The capital of Sweden is situated on an archipelago, and in recent years many fine bridges, in steel and in reinforced concrete, have been thrown across the great waterways which interrupt the normal passage of road and rail transport
THE REINFORCED CONCRETE ARCH of the Traneberg Bridge was, at the time of its completion (1934), the longest concrete span in the world. The span is 580 ft 6 in. The bridge, which consists of two decks, side by side, spans Traneberg Sound and links the island suburb of Kungsholm with Brommalandet, another Stockholm suburb.
STOCKHOLM owes her reputation as one of the most beautiful capital cities in the world partly to her situation on an archipelago. Inlets and sounds connecting Lake Malar with the tideless waters of the Baltic run their picturesquely irregular course right through the heart of the city, so that the quays, with their shipping activity, lie cheek by jowl with such buildings as the Royal Palace and Riksdagshuset (the House of Parliament). Although this makes the place most fascinating to the visitor, it complicates the city’s road and rail connexions.
In ancient days the city of Stockholm was entirely self-contained, for all of it stood on an island. This island lies in the middle of the channel flowing out of Lake Malar into the Baltic. To-day it is still called the City (Staden), but with the march of the years Stockholm has expanded on to neighbouring islands and on to the mainland. It has a population of more than 500,000. The channels and sounds which had once afforded protection became in more peaceful times obstacles to the engineers
responsible for communications in a city which was continually expanding.
For many years, town planners have been busy bridging the lesser gaps, such as those between the old city and the new parts on the northern and southern sides. Since the war of 1914-18 they have tackled the task of bridging the greater gaps, and thus of eliminating the long detours which often had to be made by traffic proceeding from one to another of the city’s more outlying suburbs. The new bridges have been planned and built by the Bridge Department of the Stockholm Harbour Board, under the management of Ernst Nilsson.
One of the most conspicuous features of Stockholm is the great Arsta Bridge across the wide waters of Arsta Sound, on the southern side of the city. This bridge is a railway bridge, but the majority of Stockholm’s great bridges carry roadways.
The second notable structure to be seen by the visitor approaching from the south is the Vasterbro, or West Bridge, a beautiful line of two great arches spanning the channel which connects Lake Malar with the Riddar Fjord, lying between the southern and western suburbs of the city. The bridge carries two wide carriageways with a double track for electric cars between them. Before its construction, the only road approach from the south to the busy quarters of the modern town was over the old bridges and through the narrow winding streets of the ancient city on its crowded island. The great increase in road transport has been responsible for the building and rebuilding of the majority of Stockholm’s magnificent bridges.
On January 26, 1931, the City Council decided to build a new road connexion from south to north-west. This involved the building of the West Bridge and of another bridge across the Pal Sound. The West Bridge and the Pal Sound (or Palsund) Bridge therefore form parts of the same scheme. The southern end of the West Bridge is situated on an island called Langholm, and it is the Pal Sound that separates this from the southern suburb of Sodermalm. The two bridges are connected with each other by a viaduct across the island of Langholm.
Altogether, therefore, the new road involved the building of three spans across intervening stretches of water — the larger two spans forming the West Bridge proper — as well as a considerable length of approach and intermediate viaducts.
One of the things which had to be continually borne in mind by the bridge engineers was the allowance they were to make for the passage of ships. Sweden’s inland navigation is extensive in places, and the eastern end of Lake Malar, across which they were to build their bridge, is traversed by vessels coming into Stockholm from the Sodertalje Ship Canal, which provides a short cut for ships less than a certain tonnage coming in from the Baltic. They had to allow for a clear headway of 94½ feet above the middle of a fairway 197 feet wide. Although they might have used cantilever or suspension construction, the designers of the West Bridge and of the Pal Sound Bridge decided to return to the old traditions of Thomas Telford and other pioneers, and used the wrought-metal arch. The West Bridge, however, does not at all resemble the bridges of the early nineteenth century.
The West Bridge is much larger, but it is not in size that the great difference lies. Its peculiarity is its extreme simplicity. There is no maze of girders and latticework. So severely simple is the design of this great steel bridge and of its smaller neighbour that from a distance it resembles a concrete structure rather than anything else. Briefly the principle of the design is as follows.
Floating Dock as Pontoon
There are six piers of reinforced concrete, set in pairs, one pair in the middle of the channel and the others on the two facing shores. Between each pier extends an enormous steel arch member built up of girder sections. Each pair of steel arches is cross-braced with K-shaped diagonals. The top of each arch supports the roadway, which is at its highest point over the main arch. Between and beyond each end of the arches the bridge deck rests on a double line of vertical steel columns, one line rising from each of the steel arch members.
On July 16, 1932, contracts were placed with the Lindholmen-Motala Company, a Swedish concern, and with the United Steelworks of Dortmund, Germany, among other firms, for the materials to be used in the West and Pal Sound Bridges. The girders for the arch members of the West Bridge, having been completed, were temporarily assembled in the steelworks at Dortmund before they were dismantled again and transported to Sweden for their final erection on the chosen site. Although some of the materials of the bridges were manufactured in foreign works, Swedish steel was used throughout.
Meanwhile, work had been proceeding on the concrete piers. These presented no unexpected difficulties. They had to be founded on hard rock, one beneath the bed of the Riddar Fjord. This is 65½ feet deep in the centre, but only about 26 feet deep where the middle piers were built. Opening as it does into the Baltic, the fjord is free from tidal fluctuations. True, the water freezes over in winter and is full of drifting ice in spring, but Swedish engineers are accustomed to this in a country where the sea itself freezes over in many places.
“CLOVER-LEAF”’ JUNCTION built to alleviate traffic congestion at Karl-Johans Torg, south of the island on which the City (Staden) of Stockholm stands. The junction is built of concrete and accommodates motor, tramway and pedestrian traffic. It is built above an underground station of the Stockholm Saltsjon Railway.
While the finishing touches were being put to the bedplates on the piers, the arch members, each in two halves, were being prepared in a shipyard about 1¼ miles away. Midway between each couple of piers, the engineers at work on the structure erected a gantry resting on the bed of the channel, where it was more than 60 feet deep. A floating dock was requisitioned as a pontoon for floating the arch members out to their future position. On the sides of this floating dock were mounted half-arch ribs, supported in such a way that the ends hung over the water. On arrival between the piers, one end of each member was lifted up by hydraulic jacks on the gantry, while the other automatically came to rest on the bedplate of the pier. As each section rose clear of the floating dock, this was towed back to the yard in readiness for the next section.
Each pair of half-arches was raised into position, the two side by side, before their opposite numbers were floated out and similarly raised to be connected to them, thus forming the two sides of the complete arch. Once the arches were in position, the erection of the columns and the superimposition of the bridge deck were simple matters. The deck was cantilevered out from one pair of vertical columns to the next until the main part of the bridge superstructure was complete.
The design of the approach viaducts and of the intervening section over the island of Langholm, between the West Bridge and the Pal Sound Bridge, was simplicity itself.
The continuous bridge deck was extended on its vertical columns, these resting on the hard bedrock instead of on the steel girders of the arch members. In addition, where the road passed over the middle of Langholm, there was a “connecting road”, consisting of a masonry causeway on the summit of the island. The road is 292½ feet long and its ends form the abutments for the approaches to the two main bridge sections. Altogether, its masonry walls and ends enclose over 5,000 cubic yards of earth and rock fill.
For the Pal Sound Bridge it was not necessary to float out the arch members to their final position. The sound is comparatively narrow and the bridge clears it at one sweep. All that was necessary was to erect the necessary falsework and gantry in the middle, to manoeuvre the two halves of each arch member into place between this and the shore piers, and then to lift them into their final position.
Some Unusual Tests
The West Bridge at Stockholm has a total length of a little over 1,972 feet, of which 1,295 feet are contributed by the main spans over the channel and the rest by the northern and southern approaches. The main span has a length of 669 feet.
The span of the smaller Pal Sound Bridge amounts to 183 feet. The width of the bridge deck throughout is 78 ft. 6 in. The footways run along each outer side, having a width of 8 ft. 2 in. each. Two tramway tracks run along the middle of the bridge with a narrow island between them and the northbound and southbound carriageways on either side. Although the middle of the channel was deeper than elsewhere (bedrock being there 120 feet below the surface), this did not trouble those responsible for the founding of the middle piers of the West Bridge. The site of these lay on rock quite near the surface, where the bottom of the Riddar Fjord shelves towards the northern shore.
The builders completed their work at the end of 1935, and the bridge was carefully tested by running two trains of tramway cars along the central tracks, a large number of heavily laden road vehicles, including a 20-tons steam roller, along the up and down carriageways, and parading 650 men up and down the footways at the side.
In addition to the main bridge system, extending from the northern abutment of the West Bridge to the southern abutment of the Pal Sound Bridge, there is a long elevated road running from the great street called Drottningholmsvagen, which leads to the centre of the northern part of Stockholm, to the northern end of the approach viaduct. This elevated road in some ways resembles the approach viaducts, but it is carried on flared-top columns of reinforced concrete.
A CLEAR HEADWAY OF 91 FEET above mean water level is provided in the centre of the Traneberg Bridge’s reinforced concrete span. One of the two decks of the bridge is more than 60 feet wide and carries a road, two cycle tracks and two footpaths. The other deck accommodates two electric tramway tracks.
Almost contemporary with this bridge system is the noble Traneberg Bridge which connects Kungsholm, the island on which stands the northern abutment of the West Bridge, with Brommalandet, on the western side of the intervening sound. At first glance it strongly resembles the West Bridge in appearance, though it contains a single great span only. It is not, however, built of steel, but of reinforced concrete and, at the time of its completion, the great twin arch, 580½ feet long, was the longest concrete span in the world.
Two separate decks are carried, side by side, by the Traneberg Bridge. One of these, a fraction over 60 feet wide, carries a road for motor and other vehicles, with footways on either side. The other deck carries a double track for electric cars. The road section is divided as follows: carriageway, 38½ feet wide; two cycle tracks, each 4 feet wide; two footpaths, one 6 ft. 5 in.
and the other 8 feet wide. The deck carrying the up and down electric lines has a width of a little over 27¼ feet.
In designing the decks the engineers aimed at keeping the weight down as far as possible, to prevent too great a strain from being imposed on the concrete arches supporting them. As in the West Bridge, the decks rest on the top of the arch, being supported on either side of the crown by perpendicular columns. Of these, four sets on either side rest on the arch members and one set rests on each of the foundation blocks. The remainder rest on the rock surface, there being fifteen on the Kungsholm side and fourteen on the Brommalandet side of the top of the arch.
There was no need, in the Traneberg Bridge, to build any piers in the water, the two foundation blocks facing each other on the opposite shores of the channel. The methods adopted by the engineers in the erection of this bridge were interesting. Building began at the end of February 1932. One of the problems of building a bridge of this type is the provision of an efficient and reasonably inexpensive temporary structure for the support of the concrete members before they have hardened. The engineers had to choose between wooden or steel construction. Local conditions put out of the question the common method of erecting timber falsework supporting moulds for the concrete right across the sound. There were several possible lines of action. First there was the erection of a frame for the arch across the sound, resting on a limited number of supports coming up from rock bottom; secondly, a wooden cantilever framework spanning the entire gap; thirdly, a bar-shaped framework which could be built up in two pieces near by, These pieces would be floated out one after the other and erected beneath the site of the future concrete arch member. The method of erection would be the same as that used for raising the steel arch members of the neighbouring West Bridge, except that the temporary support for raising the arch members consisted of a steel tower erected on the floor of two floating dock sections in the middle of the sound.
THE BRIDGES OF STOCKHOLM link the various islands on which the city and its suburbs lie. Lake Malar is spanned by the Vasterbro, or West Bridge, which links Kungsholm with the island of Langholm. The road is continued across the island and across the Pal Sound Bridge to Sodermalm. South of Sodermalm is Arsta Sound, which is spanned by the great Arsta Bridge, carrying the main line of the Swedish State Railways.
The third of these possible lines of action had one substantial advantage. Only one such bow-shaped framework for the support of the moulds would be needed and, once the framework had been erected, the work would be relatively simple. For as soon as the superimposed concrete arch member had set hard, it would be possible to move the whole of the underlying temporary arch sideways along rollers on to the site of the next concrete arch member to be erected. This method, which was finally adopted, considerably lessened the cost of construction.
The steel frame consisted of four parallel girder members, secured to one another by cross-bracing, and having a theoretical span of 551 feet. The depth of the frame was 7 ft. 8 in. These girders were built up in sections at the Motala Works, in Central Sweden, whence they were transported to Stockholm and there assembled to form the two halves of the temporary structure. Once the two halves, one on either side, were in position on the site of the future concrete arch, they were securely fastened together by a temporary coupling.
The concreting of the permanent arch above was carried out in two layers, or rings. The lower ring comprised the underside of the arch member and also its outer wall. The upper ring formed the upper and the inner sides of the member. While the concrete was being poured into the moulds through huge funnels, the steel girders of the temporary arch had to be carefully screened from the sunshine to guard against any undue expansion during the setting of the concrete.
Concreting of the southern arch member was completed by Christmas 1932, and at the beginning of March 1933 the growing structure was ready for the steel temporary arch member to be shifted sideways to the site of the northern concrete arch. This was effected with the aid of eighteen hydraulic rams, each with a thrusting power of about 330 tons.
Gradually the steel structure was lowered until it came on to the system of rollers which was to convey it to its second position. The weight of the whole temporary arch, with its equipment resting on the roller tracks, was about 1,200 tons. Once the vertical stage of its journey had been accomplished, two 100-tons hydraulic rams sufficed to push it sideways until it came under the future position of the northern arch member.
The concrete was manufactured on the island of Kungsholm, only about 1½ miles from the site of the bridge. Transport of the concrete to the site was thus simple, the concrete being carried in rotary mixers mounted on motor vehicles. For the transport of materials on the site, a travelling hoist running on a cableway was used. This cableway had a span of 991 feet and extended between two steel pylons, each with its top nearly 190 feet above the level of the sound. The crane had a lift capacity of 4 tons. Concreting took place daily in two shifts between 2.30 p.m. and 11.0 p.m., the morning shift being devoted to the transport of materials and similar work.
The testing of Traneberg Bridge was carried out with three electric cars, each weighing 33½ tons, which were driven backwards and forwards across it at varying speeds. The work was brought to a successful close in a remarkably short period. Authorized early in 1931, the bridge was completed and opened for traffic on September 1, 1934.
The bridge is strikingly handsome, and its single great arch gives a clear headway of 91 feet above mean water level in the middle. In the same way as several other Stockholm bridges of to-day, it replaced an old pontoon bridge of the type formerly so popular in parts of Europe.
In the mouth of the sound leading from Lake Malar proper, a little farther westward than the Traneberg Bridge, there lies an island called Great Essingen. Its neighbour, Little Essingen, lies between it and the island of Kungsholm, with a narrow strait between, spanned by one of Stockholm’s lesser bridges. Between Little Essingen and Great Essingen, however, the gap is wider, and the increasing importance of the larger island as a residential quarter prompted the City Council, during the nineteen-twenties, to authorize the building of a new road bridge between the two.
A STEEL, SINGLE-ARCH SPAN carries the Swedish State Railway over the southern channel of Arsta Sound. The Arsta Bridge is continued over the island of Arstaholm and over the northern channel on masonry viaducts, in one of which is incorporated a vertical-lift section to allow a headway of 102 ft. 6 in. for passing ships. The steel arch has a span of 483 ft. 6 in.
The Great Essingen Bridge is a simple road structure carrying a single carriageway and two footpaths. In the same way as its big neighbour, the Traneberg Bridge, it replaces a temporary pontoon bridge. The pontoon bridge had a lifting span in the middle, but had long proved inadequate as a link between the two Essingen islands. The new Great Essingen Bridge has four-piers founded on the bed of the sound. There are five main arch spans of 128 feet, with an approach viaduct 344 feet long on the Little Essingen side and a short approach of about 102 feet on the other. The bridge was opened for traffic on November 7, 1928.
Bridge building in its straightforward form has not constituted the only activity in the improvement of Stockholm’s road connexions, and one of the most striking features of modern engineering in the city is the new double-decked road junction at Karl-Johans Torg, lying to the south of the island on which the old city stands. Much of the road traffic entering or leaving the city on the southern side has to pass this point, and with the great increase of motor traffic in recent years, this has been a place of steadily increasing congestion.
The traffic problem has been solved here by the building of a huge “clover-leaf ” junction accommodating motor, tramway and pedestrian traffic. The whole great structure is built of concrete. Beneath it lies an underground station belonging to the Stockholm Saltsjon Railway and, adjacent to it, on the west side, is a terminal loop station serving a suburban tramway subway. The whole scheme is an object lesson in practical transport co-ordination, and one can pay no greater tribute to its designer than to say that it has made traffic blocks virtually impossible at this point, despite a complicated layout of electric tramway tracks.
An interesting example of the smaller bridges is the bascule bridge at Danvikstull. Of its kind it is a fine specimen and has several unusual features. It replaces an older structure, and forms part of the works of the Hammarby Ship Canal. The Danviks Bridge carries a motor road on its southern side and the electrified line of the Saltsjon Railway on its northern side.
The railway, similarly to all Swedish electric lines, has overhead conductors instead of third rails, and special connexions had to be provided at either end of the bascule leaf, these giving unbroken contact for the train pantographs while in the down position, yet automatically disengaging whenever the span was raised.
Provision for Motor Road
The Arsta Bridge carries the main State Railway line southwards out of Stockholm. This is one of the principal arteries of the country, and the bridge has been strengthened with a view to superimposing a motor road above the railway tracks at some future date.
Arsta Sound is an arm of Lake Malar. To-day the new Hammarby Canal runs out of its eastern end, and at the western end there is a narrow neck of water across which the old railway bridge used to run. This old bridge involved a considerable detour for all rail traffic running southwards out of Stockholm, and it was to straighten out this troublesome corner that the building of the new great bridge was undertaken.
Midway down Arsta Sound is an island called Arstaholm, dividing the main stretch of water into two channels. It was across these channels, and the island, that the planners of the new bridge decided to make their way, Arstaholm serving them rather as Inchgarvie served the builders of the Forth Bridge in Scotland. Arsta Sound, however, in contrast to the Forth, is a relatively shallow piece of water, and it was thus possible to build most of the great bridge in the form of a masonry viaduct, with two larger openings.
The three masonry viaduct sections, at either end and in the middle, are fine pieces of work, but not unusual. The railway tracks reach a maximum height of 89 ft. 2 in. above mean water level. Over the fairway in the north channel, however, the designers had to allow a minimum headway of 102½ feet for passing ships. Here, therefore, was installed a vertical lift section between two masonry towers, the towers containing the necessary electric hoists.
Over the southern channel the provision of this extra headway was not necessary but, on the other hand, a gap of 483| feet had to be bridged with a single span. The opening was filled with an arch span of steel, similar in design to the spans of Sydney Harbour, Kill Van Kull and other large modern bridges. Both openings are marked at night by flashing lamps for the guidance of shipping. On the bridge itself, the accidental running of trains into the northern opening is impossible when the bridge is lifted. All trains are electrically driven and, on the lifting section being opened, current is switched off automatically in the rail sections lying to north and south of it. Thus the most reckless driver may run past successive signals in the danger position and find his train suddenly without motive power long before it is near the fatal gap.
THE TWO STEEL SPANS of the West Bridge, across Lake Malar, have a length of 1,295 feet. The approach viaducts bring the total length of the bridge up to more than 1,972 feet. About 7,000 tons of steel were used for the main spans.
Click here to see the photogravure supplement to this chapter.
