# Bridges construction

Bridges construction is the creation of engineering structures that provide people and vehicles with unhindered transition or passage through natural and man-made obstacles: rivers, straits, channels, ravines, canyons, watercourses, valleys, other roads, for connecting similar transport systems into a single network.
Bridges are indispensable links of communication. They dramatically increase the ability to move people and goods. The appearance bridges and its associated infrastructure reduces poverty by increasing trade, revitalizing entrepreneurship and lowering prices for goods, as well as improving the living conditions of the population. Thanks to the construction of bridges, the population is served by ambulances, police, fire, rescue, repair and construction services. The developed bridges network allows the distribution of medical and educational services in previously inaccessible areas.

## The history of bridge construction

Almost all ancient peoples who left behind any written records have words denoting a bridge. Archaeologists have discovered the remains of some stone bridges from the megalithic era, made in the form of vaulted arches.
However, the very first bridges were tree trunks that accidentally fell across the stream, along which animals and hunters crossed from one bank to the other without, getting wet in the cold water. Then the people began to deliberately fell trees, peel the logs, lay them over the river and fasten them together.
Then the ancient inventors guessed to connect logs floating on the surface of the water into big rafts from one shore of river before the other shore, forming a single floating bridge. Later, boats with tarred sides began to be used instead of logs.
Since floating bridges soon get wet and sink, and are also destroyed by the rapid current of the river and ice drift, then for a more durable bridge, ancient people came up with the idea of lowering large log boxes (crib pier, log cabins) to the bottom of the river, filling them with stones, and connecting the adjacent tops of the supports protruding from under the water with long logs.
Where the spring flood annually wash away the loose banks and carry trees into the river, or where is timber rafting, the floating trees can get stuck between the bridge supports and block the powerful current of the river, which can destroy the bridge. Therefore, the distance between neighboring islands-bridge support has to be made longer than the length of the trees, but for longer spans it is necessary to build a complex strong load-bearing structure. To prevent the extended bridge span from breaking under the weight of pedestrians and transport, its girders are rely by struts or fastened to other logs, which form a single rigid frame with the girders or a spatial truss of the bridge span between adjacent supports.
Ancient craftsmen also adapted to drive wooden piles into the bottom of river, on the tops of which they then installed bridge spans. Since the depths of water barriers and the thickness of silt at the bottom can be quite large, ordinary logs for piles were built up by sequentially connecting several logs joint to joint using pipes and clamps. Piles under the bridge supports were immersed not in single ones, but in groups, the space inside of which was filled with stones, with the formation of a chain of islands across the river, pointed in form against the current of the river. In addition to the vertically driven piles, some piles were driven into the river bed at an angle, giving the entire bridge structure more rigidity.
When an even longer span was required, high support towers began to be built along the edges of the bridge, between which strong chains or ropes (later steel cables) were stretched, on which the spans of suspension or cable-stayed bridges were then suspended. The first suspension bridges made of vines appeared in the jungle. People Khasi and Jantia in India even in the XXI century cultivate living root bridge from the aerial roots of Ficus elastica. In medieval European fortress castles, drawbridge often functioned above the moat in front of the only gate.

## Priorities in bridge construction

It is preferable to build a bridge in a place where there is the shortest distance between the opposite banks of a river, stream or gorge, which significantly simplifies the process of creating any bridge. To reduce the distance between the banks of a river and, consequently, the size of the bridge, in shallow waters across a river, strait or bay, a dam can be built, and a more expensive bridge span structure can be constructed only over the remaining relatively small deep section of the river fairway to ensure the flow of water and the passage of ships.
To resist the pressure of the destructive spring ice drift, the bridge is often positioned immediately after the turn of the river, so that huge ice floes that have gained dangerous speed along the water flow would, by inertia, hit, break or rest against bend of the shore, and not against the bridge supports.
With the development of metallurgy and the advent of rail and road transport, the size and cost of bridges increased significantly. New technologies for creating bridges appeared.

## Preparatory works

The construction of a bridge begins with a comprehensive engineering-geodetic survey, which includes a comprehensive study of the section of the river selected for the construction of the bridge, usually with a minimum distance between the opposite banks of the river, strait, bay or reservoir. If possible, this distance is further reduced by creating an artificial embankment towards the opposite shore. In this case, for an unhindered flow of water and for navigation, a dredger deepens the fairway (talweg), over which a high span of the bridge is being built.
It is economically advantageous to use the available natural islands, which make it possible to reduce the length of complex and expensive spans of the bridge under construction. Artificial islands can be built on the shallows and a road can be laid the road on top of them, or only bridge supports can be erected. However, more often than not, the construction location of each support is determined by the optimal span length of the future bridge, regardless of the depth of the river or strait.
Shift work settlements and even entire factories for the manufacture of individual structures, large elements and bridge blocks appear on construction sites near bridges under construction.

## Construction pile foundation for a bridge support

Bridge supports can be located both on the shore and under water, respectively, different methods of their construction are applied to them. The improvement of their designs and the invention of new shapes and materials for them is a common phenomenon.
During the construction of the pile bridge, it is possible to reduce the cost of underwater and some coastal earthworks. Currently, instead of the previous wooden piles, various types of steel, reinforced concrete, asbestos concrete and plastic piles are used. The piles can be beat in with a vibratory hammer, or immerse into pre-drilled holes, or twist (screw piles) into the river bottom with a special pile rotator driver. Screw piles are more firmly held underground and are able to withstand not only the simultaneous passage of many heavy vehicles and trucks, but also the impacts of huge floating ice floes during strong spring ice drift.
Under the surface of the water, the tops of installed neighboring piles are united by a reinforced concrete grillage, which serves as the foundation of the bridge support and, with its sharp angle directed against the river flow, is designed to break large ice floes during ice drift.

## Caisson works

However, when building bridges, the use of piles is not always effective. For piles, the bottom of a water barrier can be either too hard — rocky or located in a permafrost zone, or vice versa, too loose — with a very deep layer of silt in which the piles are not retained. In this case, as a support the bridge, a caisson (a large waterproof box) is lowered to the bottom of the river, inside which a space frame of steel rebar is made and then the all inside of caisson is filled with cement mortar. In order for the spring ice drift not to destroy the bridge support, the caisson-formwork is given a pointed shape of an ice axe facing against the river current. Modern caissons and reinforced concrete technologies spread at the end of the XIX century and replaced the former wooden log cabins, which were drowned when filled with stones.
Before lowering the caisson under water, the bottom of the river, where the bridge support is supposed to be erected, preferably be previously cleaned by a dredger from a layer of loose silt. It is advisable to install piles into the hard soil of the river bottom to secure the submersible caisson and the foundation of the being built bridge support. The rebar frame can be constructed either inside the caisson, or separately on the bank, from where it is then moved and lowered into the caisson and to the bottom of the river. Reinforcement flooded with concrete can be not only steel, but also fiberglass, not subject to corrosion from water.
In some cases, the foundations of bridge supports are concreted without caissons, using special cement that hardens quickly under water.

## Construction of bridge span

The spans are installed on the prepared bridge supports. Methods for constructing spans depend on the shape of the bridge.
The most common, economical and quickly erected are beam bridges made of prestressed concrete. For a girder bridge, the beam is made separately on the shore. Then it is placed on a special sledge and pushed from the support beam of the slipway along the sliding path by power hydraulic cylinders until its ends connect the tops of two adjacent bridge supports rising above the water. Often push a whole string of bridge spans. Since the end of the front beam inevitably sags under its weight, an beveled end-face is attached to it, allowing it to smoothly rise along the inclined side of the beam to the top of the upcoming support. After completion the sliding, the slides are removed from under the beams, the beams are leveled and finally secured to the supports and to each other by rivets, screw connections (bolts), welding, and sometimes concreting, and then on the beams is laid the road surface.
Some beams and trusses assembled on shore are very heavy, and when pushed, they can damage the bridge supports. Therefore, for transportation, half-submerged pontoon barges are placed under them. When water is pumped out of the pontoons, they float up, elevating a truss-span structure (often in the form of an arch). Then pontoons with a truss loaded on them are towed to ready-made supports, to the tops of which this truss is lifted with lifting jacks. After the final fastening of the truss, all the numerous metal parts of the structure are protected from corrosion with special coatings.
When the spans between supports are very long, suspension bridges or cable-stayed bridges are most often built. In these cases, one high pylon is built onto each support in order to installed two parallel cables or ropes on their tops, the ends of which are then secured to the ground with guy-wires. On these cables is suspend all the structure of span, parts of which at first is transport by water, is rais to a calculated height above the water and are lining up as the single road.

## Pontoon bridges

The creation bridges is slow and very expensive. Therefore, for temporary crossings, dirt roads in sparsely populated areas, in some accidents, during a wars, during the liquidation of the consequences of natural disasters or during the repair of permanent bridges, through non-navigable water barriers is practiced the construction pontoon bridge. The same type of floating links of a pontoon bridge can be transported to the place of construction of this bridge by cars or by water, where they are connected to each other in a single chain floating between the banks. In the absence of a special pontoon park nearby, from shore to shore installations many identical-sized floating craft (boats, barges, empty reservoirs, wide pipes sealed at both ends, dried and soaked with used engine oil (to reduce wetting in water) logs from lightweight tree species, connect them to each other by ropes and on top of them put a road deck. From the surface of floating deck bridge not should protrude poorly fixed boards and nails under the weight of heavy trucks passing over it. To counteract the current, anchors are used, and in shallow water — logs stops in the bottom.
The pontoon bridge is not a reliable structure, and only slow travel is allowed on it, and only for light transport. Before the onset of cold weather, the pontoon bridge must be dismantled, otherwise in the spring it can be destroyed by an ice drift or block the rapid flow of the river and cause disastrous flooding with meltwater of the entire district.

## Bridges in the mountains

Stormy mountain rivers are usually narrow. To avoid dangerous rockfalls, avalanches and mudflows, threatening both the bridge itself and the roads diverging from the bridge, drilling and blasting is first carried out. Then the rubble is cleared with a bulldozer, moving suitable large boulders to the site where the supports of the future bridge will be built. Some stones are sawn and trimmed before they are used to build a platform for the base of the support. For greater reliability, concrete is poured into the remaining space between these stones. On top of the concreted stone blocks in the foundations of both bridge supports, a single frame is constructed, strut structures and surrounding formwork made of steel reinforcement in the form of an arched vault curved in a semicircle, which is also concreted. After the concrete has solidified, the formwork is removed, and the temporary scaffolding (if they were used) is dismantled.