THE HUEY LONG BRIDGE carries a double railway line, two 18-feet roadways and two footpaths, each 2 ft. 6 in. wide. A clear headway of 135 feet is afforded in midstream. This has necessitated long approaches which give the bridge a total length of four and a half miles.
FROM St. Paul, Minnesota, to New Orleans, Louisiana, the muddy, yellow Mississippi is spanned by twenty-nine major bridges carrying railway tracks. The twenty-ninth, near New Orleans, is the vast new structure known as the Huey Long Bridge. This is four and a half miles in length including its approaches and is numbered among the greatest bridges in the world.
Recent as is the completion of this great work, proposals for building a giant bridge across the Mississippi at or near New Orleans have been made at intervals over a long period. As far back as 1892 there were agitations for such a structure, and again shortly before the war of 1914-18. The war caused the plan to be shelved, but in 1924 it was revived.
Various factors governed the delay. Before the war the bulk of the traffic passing in and out of New Orleans to and from the West was carried by railway. The Texas and Pacific Railroad already had across the river a steam train ferry which, although not capable of dealing with an expanding traffic, was at least sufficient for contemporary needs. The nineteen-twenties, on the other hand, saw a boom period in the American motor industry. The number of cars, buses and lorries grew and grew until within a decade, there was one car for every five inhabitants in the United States. As many more people were travelling than before, the proposed bridge was to become a link necessary not only to the railway system of the State of Louisiana, but also to its highway system.
Two great main roads intersect at New Orleans. The first of these is the Old Spanish Trail, running from St. Augustine in Florida to San Diego in California. This road was formerly broken by a ferry in New Orleans. The other great road, which meets the Old Spanish Trail, is the Jefferson Highway, which runs northwards for hundreds of miles to the Canadian frontier and beyond, coming to an end at Winnipeg, Manitoba. Following roughly the same course as these two roads, the former of which, as its name implies, is the trail taken by the early Spanish settlers, are two great main lines of railway. These are the Texas and Pacific, which owned the old train ferry, and the Illinois Central.
For the consolidation of these four great arteries of transport, with their many tributary routes, American engineering science was called upon to produce the Huey Long Bridge.
After the revival of the scheme in 1924, Ralph Modjeski, a well-known consulting engineer of New York, was asked to prepare tentative designs. Considerable preparations, in surveying and in the drawing office, were necessary before the plans reached tangible form, and in their ultimate stage they had undergone a great deal of modification. The official permit for the building of the bridge was not issued by the United States War Department until December 5, 1930. This permit required that the bridge should contain a central navigation span over the fairway not less than 500 feet in length and giving a clear headway of not less than 135 feet above mean Gulf level, that is, the level of the adjacent Gulf of Mexico. It was this considerable minimum height which necessitated the provision of long approach viaducts, giving the bridge its great length.
A FALSEWORK BENT, or temporary pier of steel, being erected under the 531-feet simple girder span of the Huey Long Bridge. Bents were temporarily placed under the cantilever section as well. The bents were founded on 12-in piles driven down into the firm ground below the bed of the river. The average weight of the piles was 85 to 100 lb.
The design adopted involved a steel cantilever bridge, with deck-span girder approaches, carrying a double line of railway inside the girders and a dual carriageway for road vehicles. The carriageway was to consist of two 18-feet roadways, placed outside the girders on either side of the railway and carried by cantilever extensions of the steel beams supporting the floor of the railway track. Outside the carriageways were provided two footways, each 2 ft. 6 in. Wide.
The great central cantilever span over the fairway has a total length of 790 feet from pier to pier. In common with the other three main spans, this consists of fourteen panels. The central suspended section between the two cantilever arms is 500 feet long. The anchor span of the cantilever on the side opposite to the city of New Orleans is 529 feet long, the length of the other anchor arm being 531 feet. Beyond this second anchor arm comes a simple girder span, with the rails inside it, and also measuring 531 feet from pier to pier. The remainder of the bridge, comprising the approaches, consists of deck spans, with the railway laid on the tops of the girders instead of inside. The roadway, on the other hand, descends through the girders at a relatively steep angle.
DISMANTLING one of the steel shells used in the construction of the main piers. The shell was circular, 121 feet in diameter, and extended from above the surface of the river at high water, down to the river bed and into it for a distance of from 10 to 15 feet. After the shell had been placed inside the falsework shown, it was filled with sand, forming an island through which the caisson was sunk.
The central cantilever span is level. On the girder and anchor spans in the main section of the bridge rail and road descend at a gradient of 1 in 80. This is maintained throughout the approaches by the railway, but through these sections the gradient of the roadways is increased to 1 in 25.
Sand Island Method
The laying of the foundations involved some interesting work. To sink the piers for the main section across the Mississippi the contractors for the substructure used what is known as the sand island method. Hitherto this process had been unique in its application, having been used only in the building of the Suisun Bay Bridge of the Southern Pacific Railroad in California.
The first step was to protect the site of foundations and piers with huge “mattresses” of willow saplings. For the four main piers in deep water, each of these woven mattresses was 450 feet long by 250 feet wide. The bed of the Mississippi at New Orleans consists of a shifting mass of sand mixed with streaks of clay. A considerable depth had to be penetrated before a firm foundation for the piers could be found. The use of the mattresses kept the surface even and more or less constant in readiness for the first steps in building the caissons for the bridge piers.
The weaving of the mattresses took place in barges moored upstream from the site of the bridge, each mattress, as it increased in size, being allowed to float out over the side of the barge. On completion of the mattresses, the men in the barges ballasted the mattresses uniformly with sandstone and lowered them into the bed of the stream, sinking the upstream end first so that the current should assist in the sinking process instead of impeding it, as it would otherwise have done. They controlled the movement of the mattresses downstream with manila ropes, dropping additional ballast on to them after they had submerged. The mattresses thus sank into position on the sites of the future piers, falling through the water at a long, downstream slant. The navigators of the ballast barges controlled their craft so as to allow for the action of the current on the falling ballast, and to prevent the ballast from falling beyond the mattresses.
WILLOW MATTRESS floating on the surface of the river at the site of main river pier. This mattress was built of willow saplings and was approximately 250 feet wide and 450 feet long. The mattress was sunk to the river bed by weighting with stone while securely anchored at the upstream end. The object was to prevent scour of the river bed during the building and after the completion of the river piers.
The mattresses were finally held in position by great fluke anchors, each weighing three tons, which were laid in position approximately 600 feet upstream from the mattresses. Each mattress was equipped with three of these anchors, to which it was attached by steel cables. These cables ran right through the length of each mattress, their extremities being secured to the downstream headers of the mattresses. Each mattress, after having been submerged and brought into position, was easily located by four buoys, anchored to its four corners. Four observation points, two on either side of the river, were set up whence a watch could be kept on these buoys and signals could be transmitted to the masters of the tugs and to the engineers in the ballast barges.
With the mattresses in position, it was possible for the bridge engineers to begin operations on the piers. First they erected a ring of timber piling or falsework round the site of each pier, on top of the mattress. Inside this they built up a steel shell. On the sites of the great piers which were to support the cantilever section over the fairway the shells were each 121 feet in diameter. Those built round the sites of the piers supporting the girder span on the New Orleans side had each a diameter of 111 feet. Their heights respectively were 106 feet and 80 feet, the tops being some 16 feet above average high-water level.
The mattresses below were a temporary measure to guard against disturbance of the river bottom before the sinking of the shells. On completion of a shell the circular piece of willow mattress inside it was severed with cutting piles, driven down round the inner circumference of the shell and removed through the open top.
With the shells in position and the matting inside them removed, the engineers then proceeded to form their sand islands. First they dredged sand from a part of the river bed situated some distance above the site of the bridge. They filled the sand into the shells with the aid of derricks mounted on the top of the surrounding falsework. Each shell thus became an artificial island; hence the name given to the whole method. The shells remained steady during the filling process, and even at the deepest piers a settlement of no more than 3 feet was recorded.
On the tops of these sand islands the engineers were able to build the reinforced concrete caissons on which the piers were ultimately to rest. Each caisson was built up in the dry, on top of the sand, and then sunk through the shell towards the river bed by its own weight, the sand being simultaneously dredged out. Such was the nature of the shifting silt that the engineers had to penetrate to a depth of 170 feet below the mean level of the Gulf of Mexico.
The caisson walls were built on the cellular principle, each containing fifteen dredging wells. On the sites of the main cantilever towers the caissons, oval in section, were each 102 feet by 65 feet thick, the thickness of the walls being approximately 4 feet. On the sites of the piers supporting the girder span beyond, the caissons were 93 ft. 6 in. by 53 ft. 6 in., the walls being 3 ft. 6 in. Thick.
The engineers engaged on the caissons alternately sank them into the silt and applied fresh concrete. Each of the larger two piers supporting the cantilever span required 24,000 cubic yards of concrete and 450 cubic yards of granite masonry work. On top of each caisson was built a timber cofferdam. The object of this was to exclude water from the top of the caisson, after it had sunk below the surface of the river, until the upper pier had been superimposed and completed.
A certain amount of trouble was occasioned by blowouts from the bed of the river. Silt from the river and pieces of the sand island fill would form a bed of quicksand beneath the caisson and, being subjected to enormous pressure, would shoot up through the dredging wells to the surface. When this happened, the weight of the caisson over the suddenly vacated space below made it settle violently, causing more or less damage to the temporary works round and above it.
THE CENTRAL SPANS of the Huey Long Bridge during construction. Half of the main cantilever span has been completed and part of the strain is taken by the bent erected under the completed girder span until the other cantilever arm is finished.
The worst blowout occurred at the fourth pier, that is, the one which was to support the New Orleans end of the girder span. The lower cutting edge of the caisson wall was entering a stratum of good sand at a level of 165 feet below mean Gulf level. Here, however, it encountered an unexpected vein of hard clay which set up considerable friction. To disperse this clay the engineers inserted a number of small charges of dynamite in it. When the charges had been fired, the entire caisson gave a sudden lurch and dropped 3 feet, while a surge of quicksand shot up into it, rising to a height of 60 feet in the dredging wells and blowing out the cofferdam above. The steel shell of the sand island sank abruptly with the caisson, pulling down and destroying a quarter of the surrounding timber falsework. This was the worst and last of the accidents due to blowouts. The engineers cleaned out the caisson, repaired the falsework and carried the caisson down another 2 feet before sealing it. This pier was completed in August 1934.
The final operation took place when the concreting of the piers had reached a safe distance above average high-water level. This operation consisted in removing the steel shell which had enclosed the artificial sand island and reinforcing the base of the pier against scouring action by the river water. Once the foundations had been sunk into and safely through the shifting bed of the Mississippi, it was possible to go forward with the steel superstructure, which to the ordinary observer is the most imposing part of the bridge. The girders and steelwork generally of the main spans were erected by the American Bridge Company of New York. The building of the cantilever section across the fairway was difficult, but it was carried through without delay or accident. As the U.S. War Department would not permit the navigational opening to be less than 500 feet at any time, the greater part of the span had to be built, as the Forth Bridge had been built, without any supporting falsework. The only intermediate support allowable was close to the two main piers.
The engineers engaged on the superstructure began operations on the tops of the piers, each group working outwards to meet the next. Thus from the cantilever towers they built out simultaneously on either side, the inner arms of each cantilever being balanced by the anchor arms. Instead of the more usual travelling derricks, which proceed along the bridge as it is assembled, they used fixed steel cranes mounted on the tops of the piers, these having masts no less than 113 ft. 6 in. long and booms 95 feet in length. So long as the two arms of the cantilever balanced each other, all was well and no further support was necessary. When, however, it came to erecting the middle, or suspended span between them, it was another matter. The weight of an increasing section of this span, had it been supported by its parent cantilever alone, would have set up considerable unequal stresses in the parent cantilever which the weight of the anchor span could not have counteracted.
To guard against this, the engineers erected a steel bent, or temporary pier, rising from the bed of the river to the level of the bridge girders as far out as possible from each pier supporting the cantilever section. These bents, temporarily subjected to tremendous weights, had to be absolutely firm while they lasted. In effect they served the purpose of piers for the time being. When the two halves of the suspended span met and were united, the whole cantilever span with its two anchor arms became self-supporting and the bents could be removed.
BUILDING THE WEST APPROACHES to the Huey Long Bridge. Reinforcing steel has been set in place in preparation for concreting the upstream or westbound roadway A travelling A-frame derrick is at the forward end of the approach. The difference of gradients between the railway deck and the highway deck is apparent.
The engineers founded the bents on piles driven down into the firm ground below the bed of the river. Each pile was 12 in. thick and the weight of the piles averaged 85 lb. to 100 lb. These piles were spliced together in varying lengths, the maximum being 75 feet. The next component part of the bent consisted of a “cage”, built up of structural steel girders. The cage was mounted between two large pontoons. The pontoons in turn were towed to the site of the bent and anchored firmly on either side of it, so that the cage was directly over the piling. The engineers then lowered the cage on to the top of the piles and bolted it firmly in position. Each bent below the cantilever arms was founded on forty piles. These were arranged in two rows, the distance between them, centre to centre, being 2 feet.
From the cage rose the bent itself. This was built in the form of a strong steel trestle, stoutly reinforced with cross-bracing and rising perpendicularly to the level of the bridge deck. For the central support of the simple girder span on the New Orleans side of the cantilever section a double bent had to be raised. For it the engineers used the same principle as that which they had used for the bents supporting the cantilevers, but they founded the double bent on eighty instead of on forty piles in the river bed. The erection of the girder span was the subject of considerable delay through natural causes. The engineers drove their eighty piles and had them ready to receive the cage by March 1935. In that month, however, came the spring rise of the river, caused by thaws spread over the vast extent of its basin in the north. The rise was particularly severe during that year.
Flooded March came to an end and flooded April succeeded it. April is always the month of high water in the Mississippi Valley. The floods increased and work on the girder span remained at a standstill. There was nothing to be done until the central double bent could be erected; so the gaunt, unfinished span remained open. May followed April, but at first there were only slight signs of a diminution in the flood. Not until June were the engineers at last able, with perfect safety, to manoeuvre their pontoons out into the stream, anchor them firmly without fear of dragging and set the supporting cage in place on the piling.
Built in Less than Three Years
After this, work on the great bridge progressed speedily. The huge girders grew out from their piers, and the structure rapidly assumed its final form.
Construction work on the approach spans was a relatively simple matter. Deep foundations were not so necessary here, away from the scouring action of the river, and there was no need to keep wide passages open for the movement of river navigation. It was possible for the engineers to give continuous support to the ever-lengthening girders by placing bents under every other panel, each span being temporarily supported on a steel and pile foundation. On the top of the highest pier at either end the engineers mounted a big travelling crane, carried on four undercarriages fitted with double-flanged wheels.
At first, while the initial panels were being built out, these cranes remained on the tops of the supporting towers. Then, as the girder reached the first bent, the crane moved on to it, and building of the next panel began. Thus work on the two approach sections was carried out progressively, the two cranes, one at either end of the bridge, moving slowly down the gradient towards the extremities, building up their paths before them as they went. The substructural work was relatively light. Within a comparatively short time the travelling cranes on the approaches reached the abutments, with the girder spans, railway and road decks completed in their wake.
Thus the spanning of the Mississippi almost at its lowest point, a bare three and a half miles from the centre of New Orleans, was accomplished in a fraction under three years, the bridge having been begun in January 1933. The whole of the superstructure consisted of steel, concrete and a certain amount of granite masonry. For the railway flooring the engineers used silicon steel, as well as for nearly all the girder work. Heat-treated carbon steel was used for eye-bars, and some of the struts and hangers were of medium carbon steel.
On December 16,1935, the great work was completed and was opened for traffic, being named after Huey Long, then Governor of Louisiana.
THE COMPLETED BRIDGE across the River Mississippi. The central span is 790 feet long, of which 500 feet comprise a suspended span supported by cantilever arms. On the approach spans rail and road descend from the level central span at a gradient of 1 in 80. The railway continues at this gradient along the approaches but the roadway steepens to 1 in 25.