|Tower Bridge, London||
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|BHA have been involved in the engineering aspects of Tower Bridge since designing the present electro-hydraulic drive system in 1974 . The article below, from a 1930's partwork, describes the building of the bridge and gives some details, albeit in the imperial measure of the day. Tons are imperial long tons, approximately equal to a metric tonne; one foot is 0.3048 metre.||The hydraulic motors, installed in 1974/75, drive the bridge through gears and the original final pinions which engage the arcuate racks attached to the moving bascules. In the horizontal position, some load is taken by resting blocks beneath the bascules. Work is going on to develop a way of transferring all the load to these blocks, as the original designers are believed to have intended.|
|click here for a section through the bascule|
The problem of building a bridge over a busy river with low banks so that shipping is not obstructed is one that taxes the resource and ingenuity of the engineer. He surmounts the difficulty by resorting to the opening type of bridge, of which the main types are the drawbridge or bascule bridge, turning about a horizontal axis ; the swing bridge, turning about a vertical axis ; the rolling lift bridge and the vertical lift bridge.
One of the most famous examples of the bascule type is the Tower Bridge, which spans the River Thames just below London Bridge. It is the most distinctive of London's bridges and its construction was a masterly engineering achievement. The building of the Tower Bridge came about because the development of cross-Thames traffic had far outstripped the capacity of the existing bridges.
By the year 1870 the position had become serious, and between 1874 and 1885 some thirty petitions from various public bodies were brought before the authorities urging either the widening of London Bridge or the building of a new bridge.
A two days' census taken during August 1882 showed that the average traffic for twenty-four hours over London Bridge -which at that time was only 54 feet wide-was 22,242 vehicles and 110,525 pedestrians. A committee was appointed to consider the matter and to report upon the different plans that had been proposed.
These included schemes for low-level bridges with swing openings of various kinds, and high-level bridges with inclined approaches or with lifts at either end. There was also a proposal for a railway line to be built at the bottom of the river and to carry a traveling staging with its deck projecting above high-water level. Proposals for a subway and for large paddle wheel ferry boats were also considered. one of these schemes was approved.
In 1878 Horace Jones, the City architect, put forward a proposal for a low-level bridge on the bascule principle - that is, a bridge on a level with the streets with two leaves or arms that could be raised to let ships pass up and down the river and lowered to let vehicles pass to and from across the waterway. Successful bridges of this type already existed, though on a much smaller scale, at Rotterdam and Copenhagen.
"Bascule" is derived from the French word for see-saw," and the bascule bridge is a kind of drawbridge which works on a pivot and has a heavy weight at one end to balance the greater length at the other. This was the type of bridge finally decided upon, and it has proved a great success.
The Tower Bridge is, perhaps, the most famous bascule bridge in the world, and its working from the day it was first opened to the present has been perfect, far exceeding the hopes even of its most enthusiastic advocates. An Act of Parliament empowering the Corporation of the City of London to build the bridge was passed in 1885.
Horace Jones was appointed architect and was knighted, but died the same year, and Mr. (afterwards Sir) John Wolfe Barry was appointed engineer. The work was divided among eight different contractors Among them Sir John Jackson was responsible for the piers and abutments, Sir William Arrol for the steel superstructure, Sir W. G. Armstrong, Mitchell and Co., Ltd., for the hydraulic machinery and Perry and Company for the masonry superstructure.
Work was started on the bridge in April 1886, the foundation stone being laid, on behalf of Queen Victoria, by the Prince of Wales, afterwards King Edward VII. The bridge was to have been finished by 1889, but difficulties arose and Parliament was twice asked to extend the time for the completion of the work.
It did so, and the bridge was eventually opened on June 30, 1894, having cost about £1,000,000 sterling to build, a remarkably small sum for such a bridge in such a position. The total length of the bridge, including the approaches, is half a mile. The roadway has a width of 35 feet and on either side of it is a footway 12.5 feet wide.
The total height of the towers on the piers, measured from the level of the foundations, is 293 feet.
140 Feet Headway for Ships
In building the bridge there were used about 235,000 cubic feet of Cornish granite and Portland stone, 20,000 tons of cement, 70,000 cubic yards of concrete, 31,000,000 bricks and 14,000 tons of iron and steel.
The bridge is a combination of the suspension and bascule type. The width of the river between the abutments of the bridge on the north and south sides is 880 feet. This is crossed by three spans. The two side spans, each 270 feet long, are of the suspension type. They are carried on stout chains that pass at their landward ends over abutment towers of moderate height to anchorages in the shore. At their river ends the chains pass over lofty towers which are themselves connected at an elevation of 143 feet above high water. Heavy tie bars, at the level of the connecting girders, unite the two pairs of chains so that one acts as anchorage for the other at the centre.
The central span has two high-level foot ways side by side, and one low-level roadway. High-level girders carry the upper footways, which are reached by hydraulic lifts or staircases in the main towers. The roadway, or central opening span, is 200 feet long and consists of two bascules or leaves.
The Tower Bridge Act laid down that when the bridge was open there should be a clear headway at high tide between the water and the high-level footways of 135 feet and a headway of 29 feet when the bridge was closed. These dimensions were exceeded in practice, the open height being 5 feet and the closed height 6 in. greater than had been prescribed. This was above high-water level. The greatest extreme between high and low tide at Tower Bridge is 25 feet.
The Act further stipulated that the piers were to be 185 feet long and 70 feet wide. There was also a clause making it compulsory to maintain at all times during the building of the bridge a clear waterway 160 feet wide. This stipulation made it impossible for the two piers to be built at the same time, because the staging would have occupied far too much of the river space. As the use of timber cofferdams was prohibited, the builders had to rely on caissons. The restricted area which they were allowed for their staging, 130 feet by 335 feet, did not permit the use of one caisson extending the full length of a pier.
The builders therefore adopted a system of small caissons covering the area of the pier. By this means it was possible while building one of the piers to be working also at the shore side of the other. Had both piers proceeded simultaneously a saving of thirteen or fourteen months might have been effected.
The piers of the Tower Bridge are much more complicated structures than the piers of an ordinary bridge. In addition to supporting the towers carrying the overhead girders for the high-level footways and the suspension chains of the fixed spans, they also house the counterpoise and the machinery which operates the bascules.
The caissons used for securing the foundation of the piers consisted of strong boxes of wrought iron, without either top or bottom. To secure a good foundation it was found necessary to sink them to a depth of about 21 feet into the bed of the river. There were twelve caissons for each pier. On the north and south sides of each pier was a row of four caissons, each 28 feet square, joined at either end by a pair of triangular caissons, formed approximately to the shape of the finished pier. There was a space of 2.5 feet between all the caissons, this being considered the least dimension in which men could effectively work. The caissons enclosed a rectangular space 34 feet by 124.5 feet. The space was not excavated until the permanent work forming the outside portion of the pier had been built, in the caissons and between them, up to a height of 4 feet above high-water mark.
The method adopted in building and sinking the caissons was unusual. First came the building of the caisson upon wooden supports over the site where it was to be sunk. The caisson was 19 feet in height and it was divided horizontally into two lengths. The lower portion was known as the permanent caisson and the upper portion, which was removable when the pier was completed, was called the temporary caisson. The object of this upper portion was simply to keep out water while the pier was being built. When ready the supports were removed and the permanent caisson lowered to the river bed (this had previously been levelled by divers) by means of four powerful screws attached to four lowering rods.
After the caisson had reached the ground various lengths of temporary caisson were added to the permanent section, till the top of the temporary portion came above the level of high water. The joint between the permanent and the temporary caissons was made tight with india-rubber. Divers working inside the caisson excavated first the gravel and then the upper part of the clay forming the bed of the river. As they dug away the soil, which was hauled up by a crane and taken away in barges, the caisson gradually sank until its bottom edge penetrated some 5 feet to 10 feet into the solid London clay. London clay is a firm watertight stratum, and when the desired depth had been reached by the caisson it was safe to pump out the water, which up to this time had remained in the caisson, rising and falling with the tide through the sluices in the sides.
The water having been pumped out, navvies were able to get to the bottom of the caisson and to dig out the clay in the dry. Additional lengths of temporary caisson were added as the caisson sank, so that at last each caisson was a box of iron 57 feet high, in which the preparation of the foundations could be made. The caisson having been controlled from the first by the lowering rods and screws, its descent any farther than was desired was easily arrested by the rods when the bottom of the caisson was 20 feet below the bed of the river. The clay was then excavated 7 feet deeper than the bottom of the caisson, and outwards beyond the cutting edge for a distance of 5 feet on three of the four sides of the caisson. In this way not only was the area of the foundations of the pier enlarged but, as the sideways excavation adjoined similar excavations from the next caissons, the whole foundation also was made continuous.
All the permanent caissons, with the spaces between them were then completely filled with concrete, upon which the brickwork and masonry were begun in the temporary caisson and carried up to 4 feet above high water. The preparation of the foundations was a long and troublesome task because of the extent of the river traffic, which made it difficult to berth the necessary barges. On two occasions "blows" occurred which hindered the operations. When the cutting edge of one of the caissons had reached a depth of 16 feet beneath the river bed, water rushed into the caisson through a rent in the clay. The caisson had to be lowered still further to seal the opening when the water was pumped out.
The second blow was due to one of the stage piles between the caissons having been driven in aslant. As the caisson went down its cutting edge came in contact with the pile and thus loosened the clay in the immediate neighbourhood. Divers were sent down to ascertain the damage and the pile was re-driven. The full extent of these handicaps was underestimated and thus this section of the work occupied much longer than had been expected. Finally there emerged four feet above high-water mark two gigantic piers of concrete, granite and bricks able to withstand without settlement a load of 70,000 tons. From the river bed upwards the piers are faced with rough picked Cornish granite, in courses between 2 feet and 2.5 feet thick. The piers called for the excavation of 30,000 cubic yards of mud, silt and London clay. The material consumed in the piers was 25,220 cubic yards of cement, 22,400 cubic yards of bricks and 3,340 cubic yards of Cornish granite. The cost of the piers was £111,122. As soon as the piers had been finished the building of the towers began.
Stone Over Steel
Because of the fine masonry work of these towers, Tower Bridge is often mistaken for a stone bridge. It is a steel bridge, however, just as much as is the Forth Bridge, and it depends entirely for its strength upon the steel columns and girders of which it is composed. As the authorities insisted that the design of the bridge should be in keeping with its surroundings, the steelwork is faced with masonry whose architectural character is made to harmonize with the general style of the Tower of London close by.
The masonry is Cornish granite and Portland stone, backed with brickwork. Each of the steel towers consists of four octagonal columns, with a diameter of 5 ft. 6 in., connected at a height of 60 feet above the piers by plate girders, 6 feet deep. Across these are laid smaller girders which carry the first landing. Twenty-eight feet higher is the second landing, similarly built, and at an equal distance above that is the third landing, leading to the high-level footways Each column rests on a massive granite slab previously covered with three layers of specially prepared canvas to make the pressure even and the joint watertight. The columns are keyed to their foundations by great bolts built into the piers.
All four columns in each pier are braced diagonally to resist the wind pressure, which is calculated at a maximum of 56 lb. to the square inch, a pressure several times greater than has ever been registered in the locality. It was important that precautions should be taken to prevent any adhesion between the masonry and the steelwork of the towers. With this object the columns were covered with canvas as the masonry was built round them, and spaces were left in places where any later deformation of the steel work might bring undue weight upon the adjacent stonework. The masonry covering forms an excellent protection against extremes of temperature.
All parts of the metal not accessible for painting purposes after the bridge was completed were coated thoroughly with Portland cement. Manholes were provided in the steel columns to make it possible to paint the interior whenever it became necessary. The abutments of the bridge, which were built by means of cofferdams in the usual manner and without difficulty, have similar but shorter towers.
The towers finished, workmen tackled the high-level footways. These are cantilever structures, each with a suspended span. They were built out from either tower simultaneously. The footways are cantilevers for a distance of 55 feet from either tower and suspended girders for the remaining distance of 120 feet between the cantilever ends. The building of these cantilevers attracted a great amount of attention on the part of the public, who watched their gradual approach with keen interest. Every care was taken to prevent rivets, fragments and tools from falling into the river below, to the peril of passengers in passing vessels.
Intricate Suspension Chains
Along the upper boom of the footway run the great ties connecting the suspension chains at their river ends. Each of the two ties is 301 feet long and is composed of eight plates 2 feet deep and 1 inch thick, ending in large eye-plates to take the pins uniting them to the suspension chains. The making of these chains was one of the most interesting and at the same time most delicate parts of the whole undertaking. Each chain is composed of two parts, or links, the shorter dipping from the top of the abutment tower to the roadway, the longer rising from the roadway to the summit of the main tower. The links have each a lower and upper boom, connected by diagonal bracing so as to form a rigid girder. They were built in the positions they had to occupy, supported on trestles, and were not freed until they had been joined by huge steel pins to the ties crossing the central span and to those on the abutment towers.
The boring of the pin holes was a matter of great delicacy and considerable difficulty. The holes in the eye-plates of ties and chains had been bored to within 0.5 in. of their final diameter before leaving the contractor's works at Glasgow, and the finishing touches were added when the plates were in position. The labour of enlarging all the holes to their full diameter was equivalent to boring a hole with a diameter of 2 ft. 6 in. through 65 feet of solid steel. Most of this boring had to be done in somewhat awkward positions at the top of the main towers and abutments, whither it was necessary to transport engines, boilers and boring tools.
The outstanding feature of the bridge is its opening span, consisting of two bascules or leaves. Each leaf consists of four parallel girders 13.5 feet apart and about 160 feet long. When lowered the leaf projects horizontally 100 feet towards the opposite tower, spanning exactly half of the opening. The point of balance is a solid pivot, with a diameter of 1 ft. 9 in. and a length of 48 feet. It passes through the girders 50 feet from their shore ends. The pivot is keyed to the girders and rotates on roller bearings carried by eight girders crossing the piers horizontally from north to south, themselves borne on girders under their ends.
The chief difficulty attending the erection of the bascules was due to the condition that compelled the contractors to leave a clear way of 160 feet between the towers. In other circumstances the girders might have been completed before being brought into line and connected together. As it was, the engineers first built the portions on the shore side of the pivot, added a short section of the riverward steelwork and launched the incomplete girders from the main stage close to the piers into the bascule chambers. A steel mandrel (cylindrical rod) was inserted to carry their weight while they were turned into a vertical position. The mandrel was then withdrawn to make room for the permanent pivot, which weighed 25 tons. The outer ends were added to until a point 53 feet from the pivot had been reached. Work in this direction then stopped until the raising and lowering of the leaves for purposes of adjustment had been concluded.
After that the girders were completed vertically. The leaves, each of which weighs about 1,200 tons, are moved by toothed pinions, engaging with steel quadrantal racks riveted to their two outside girders. The accurate attachment of the racks was a somewhat difficult business because of the confined space in which the men had to work. To preserve the balance of the bascule it was necessary to load the shorter, or inner arm with counterpoises, consisting of 290 tons of lead and 60 tons of iron enclosed in ballast boxes at the extreme ends of the girders. The function of the raising gear is merely to overcome the inertia of the 1,200-tons leaf and the friction caused by wind pressure on the exposed surface. In designing the hydraulic machinery allowance was made for a wind pressure of 56 lb. to the square foot.
Opened and Shut in Five Minutes
The source of power is a building on the east side of the southern approach, where are stationed two large water accumulators with 20-in. rams loaded to give a pressure of from 700 lb. to 800 lb. per square inch. The engines are duplicated on either pier to provide against the possibility of breakdown. The operations of opening and shutting the bridge are safeguarded by every possible means. When the leaves are brought together bolts carried on one leaf are locked by hydraulic power into sockets on the other leaf. In the event of anything going wrong with the opening and closing mechanism there would be no danger of disaster, for the leaves would be brought gently to rest in either the vertical or the horizontal position.
The whole process of opening the bascules, allowing a ship to pass and bringing them down again for the resumption of road traffic takes only five minutes. Thus the large hydraulic lifts, which go to the top of the tower to the overhead footway with eighteen passengers in one minute, are rarely used. It has been found that the interruption of traffic is so brief that pedestrians do not take the trouble to go up and over the footway, but wait for the lowering of the bascules.
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