Road and rail communications in Denmark are interrupted by numerous straits and other waterways which have to be bridged or ferried. The Little Belt Bridge and the Storstrom Bridge are recent examples of the surmounting of difficulties in crossing arms of the sea
STORSTROM BRIDGE, nearly two miles long, is one of the longest bridges in Europe. It carries a railway and a road across the Storstrom, a strait dividing the Island of Zealand from the island of Falster. As about 15,000 ships yearly pass through the Storstrom, three arch spans have been arranged in the centre of the bridge to give a clear headway of about 85 ft. 3 in.
THE Kingdom of Denmark, more than most other countries in Europe, is ill endowed by Nature for the development of her land transport. Although she is not isolated from the Continent - being joined to Germany through Schleswig-Holstein - her own territory, apart from the peninsula of Jutland, consists of a series of islands. There are the two major islands, Zealand and Funen. Zealand contains the capital, Copenhagen, and Funen the important, town of Odense. Then there are smaller islands such as Palster and Laaland, and a number of islets.
These islands, which help to make up the bulk of Denmark, are separated by various salt-water sounds, some of them narrow and others wide enough to necessitate lengthy crossings. Such a broken-up country is faced with serious disadvantages where land transport is concerned. When railways were introduced, their builders had to be content with a series of “bits and pieces”, instead of making a homogeneous national system such as other countries could boast. Road transport was in exactly the same position, and the motorist has found himself similarly handicapped.
To travel from Thisted, in north Jutland, to Copenhagen in the old days necessitated being ferried across three of these sounds, namely, the Lim Fjord, the Little Belt and the Great Belt. The important through route from Copenhagen to Gjedser, the port for Berlin, was broken by the Storstrom, dividing Zealand from Falster. The Storstrom, in its turn, is divided by the small island of Masnedo. Some improvement was effected by the State Railway authorities, who introduced train ferries across the intervening stretches of water. But a train ferry, though it may convey all kinds of road motor vehicles as well as railway cars, has a strictly limited capacity.
In the years following the war of 1914-18 the Danish Government finally decided that wherever possible the building of giant bridges should remove the disabilities inflicted upon the country by Nature. The first fruits of this policy are the Storstrom Bridge, between Masnedo and Falster, and the Little Belt Bridge, connecting the island of Funen with the mainland of Jutland.
The appropriate department of the Danish State Railways at first projected the building of a bridge across the Storstrom for rail traffic only. A railway bridge, built in 1884, already connected Zealand with the island of Masnedo, on the northern side of the Storstrom. The requirements of modern transport, however, demand something more than a bridge for the railway only, and the original design was not adopted. In 1932 the British firm of Dorman Long and Company, Limited, made an offer to the Danish Government and submitted a design of its own. This second design was still not quite suited to Danish requirements, and a third was prepared by the State Railways, which allowed for a single railway track, a motor road and a passage for cyclists and pedestrians. This was the final design. The contract was awarded to Dorman Long and Company, with Professor Anker Engelund
responsible for the design of the structure. There were two interesting stipulations. Dorman Long and Company made their offer on condition that the bridge was to be a steel structure. The Danish State Railways in their turn stipulated that Danish labour was to be employed as far as possible.
CONCRETE ARCH VIADUCTS form the approaches to the bridge across the Little Belt, between Fredericia, on the peninsula of Jutland, and Middelfart, on the island of Fiinen. The Jutland approach has five spans and an overall length of 703 ft. 6 in. Each span consists of four parallel arches, two wide ones to carry the railway tracks and two narrower ones supporting the road.
Professor Engelund and his colleagues, no less than the contractors, were faced with a tremendous task. The spanning of the Storstrom itself involved the building of a bridge nearly two miles in length, apart from embankment approaches, and a new bridge was also to be built across the narrow sound between Zealand and Masnedo in place of the structure which dated from 1884.
While the British firm was to be responsible for the main part of the bridge, the piers and superstructure, a Danish company, Christiani and Nielsen, Limited, was awarded a subcontract for sinking the foundations of the piers. Conditions were found to be comparatively good. To ascertain the nature of the underlying strata on which the bridge was to be founded, preliminary borings were taken into the bed of the Storstrom. The depth of the channel on the line to be taken by the bridge nowhere exceeded 46 feet. It averaged only 22 feet and exceeded 30 feet over a distance of 2,000 feet only.
On the shelving shores the bottom was found to consist of layers of mud and sand, but the bed of the channel was formed by a layer of grey glacial boulder clay, varying in thickness from 13 feet to 33 feet and having a substratum of sandy clay. Beneath the clay was a thin layer of sand and gravel lying on top of a thick stratum of chalk. This chalk was struck at depths varying from 65 feet to 75 feet. In the middle of the Storstrom, however, the ancient glacier had carved out a channel for itself, so that the top of the chalk layer was here 200 feet below the surface, with a thick layer of brown clay and sand above it.
Precautions Against Pack Ice
Difficulties due to the rise and fall of the tides in these waters are slight. The Baltic is almost tideless, and the Storstrom, which forms an arm of the Baltic, shows a tidal range in normal conditions of about 8 in. to 13 in. In the last fifty years the extreme flood tide variation has not exceeded 6 ft. 6 in. On the other hand, the Storstrom is afflicted with pack ice during the hard Scandinavian winter and in the shallows on either side the ice may mount up to a thickness of 23 feet.
The designers of the Storstrom Bridge, and of the accompanying spans across the small Masnedsund, between Zealand and Masnedo, had to allow for this and to give their piers sufficient strength to withstand all potentially damaging effects from the driving up of the pack ice. The flow of the channel is not strong, and normally does not exceed 7 feet a second, large waves being unusual except under the influence of a westerly or south-westerly gale. The line of the bridge takes a south-south-westerly direction from the Masnedo abutment.
After the preliminary borings, the engineers appointed by the Danish Government decided that the bearing power of the underlying boulder clay was sufficient to support the weight of the bridge piers at a maximum pressure of 3 tons 4 cwt. to the square foot. They estimated that it would be necessary to sink the foundations to a depth of about 40 feet, the maximum depth in the middle of the channel being 55 feet.
There were other considerations governing the design of the bridge. About 15,000 ships pass through the Storstrom yearly on their way between the Baltic ports and the Kiel Canal, and on coastal services to and from places on the Great and Little Belts. The vessels range from small motor coasters to large ocean-going steamers. After a deliberately lengthy period of observation the Danish Government engineers came to the conclusion that the clear headway of the bridge’s central spans should be fixed at a fraction over 85 ft. 3 in.
FROM THE MASONRY PIERS of the Little Belt Bridge the spans were cantilevered out, each section being built out until it met its neighbour. The Little Belt Bridge spans a sound known as the Little Belt, between the island of Fünen and the peninsula of Jutland, Denmark. The main piers of the bridge are 47 ft 1 in wide and are built on specially designed caissons. The bridge contains about 50,000 tons of steel and 3,955,258 cubic feet of concrete.
The final design of the Storstrom Bridge, therefore, consisted of fifty spans, of which forty-seven were approach spans on either side of the fairway, rising at a gentle gradient to the three great spans in the middle. The middle spans were designed in the form of three polygonal arches supporting steel plate girders, the central one 447 ft. 5 in. and the other two 335 ft. 7⅝ in. long each. For the approach spans the designers provided deck cantilever girders. All spans rest on concrete piers and the design of the adjacent Masnedsund Bridge is similar to that of the Storstrom approach spans.
The railway track rests squarely on one side of the succession of girders; then comes the roadway, built of concrete on a base of reinforced concrete, 18 ft. 4½ in. wide, and with a 2-in. camber. Finally, on the side remote from the railway, comes an asphalt way about 8 feet wide for pedestrians and cyclists. This cycle track and half the roadway overhang the piers on the outside, being balanced by the railway tracks and the heavier weights carried by them.
Movable Steel Cofferdams
For the foundations and the building of the piers above movable steel cofferdams were used, instead of the more usual method of sinking sheet steel piles to form the cofferdam. First of all relatively small piles were sunk into the bed of the channel on the site of each pier. Then the cofferdams were built up ashore. Each cofferdam consisted of an oval ring of steel, free from all obstructions inside, and containing watertight compartments with facilities for admitting or expelling water according to the need for sinking or raising the whole unit. The reinforcement of each cofferdam was effected by cross-bracing inside the double steel walls. Each unit contained the necessary cranes, pumps and an independent supply of electricity for power.
The engineers hung sheet steel piling round the outside of the cofferdam and towed it out into the channel until it was directly over the piling which had been driven on the site of the pier in advance. They then admitted water ballast to the cofferdam, allowing it to sink until it rested on the small piles below. The procedure now was to pump out the water enclosed by the interior
of the unit, but it was first necessary to take precautions against an inrush of soil from the surrounding bed of the channel. This was the reason for the sheet piling. As soon as the cofferdam was in position, this piling was driven down into the clay bottom to depths of from 25 feet to 40 feet, thus forming an effective barrier against scouring. After the cofferdam had been dried out, the engineers proceeded with the excavation for the pier concerned and poured in the concrete, this being provided by a concrete plant mounted on a barge laid alongside.
Having brought the level of the concrete foundation up to some 9 ft. 10 in. below the surface, the workers in the cofferdam allowed it to dry and then removed their cofferdam from the site. This they effected by filling it with water to balance the water pressure outside the walls, then pumping the water ballast out of the interior compartments. They then jacked the entire unit clear of the concrete and sheet piling and tilted it by admitting water ballast to the compartments on one side. Having been tilted, the opposite edge of the cofferdam just lifted clear of the concrete foundation. It was then possible to tow it away and prepare it, in the same manner, for the building of the next pier foundation.
THE KINGDOM OF DENMARK comprises a number of islands, large and small, and the main peninsula of Jutland. Copenhagen, the capital, lies on the island of Zealand. The Storstrom separates Zealand from Falster. An islet called Masnedo, in the Storstrom, is linked with Zealand by the Masnedsund Bridge and with Falster by the Storstrom Bridge. The important town of Odense is on the island of Funen, separated from Jutland by a stretch of water known as the Little Belt. Near the site of the former ferry between Fredericia and Strib, near Middelfart, the Little Belt Bridge has been built to carry road and rail traffic.
The next stage consisted in building a reinforced concrete caisson on shore and surrounding it with granite facing. When this was set, it was fixed firmly between two barges and towed out into the channel. It was then lowered on to the top of the already completed concrete foundation, thus bringing the height of the pier up to 8 feet above water level. The engineers next pumped out the water enclosed by the caisson and filled it up with concrete. The builders of the piers found that this method answered well. Certain piers, however, were exceptions, where the strength of the clay below was not sufficiently great to allow the water to be pumped out of the cofferdam without a great deal of sheet piling being used. So another type of cofferdam was adopted for these piers. This modified unit was larger, and so arranged that the sheet was dropped down on the inside, original type of cofferdam having had the same outside dimensions as the foundation of the pier, this type was sufficiently large to surround the site of the foundation. The foundation workers were thus able to lay the whole of their concrete base within the area surrounded by the cofferdam, which thus afforded additional protection from scouring.
In shallow water, where there was not a sufficient range for floating the cofferdam unit, and also for laying the foundations of the large piers supporting the high spans over the fairway, a third method was adopted. This consisted simply of normal open cofferdam construction by sheet steel piles. The engineers sank the piling to a sufficient depth to exclude the percolation of water without the use of concrete water seals and, having pumped the water out of the inside of the cofferdam, carried out the raising of the piers in the dry. Of the three methods, the first was used by the engineers, on twenty-seven of the forty-nine piers in the Storstrom Bridge, the second on fourteen and the open cofferdam on eight piers.
The abutments at either end of the bridge are of concrete, with slab floors and tops connected by the outer and intermediate walls, road and rail being carried side by side on the upper slab, as on the deck of the bridge itself.
The winter climate of northern Europe is distinguished by low temperatures and the summer weather can be exceedingly hot. The designers of a great work such as the Storstrom Bridge, therefore, have to make special allowances for expansion and contraction of the superstructure in climatic extremes. The girders are supported on fixed and expansion bearings alternately, providing for their longitudinal movement under stresses imposed by changing temperature.
The deck cantilever spans on either side of the high central arches over the fairway consist, alternately, of anchor arms fixed to the tops of the piers, and suspended spans between them. The suspended spans are supported on the cantilever arms of the anchor spans by pendulum bearings, with expansion hinges at both ends of the bottom laterals and on the top laterals at the expansion end bearings. Fixed hinges are installed at the fixed bearing end. A curious effect was to be observed by any one standing broadside on to one of the expansion bearings during construction, an apparent break being visible in the line of girders.
DECK CANTILEVER SPANS of the Storstrom Bridge consist, alternately, of anchor arms fixed to the tops of the piers and suspended spans between them. The suspended spans are supported on cantilever arms 29 ft. 2 in. long, projecting from the anchor spans. The anchor spans are 189 ft. 7 in. long and the suspended spans 145 ft. 10 in. Long.
THE THREE CENTRAL SPANS of the Storstrom Bridge allow sufficient headway for the passage of shipping. To build these spans wooden trestles were erected between the piers. A 500-tons floating crane lifted the floor girders into position, resting them alternately on the trestles and on the piers. When the floor girders were in position the arch girders and hangers were built in to support them.
Each anchor arm has a length of 189 ft. 7 in. and each cantilever arm a length of 29 ft. 2 in. The whole anchor span with its two cantilever arms has a total length of 247 ft. 11 in. The length of a suspended span between opposing cantilever arms amounts to 145 ft. 10 in. The top and bottom main girder flanges are braced laterally. On the approach spans the rails are laid in ballast in the ordinary manner, the ballast being contained in a concrete trough. Where they cross the three big spans in the middle, however, to reduce the thickness of the floor, the sleepers are placed directly on the stringers, or longitudinal girders supporting the flooring.
LAUNCH OF A CAISSON for the foundation of one of the piers of the Little Belt Bridge. The caissons measured 147 feet by 79 feet, varying in depth from 50 feet to 60 feet. A ring of vertical tubes was built round each caisson; the tubes ended in steel cutting edges. The bottom of the caisson was shaped to the contour of the sea bed. It was therefore built on shore, bottom upwards, launched down a slipway, and was later capsized.
The girders were made in England and transported in lengths varying from 39 feet to 46 feet to Dorman Long and Company’s yard on the island of Masnedo. Here the engineers erected each successive span out of the girders. For placing the spans in position they used an enormous floating crane with a lifting capacity of 500 tons. First the girder was moved down from the shore along two parallel slipways. The crane then took it in hand, and was in turn towed out to the piers ready to receive the span. One of the most powerful floating cranes in the world, it consisted of two steel towers, each resting on a huge barge, and each with the top brought out in a cantilever form to take the lifting tackle. The towers were 150 feet high and the barges supporting them each 164 feet long and 26 feet wide. The lifting tackle consisted of nine sheave steel blocks on each tower, operated by steam engines supplied from boilers mounted on the decks of the barges.
For the placing of the wide spans over the fairway a somewhat different method was adopted. Here the engineers erected timber trestles between the widely spaced piers and, by means of the giant crane, lifted the floor girders into position, resting them alternately on the finished piers and on the trestles. With the floor girders in position, they then proceeded with the erection of the arch girders and hangers which were to support them when the span was completed.
The material used for the girders is “Chromador” steel, steel containing a percentage of chromium and manganese and with a minimum yield point of 51,200 lb. per sq. in. Work on the bridge began in the autumn of 1933 with preliminary operations on the concrete foundations for the piers.
Claims of Motor Transport
Dorman Long and Company began manufacturing steel for the superstructure at their Middlesbrough works in February 1934, and erected the first span early in 1935.
The old structure carrying the railway across the sound between Zealand and Masnedo was a swing bridge with two relatively large openings for the passage of shipping. It was not suited for incorporation with the new works and the Danish Government produced plans for an entirely new bridge to be built alongside. The Masnedsund is similar in natural features to the Storstrom projier, though on a much smaller scale. It was not possible for the new bridge to have high girders in the middle for the passage of shipping, because of the long downward slope from the navigation spans of the Storstrom Bridge.
The Masnedsund Bridge, accordingly, was designed as a simple girder structure of six spans, one of these being a rolling bascule span. Each of the fixed spans has a length of about 103 feet, and the rolling bascule has a length of about 93 feet, giving an opening of a fraction over 82 feet for the passage of shipping. This bridge was the first of the two to be completed, all the spans having been placed in position before the autumn of 1935. The spanning of the Storstrom proper was completed early in 1937.
The bridge which spans the Little Belt between Fredericia and Middelfart forms a vital link in the main rail and road route from Copenhagen to Jutland and parts of North Germany. Over it all the traffic between Copenhagen ,and Great Britain by way of Esbjerg is carried.
A GIANT FLOATING CRANE of 500 tons capacity was used to lower the suspended spans of the Storstrom Bridge into position between the anchor arms. The crane consisted of two steel towers, each of which rested on a huge barge. The towers were 150 feet high and the barges 164 feet long and 26 feet wide. The lifting tackle was operated by steam engines; their boilers were mounted on the decks of the barges.
Here again it was originally intended, according to plans made in 1924, to build a bridge for railway traffic only. Subsequent discussion emphasized the claims of motor transport as well. An Act of Parliament was obtained in 1927 for the construction of a combined rail, road and footbridge across the narrow neck from Middelfart, some way below the ferry passage between Fredericia and Strib. Some of the natural conditions were similar to those prevailing on the Storstrom. There were the same slight variation in the tides and the same difficulties to be encountered in winter from drifting pack ice, which came down from the Kattegat and tended to jam in the narrow neck of the Little Belt. The channel itself, on the other hand, was different. It was considerably narrower, with a maximum depth of 133 feet, and was subject to a strong current. Below the bottom lay a thick bed of compact and watertight clay and on this the engineers founded their piers. The design consisted of a continuous cantilever bridge of steel, supported on concrete piers, with arched approach viaducts of reinforced concrete at either end.
The continuous passage of shipping through the Little Belt necessitated the allowance of a clear headway of 108 feet, which involved building 240-feet piers penetrating to a depth of 80 feet. The central piers were placed on either side of the deep channel in the middle, where the depth of the water varied from 75 feet to 100 feet. The height of the structure at these two central piers is 320 feet. The finished bridge has a central span of 721 ft. 11 in., with side spans of 541 ft. 5 in. and end spans of 451 ft. 1½ in.
The approach sections consist of two beautiful concrete arch viaducts, each span consisting of four parallel arches, two wide ones carrying the weight of the railway tracks and two narrow ones bearing the road. The approach from the island of Funen consists of three such spans, having a total length of 454 ft. 1½ in; on the Jutland side there are five spans with an overall length of 703 ft. 6 in. The main piers are 47 ft. 1 in. wide and carry a carriageway 18 ft. 1½ in. wide, a footpath 7 ft. 10 in. wide and a double railway track with an overall width of 31 ft. 7½ in. The Little Belt Bridge contains about 50,000 tons of steel and 3,955,258 cubic feet of concrete.
An interesting feature of the building of the Little Belt Bridge was the way in which the caissons for the piers were built and sunk into position. Each caisson measured 147 feet by 79 feet and varied in depth from 50 feet to 60 feet. Round the edge of each caisson the designers arranged a ring of vertical tubes, made of reinforced concrete and ending at the bottom in steel cutting edges. The bottom of the Little Belt, at the site of each pier, was sounded with minute care to get its exact contour. The engineers working on the caissons then shaped the bottoms of these to correspond. To do this, they had to build each caisson upside down, on shore, and then launch it, still in an inverted position, from a slipway.
When the caisson was afloat, they neatly capsized it so that it floated in an upright position. This they did by filling the tubes on one side with ballast, which automatically ran out again as the open ends of the tubes turned downwards, and by a careful regulation of water ballast in various chambers in the hollow caisson walls.
As soon as each caisson was resting on the floor of the channel with its bottom downwards, the engineers pumped out sufficient water ballast to allow it to float, its relative density being slightly less than that of an equal volume of water. A floating concrete plant was brought alongside, and they began to build up the caisson walls on top, simultaneously moving the caisson gradually farther and farther into deep water towards its final site. Gradually it sank into place. The builders then excavated the ground through the hollow tubes round the bottom of the caisson. When these tubes, with their cutting edges, had reached the intended depth, they were filled up with concrete.
These unconventional methods were used in building all the underwater foundations for the piers, and they answered perfectly well.
Early in 1935 the bridge approached completion, and on May 14 of that year King Christian of Denmark arrived in the Little Belt on board the Royal yacht. The yacht dropped anchor at Middelfart, the king went ashore and rode across the completed bridge in a diesel train, by which he symbolized the formal opening of the bridge.
BUILDING A PIER for the Little Belt Bridge. For the foundation of each pier a cellular cofferdam was sunk and filled with concrete. The piers were built of concrete and from them the spans were cantilevered out until they met.