found so invaluable later in the war.

The British army, unlike most European armies, had no specialized bridging units. All the field units of the engineers carried out the annual course of bridging as part of their normal duty. This course was held wherever possible on the banks of a tidal river, and work was mainly concentrated on the pontoon drill which enabled the sappers to handle the material with great celerity. But the training also included practice with various forms of light improvised bridges, and the crossing of rivers by means of barrels, tarpaulin rafts, spar and timber trestles, and the construction of light suspension bridges. Little was done in the way of heavy bridging, but all units were taught the use of spars as derricks and sheers for launching girders and moving heavy loads, and a certain amount of pile-driving and heavy trestle work was done. The officers' theoretical course included the design of timber and steel girder bridges of all types, and some gained practical experience in bridging works in India and elsewhere abroad in the course of their employment in peace on the public works. Never, however, before the World War of 1914-8 had the problem to be solved been of such a varied and complex nature. The immense advance in the use of mechanical transport of all kinds, from motor-cars to steam traction engines, the greatly increased weight of artillery in the field, and finally the coming of the tank, demanded the use of heavy road bridges not far short of railway bridges in strength.

On the other hand, owing to the ease with which destruction can be carried out by means of modern explosives, advancing troops were more frequently than ever before confronted with the problem of crossing a river or canal when all existing bridges had been destroyed, approaches broken up by explosives, and the river and its environs defended by artillery and machine-gun fire. In such a case pontooning was clearly impracticable, and other means had to be devised by which the infantry could be given a footing on the opposite bank to form a bridge-head to cover regular bridging operations.

For these fighting bridges, which were practically the most important because without them no advance could be made, no standard equipment existed. Each field company improvised its own solution to the problem after reconnoitring the crossing to be forced. Usually the material could only be carted to within a mile or so of the site, and had to be carried by hand the remaining distance across shell-pitted ground, or marshland intersected by dykes. Lightness and extreme portability were thus essentials of the design. Then the material might suffer from shrapnel fire whilst en route or when lying hidden behind a bank or wall, and might be pierced by machine-gun bullets whilst actually Double chewed cleats Riban&t Saddle beam Nook bolt LJ Heavy Bridge.

?'L?? Pontoon baulks 3 Ribands alternately Pontoon Baulks å Ribands alternately. Hook bolt. /. Treble chessed r 9"- 3 cleat. Svddle beam. Thwarts.

Longitudinal Section.

being placed, hence strength and impermeability were required. Lastly, the bridge had to be put together in the dark in perfect silence, exposing as few sappers as possible on the bank, so that simplicity and interchangeability of parts were essential.

CPO strongly so that pack-animals can be got across with ammunition and supplies; these pack-bridges usually took the form of rough improvised trestle or pile bridges, but in some cases tarpaulins lashed round a wooden framing were used as floating supports in the same fashion as the waterproof sheets above mentioned.

Note .-Framing about 5 x-Ys.Bottomsame form as B but wider. All Frames to be lashed together, not nailed 4 Carpenters 2%z hours per raft. '[[Plan ' 'Frame "A" Elevation]] ' -Handles slightly rounded /3x/0. Trench She/ter Side View I¦I Elevation Of -Groove for aura Cylinders binding - Saddle Beam 4 X 1;4 '6 ' 3 9`-Elevation Of Cork Floats e" - - Elevation Of Petrol Tins Fig. 13.

The lightest and least vulnerable pattern evolved was probably the cork-float footbridge with light wooden footboards hooked over the saddles of the float and interlocking. A pattern of this type is shown in fig. 13, which also shows the employment of captured German canister floats and of petrol tins to support these light footbridges. A petrol-tin raft was used by the engineers of the British 25th Div. for the crossing of the Sambre-Oise canal near Landrecies in 1918; in this case each raft consisted of two floats each of eight petrol tins laid flat and built into a wooden crate for carriage. Eighty of these rafts were carried for 3,000 yd. under fire to the canal bank, and each when launched carried across a man with full equipment. When sufficient men had been ferried across by this means to secure a foothold on the far bank the rafts were connected by light footboards to form a bridge 55 ft. in length.

A form of light ferry-boat which was very useful was made as shown in fig. 14 by tying the standard-size waterproof trench shelter, or bivouac sheet, measuring 13 ft. by io ft., over a light wooden framing made in parts for easy transport. In the little boat thus formed six men could squat, and be pulled across by a rope worked by a sapper who had swum to the far bank or paddled across in the first boat, another man on the near bank pulling the empty boat back; and considerable numbers of infantry could thus be put across even before a light footbridge could be constructed. The boats also formed a very serviceable footbridge when connected together as illustrated in fig. 3 (plate). In a case where a crossing could be effected at a canal lock or other point where the width to be spanned was not more than about 20 ft., a light trussed timber bridge was built up complete, and carried or rushed forward from undercover on wheels, and launched across the gap by the sappers, somewhat as a fireescape is handled. Similar devices have often been used in the storming of a fortress for the crossing of the ditch. A notable example of this method was the crossing at a lock on the Sambre-Oise canal made by the British 1st Div. on Nov. 4 1918.

Another notable piece of front-line work was the construction of a crib causeway,' built of railway sleepers bolted together and sunk in the bed of the river, to carry tanks across the river Selle in the first line of the assaulting troops (1918). This was kept just below waterlevel for concealment, and was built in the nights just preceding the attack under the nose of the enemy holding the opposite bank.

As soon as a foothold on the opposite bank has been gained by the infantry, and the enemy's machine-guns put out of action, the next step for the engineers is to establish the crossings more ? '0? - - ?

I I s 2; 0, I Frame"8" Fig.

For the crossing of minor streams and dykes often met with before or after the main crossing, various devices were used to suit the varying conditions. Plank or light footbridges of the pattern shown in fig. 13 were often sufficient to carry the infantry, but where the span exceeded to ft. light trussed bridges of timber, strutted and tied with hoop iron or stout wire, were made up to about 15 ft. in span. Above this limit some form of intermediate support in the form of a float or trestle became necessary. For marshland, muddy ravines, or shell-pitted ground, mats of canvas and wire netting stiffened with wood battens and rolled up for convenience of carriage were found very useful to give a foothold. For horse traffic, corduroy mats of timber bound together with wire and picketed down in place were used, as also were the artillery " trench bridges," 12 f t. in span with timber bearers and 12 in. flooring, made up in sections 3 ft. 6 in. wide to be laid side by side. These were a little heavy for hand carriage; but in most cases they were issued to the artillery before the advance and carried by them in their limbers to be laid down where required.

Next, it becomes necessary to bring forward the field artillery into position on the far bank. For this work the pontoon equipment is invaluable, as it enables a bridge for horse transport to be made across a river more quickly than it is possible by any other means, and the peace training of the British engineers in pontooning work justified itself in the fine work done, notably in the advance across the Aisne in Sept. 1914. The field companies of the New Army were likewise instructed in and equipped for pontooning work, and the material was used to advantage on nearly every waterway on the entire front in France, on the Piave, on the rivers of Palestine, and in Mesopotamia.

1.

Pontoon Bridge with Tidal Ramp and " Cut."

5.

Span Bridge over Escaut Canal on Cambrai - St. Quentin Road.

2.

Bridge over Moat at Conde.

6.

Hopkins Bridge-185-ft. Span.

3.

Footbridge Supported on Ground Sheets, Round Frame.

7.

Hopkins Bridge at Pont de Nieppe.

4.

High Trestle Bridge.

8.

Inglis Pyramid Bridge.

Figure I (plate) illustrates the type of bridge built with pontoon equipment across a tidal estuary in which the standard service trestle with adjustable transom is used for the bays nearest the shore; that part of the bridge which will ground on the fall of the tide is carried on barrel-piers strong enough to carry the load when grounded, and the floating portion is composed of pontoons. A " cut " is formed in the bridge by disengaging the central floating portions and allowing it to swing on the tide or stream so that vessels may pass freely along the channel. The bridge is reformed by pulling up on the anchor cables until the cut portion regains its position in bridge. The pontoon bridge shown is the normal bridge capable of carrying columns of infantry in fours, field guns, horse transport, and light cars up to 2-ton axle loads. Where a pontoon bridge has to be built to carry heavy mechanical transport, siege artillery tractors and other heavy loads it is necessary to use more pontoons and group them in the form of rafts as shown in fig. 12, the medium bridge being designed to carry 8-ton axle loads and the heavy bridge 16-ton. The roadway from saddle to saddle of the rafts is carried by heavy steel joists on which two or three layers of chesses are laid.

As the pontoon equipment is always required to move on with the army other types of bridge are substituted for the pontoon bridges as soon as practicable, and these in the late war usually took the form of timber trestle bridges of tree trunks or any other timber found available in the locality. For heavy loads these bridges were constructed of stout squared timber as in fig. 4 (plate), and with a roadway carried on heavy steel joists were capable of carrying all traffic. Where the bottom was soft piles were used in place of trestle piers to support the spans, as a trestle is very liable to sink or tip in soft mud or on an irregular bottom and so throw the roadway out of level. Pile-driving is, however, a slow operation, and plant for this purpose had to be improvised in the field, as no satisfactory portable apparatus has yet been standardized for army purposes.

These heavy timber bridges necessarily take some time to prepare and erect and are not very suitable for extreme loads, and after some war experience it became evident that for a general advance on a large scale the army must be equipped with steel girder bridges to carry the heaviest loads, and capable of transportation in small portable sections and speedy erection on the site. Many types of these bridges were designed to suit the various spans likely to be required, and held in reserve ready for dispatch to the most convenient railhead. Bridging schools were formed to train officers and men in the use of this heavy bridging material, and, when the advance came to be carried out, the corps and army engineers were able to replace the light bridges made by the divisional field companies so rapidly that, .almost as fast as the fighting troops could gain ground, the heavy artillery, mechanical transport, and all the other heavy traffic were able to follow up.

Where intermediate support could be obtained on firm ground, piers were often built up of skeleton steel cubes 3 in. by 3 in. by 3 in., each capable of supporting a weight of 40 tons and built up with timber crib work to form single, double or treble cube piers as required. A bridge consisting of a series of comparatively short steel spans could then be built on these piers. The bridge of this type illustrated in fig. 2 (plate) has two spans of 30 ft. and one of 18 ft. on piers about 15 ft. in height.

For larger spans a very useful bridge was the 60-ft. span Warren girder of which an example is shown in fig. 5 (plate). The inadequate support given by the abutments of the broken bridge is here reenforced by the use of a heavy timber trestle pier on the towpath.

For larger semi-permanent bridges on the main routes great use was made of the " Hopkins " bridge, which was a girder bridge made in two sizes capable of erection in spans to any multiple of 15 feet. The lighter type was suited to spans of 60 to 90 ft., and the heaviest design for spans over loo feet. This was normally used for spans of about 120 ft., but in fig. 6 (plate), representing a bridge over the dry Canal du Nord, the span is 180 feet. The loading must of course be calculated according to the span adopted, 150 ft. being the limiting span at which this type will carry 35-ton tanks singly.

The special feature of the design of this bridge is that of great portability, the heaviest piece weighing only 102 cwt., so that the whole bridge may be carried in G.S. wagons if required. Usually, however, the bridge was delivered on site b y lorries, the 120-ft. span being carried in 35 lorry loads. The bridge is built up upon the near bank in extension of the centre line of site and all the parts bolted together to complete the two main girders with cross bracing. The construction of the abutments usually proceeds simultaneously with the erection of the girders.

The method of launching this bridge is shown by fig. 7 (plate), which shows a 150-ft. span being got into position at Pont de Nieppe, near Armentieres. The flooring, consisting of rolled steel joists as cross girders and longitudinals, with timber decking laid crossways, is added when the bridge is in position.

Another very clever design of bridge specially adapted for the military requirement of speed in erection is the " Inglis " bridge. This bridge in its pyramid form is illustrated in fig. 8 (plate), but the rectangular form afterwards designed is better suited for mechanical transport.

The particular feature of this bridge is the absence of any bolting or riveting of joints. The steel tubes of which the girder is composed have merely to be fitted into the special junction boxes carried on the ends of the transoms and stiffeners, and are held in place by pins secured by split pins. The launching of the bridge is most quickly done by constructing the bridge in skeleton parallel to the river with enough counter-weight on the tail to enable it to be swung on a special trolley or carriage as shown in fig. 9 (plate).

The bridge, when in place, is then lowered from its carriage and decked over, and lastly the tail is dropped to form an approach as in fig. io (plate) in which a tank is shown crossing the bridge. This bridge. can carry a tank over a gap of 105 feet. Where a wider river than this has to he dealt with the bridge is carried on special heavy pontoons (fig. 15), or four bays of the bridge may be used on three of these pontoons as a raft, which is then warped across the river. The projecting bay forms the landing stage for the tank (fig. 16).

11?? ' MATjYAvaOI?h???Il? ?? ,.; FIG. 15. - Inglis Rectangular Tubular Bridge Mk II. combined with the heavy pontoon.

FIG. 16. - A 35-ton Tank being ferried across a river on a raft.

The construction of bridges to carry mechanical transport always involves work on approaches, sometimes of considerable length, to carry this traffic on and off the bridge to the main road, and the officer selecting the site has to take carefully into account the time which will be entailed in this construction, as well as the best span or combination of spans to use for the bridge itself. For instance, on a high level site it may sometimes be advantageous to build several smaller spans supported on timber trestles or steel-cube piers to reach the main span so as to save the delay of filling a high embankment approach. Usually the time for constructing a permanent macadam approach road to the bridge would be too great, and the common form of approach to a bridge for heavy traffic was a road of beech slabbing cut in the forests to a thickness of 2 in., about i ft. in width and 10 ft. in length. These slabs were best laid for a single roadway in herring-bone fashion, so as to make a road of about 15 ft. in width, the slabs being spiked to longitudinal sleepers and secured by a heavy timber curb along both sides of the road. It is important that the immediate approach to the bridge should be laid out in true alignment and level with the bridge decking, which also should be as even as possible, so that stresses due to impact are reduced to a minimum, and traffic is able to reach the bridge, and move clear of it without special effort.

In mountainous country where pack transport has to be chiefly used, and in theatres of war where still more primitive conditions of transport exist, the field suspension bridge (fig. 17) is the most common form of bridge for any considerable span. Suspension bridges have been built in the field to carry lorries, but usually they are only required for pack or even foot traffic. The best materials to use for the cables are chain or steel wire ropes; but telegraph wires are frequently used, and hemp ropes, thongs of hide, or ropes of creeper or grass, have been employed.

Aerial ropeways, too, have been of great value in mountainous countries for the supply of ammunition, stores and water, to save transport up a long steep incline, or as a temporary means of corn a.' Side Elevation ? _2-.-; i. FIG. 1 7.

.--ro munication across a deep gorge or wide river. Many forms of floating bridges have also been constructed from local boats or barges where the pontoon equipment has not been available.

In uncivilized countries the chief problems for the bridgebuilder are to devise the best use to which to put the scanty supply of materials available, and to adapt the local resources of the country to advantage, knowing that the transport difficulties render it impossible to obtain all he would desire. But, great as is the task of bridge-building for an army in undeveloped countries, greater still is the work of reconstruction during an advance in a highly developed theatre of war such as France. There the accumulation of means of attack and defence on a grand scale is made possible by the fulness of the communications, yet at the same time each of these many lines of communication is sensitive at every river-crossing. Almost without exception these bridges are destroyed by the enemy on his retirement, and an army cannot safely push on its advance without its full equipment of battle means and without clear routes for its supply transport. Hence it is no exaggeration to say that in the final campaign of 1918 in France the power of the British army to advance depended on the speed with which the Royal Engineers could construct bridge-crossings and roads.

During the period Aug. - Nov. 1918 no less than 539 heavy bridges were erected on this front alone, of which 326 were standard steel bridges and 213 of heavy timber or salved material, not taking into account the innumerable light improvised crossings and footbridges by which the leading infantry were enabled to attack, and the pontoon and light trestle bridges for field artillery and horse transport.

For such a task executive energy, organization and technical skill are equally, and each in the highest degree, necessary. And to these qualities of the military bridge-builder must be added, for the work in the forward zone, that of personal devotion under fire. It is significant that of the Victoria Crosses awarded to officers and men of the Royal Engineers in the World War more than half were won by acts of conspicuous gallantry in the construction and demolition of bridges. (E. N. S.)

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