the channel. The bridge is reformed by pulling up on the anchor cables until the cut portion regains its position in bridge. The pon- toon 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 i6-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 re- quired. 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 6o-ft. span \Varren girder of which an example is shown in fig. 5 (plate). The inadequate support given by the abutments of the broken bridge is here reen- forced 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 100 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 io| cwt., so that the whole bridge may be carried in G.S. wagons if required. Usually, however, the bridge was delivered on site by lorries, the-iao-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 iso-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. 10 (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 be 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).
Fro. 15. Inglis Rectangular Tubular Bridge Mk II. combined
with the heavy pontoon.
Wire row to far balk
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.
SIDE ELEVATION
FIG. 17.
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 com- mon 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 com-
