Engineering skill is required not only in the building of a great cathedral, but also in the rebuilding or strengthening of many structures which have weakened in the course of centuries
THE BELFRY OF LIVERPOOL CATHEDRAL in course of construction. At a height of about 180 feet above the floor of the cathedral is a concrete girdle in the tower. The concrete girdle carries the steel girders which are designed to support the ringing chamber. At this stage of the work the choir and transepts had been completed, but the central tower and the nave were still to be built.
IN building and restoring cathedrals the engineer is called upon to carry out many daring and ingenious feats. The building of the belfry in the great central tower of Liverpool Cathedral presented a complex structural problem, and engineers were called in to design and carry out the work.
The belfry, the highest and largest in Great Britain, if not in the world, is designed to carry a peal of thirteen bells, and the floor is 180 feet above the floor of the cathedral. Nothing of this kind had ever been attempted before, and the builders could obtain little guidance from previous work. The task which faced the engineer was how to distribute an exceptionally heavy load evenly upon the four corner piers without putting too much strain upon the foundations. The satisfactory solution of the problem, however, does not by any means exhaust the engineering wonders of this magnificent building rising on St. James’s Mount, almost in the heart of the great metropolis on the Mersey.
Cathedral building involves specialized engineering. From the time the foundation stone is laid until the last block is raised in position, the work has to be carefully watched and checked. Stresses and strains have to be worked out to a nicety. The foundations and the great buttress walls have to be designed to carry exceptionally heavy loads. Above all, the finished fabric must be imposing and stately, worthy of a place among the architectural masterpieces of the world.
Liverpool Cathedral will be the largest and loftiest of Great Britain’s Anglican churches, easily eclipsing in size St. Paul’s Cathedral and Westminster Abbey in London. The floor of St. Paul’s has an area of 87,400 square feet, compared with Liverpool’s 100,000 square feet. The only cathedrals that can claim the distinction of being larger than that at Liverpool are Seville Cathedral in Spain, Milan Cathedral in Italy, St. John the Divine in New York (still under construction) and the Roman Catholic cathedral now also being built at Liverpool. Though the area of St. Peter’s in Rome - 227,069 square feet - is twice that of the cathedrals mentioned, this great shrine in Rome is not a cathedral church.
A century or more may be spent in the building of a cathedral; the work can proceed only as funds permit. Work was begun on Liverpool Cathedral in 1903 and it has continued without interruption ever since. It is about two-thirds complete. The central tower, its principal architectural feature, is scheduled to be finished in 1941, when work should begin on the building of the nave. Then the cathedral will stand complete. Work on the Roman Catholic cathedral at Liverpool began in 1933, and New York’s cathedral has been in the builders’ bands since 1892. The architect of the Anglican cathedral at Liverpool is Sir Giles Gilbert Scott, and the style of the great fabric is Gothic, but it is Gothic of no particular country or period. The architect’s aim was to design a building which, while conforming to English liturgical requirements, would accommodate with in hearing of the preacher a large congregation - the cathedral will have accommodation for 3,500 worshippers - and at the same time produce in the individual a sense of reverent, awe.
In general plan the building consists of a great central space crowned by a noble tower rising to a total height of 347 feet, or 331 feet above floor level, and bounded by nave, choir and transepts. The combined area of the central space and transept crossings is 15,180 square feet, nearly twice the area under the dome of St. Paul’s Cathedral. From the extreme end of the nave to the farther end of the Lady Chapel is a total length of 619 feet, a length exceeded only by that of St. Peter’s, which is 718 feet long. First came the building of the Lady Chapel with the necessary vestries, followed by the Chapter House, choir and eastern transept. As part of the site consisted of loose earth and stone it was necessary to go right down to the solid rock because of the heavy load which the walls would have to sustain. The foundations for the main piers were carried down to a depth of 54 feet below the ground floor level, and were 45 feet square at the bottom, gradually tapering to 25 feet square at the ground level, the excavations being strongly timbered to prevent the sides from slipping.
Built on Underground Bridge
WHEN all the soft rock had been excavated and the hard stone reached, the excavations were filled in with Portland cement concrete. The foundations of the inner piers and walls vary in depth from 20 to 30 feet with the hardness of the rock. The foundations were carried up in blue Staffordshire bricks, built in cement to the correct levels to receive the walls of the superstructure. The cathedral is built upon what may be described as an underground bridge rising from the solid rock. To complete the foundations, it was necessary to excavate and remove 70,000 tons of earth, to mix and place in position 26,000 tons of cement concrete, to build in cement 1,250,000 blue bricks and 400,000 St. Helen’s bricks, and to set in position 6,000 cubic feet of stone This work was carried out in eighteen months.
When the foundations had been secured, work upon the superstructure was begun. A local sandstone of a warm rose tint was selected as the stone for the building. The contractors took over the Woolton Quarries, some five miles from the site, and laid down the necessary machines to produce this stone in large quantities.
In addition to the machinery at the quarry, a masons’ yard, with an extensive plant, including four large electrically driven frame saws, has been established at the cathedral site. After the stone has been roughly cut by the saws, it passes to the masons, whose duty it is to cut and dress the stones to the desired size. This work has to be done carefully. In the tracery of the windows and groining of the roof, every stone is of a different size and shape.
THE MASONS’ YARD at Liverpool Cathedral. The cathedral is built of local red sandstone obtained from quarries five miles distant. The stone is brought to the yard at the cathedral site, and is roughly cut by large electrically driven frame saws. It is then chiselled by the masons to the desired dimensions. The larger blocks weigh as much as 5 tons or more.
The choir is flanked by aisles, the construction of which is one of the triumphs of the design, these aisles being tunnelled through the buttress walls. From the floor to the apex of the vaulted roof the height is 116 feet, compared with the 102 feet of Westminster Abbey and the 99 feet of York Minster. Sufficient decoration has been introduced to relieve monotony, the general effect being enhanced by simplicity and the absence of excessive ornament.
Many features of the great fabric have been treated as part of the building, and not merely as incidental furnishings. The reredos, or altar screen, is of red sandstone, partly gilded, and is structurally part of the wall. The architect felt that it should not be treated as a detached feature, but rather as an integral part of the building, as it forms the culminating point in the whole design. Above the screen towers the great east window, the overall measurement being 76 feet by 44 feet, making it the largest in Great Britain.
The construction of the vaulting over the intersections of the transepts called for the display of considerable ingenuity. On one side the vaulting rests on three arches, but on the other there is a single supporting arch, extending the full width of the building. Specially strong timbers were necessary to carry the vaulting ribs over the crossing, and large upright baulks of timber were erected from the ground, to support the platform on which the centering was built.
The strength of this timbering may be estimated from the fact that the weight of the vaulting was about 500 tons. The size of the stones for the centre bosses of the choir is 5 feet by 5 feet by 4 feet and, when they were ready for fixing, each weighed over 3 tons. The central boss to the large groining over the crossing required a stone 7 feet by 7 feet by 4 feet, weighing over 5 tons. The material used in choir and transepts included 823,500 cubic feet of stone, 6,500,000 bricks and 2,650 tons of cement.
Marble on Concrete
Special attention was paid to the lighting and heating of the building. The whole of the space below the transept arms and crossing was excavated to a depth of more than 20 feet, to provide space for the maze of hot air pipes and other ducts, and for the heating apparatus and electrical machinery. The ducts, flues and heating chambers are built throughout of reinforced concrete.
The main floors of the transepts, crossing and choir are formed of reinforced concrete slabs supported on large reinforced concrete beams, a few of which were made hollow inside, to form a main duct for hot air. The concrete was finally covered with a marble flooring. Some 2,100 tons of cement concrete and forty-nine tons of steel reinforcements were used in connexion with the heating scheme.
The placing of the belfry in the tower involved the building of a ringing chamber, a sound-damping chamber and a bell frame between the levels of 179 feet and 215 feet. This was a task for the engineer, and the work was entrusted to Mr. Burnard Geen, M.Inst.C.E. His problem was to keep the load on each of the tour corner piers as nearly as possible equal. He surmounted the difficulty by building a concrete girdle above the main vault. This girdle virtually forms the base of the tower, although it stands some 180 feet above the floor of the cathedral. Upon this concrete girdle rest massive steel girders which carry the weight of the ringing chamber and of the massive bell frame.
BUILDING THE TOWER VAULT, 176 ft 6-in above the transept crossing. This photograph was taken in June 1936, when the centering had been partly removed. The weight of the vaulting is about 500 tons.
The building of the concrete girdle - which has a total external peripheral length of 344 feet - presented some difficulties, and special precautions were taken to minimize shrinkage. The girdle is 6 ft 1½-in deep and 6 feet wide, with recessed sides to provide a bond with the enclosing brickwork of the tower walls. The concrete was reinforced with mild steel bars 1½-in in diameter. As it was impossible to pour all the concrete at one time, the girdle was divided into forty-four sections, each section being cast as a single block. The work was difficult at times, as the force of the wind at this elevation was sometimes great and, apart from the brick walls and the narrow platform upon which the men had to work, there was no protection. Altogether some thirty-two tons of steel bars and 600 tons of concrete were used in the girdle.
Steel girders bridge the huge space within the tower walls, and these girders are not merely ties joining the walls together. The tying of the walls is performed by the concrete girdle on which the steel girders rest. In plan the four girders form a St. Andrew’s Cross; they are 21 feet deep and have a span of 79 feet. The total weight of the finished steelwork is some 220 tons.
Upon this steelwork is built the ringing chamber, 22 feet in height. Above it comes the sound-damping chamber, 10 feet high, an insulated space designed to deaden the sound conveyed to the ringers. The peal of bells, thirteen in number, will be hung on a circular reinforced concrete frame in the centre of the tower. The arrangement of the bells in a circle is entirely new and offers considerable advantages, but is practicable only because of the great size of the tower.
This belfry section of the tower presented an unusual problem. The approximate weight of the bells is 36 tons, including a Bourdon bell weighing 12-14 tons, but the total load added to the foundations by the girdle constructional work, the three floors and the bell frame is nearly 2,000 tons. At a height of 179 feet above ground floor level the four corners of the tower, consisting of brickwork with stone facing, are approximately 22 feet by 16 feet. Through each corner provision is made for a lift or staircase, and the walls between the corners are reduced in thickness as far as possible, to save weight on the foundations.
The building of the tower necessitated the use of elaborate temporary scaffolding and special cranes and derricks for lifting the steel members, heavy stone and other material into position. The stones for the upper walls of the structure were raised by twin cranes with 75-feet jibs. At a radius of 52 feet these cranes have a maximum lift of 3 tons, which gradually decreases until at a radius of 65 feet the maximum safe load is 1 ton. The rate of lift is approximately 140 feet a minute with a load of 1½ tons.
As the tower rose in height an electric lift was installed for the use of the workmen. The lift shaft, of steel, has been placed against the north face of the tower and carried up to a height of 230 feet to a gangway leading to the interior of the tower through one of the louvre windows. The cage holds twelve men and ascends at a rate of approximately 100 feet a minute.
The tower is rising at the rate of slightly over a foot a week. Two hundred men, more than half of them masons, are engaged upon it. The cathedral is being built entirely by voluntary subscriptions, and at the time of the dedication of the building in 1924 a sum of £1,000,000 had been spent upon it. Another £1,000,000 will probably have been spent before the great fabric is finally completed. The finished parts of the building - the Lady Chapel, Chapter House and choir, with their various vestries - are open and used for public service.
ONE OF THE VAULTS over a transept of Liverpool Cathedral. The vault is fashioned entirely of timber, its strength lying in the massive ribs. The vaults owe their stability entirely to their design and there is no concealed metalwork in them at all.
Unfortunately the great cathedrals are liable to structural troubles. These are generally due to faulty foundations which cause the piers and buttresses carrying heavy domes and roofs to subside. If such faults were not attended to they would ultimately cause the collapse of the building. In 1925 St. Paul’s Cathedral, London, was deemed a “dangerous structure” by a committee of eminent engineers and architects who had been called in to examine the building. There were serious subsidences of the piers and the dome was out of the perpendicular. The greater part of the building was closed to the public for a period of five years to enable extensive reconstructional work to be carried out.
An architectural feature of St. Paul’s is its great dome, 145 feet in external diameter and 420 feet in circumference. St. Paul’s has three domes - an external one, an inner one and an intermediate conical dome of brick which bears the weight of the lantern, ball and cross which surmount the structure.
These three domes and the galleries they carry represent a weight of about 70,000 tons. Sir Christopher Wren placed his great dome on four bastions and eight piers, the main weight falling on these piers, which are arranged in pairs. At crypt level the piers are 43 feet long and 20 feet wide. Instead of being built up solid, however, they have merely a thin veneer of Portland stone from 4-in to 6-in thick. Their interior originally consisted of debris and rubbish from the earlier building on the site, dumped in without any attempt to bed the material or to form a solid whole.
The eight piers were found to have subsided, the degree of subsidence varying from 2-in to 6½-in. They were also badly cracked, and the dome was 5¾-in out of the perpendicular. This threw an undue strain upon the walls, which are by no means thick. They are not strengthened by buttresses, as are so many cathedral and church walls. They run up a considerable distance; then comes a series of niches. Into these niches, or crevices, run the great arches that span the nave, choir and two transepts; and, to counterbalance their tremendous thrusts, smaller arches are made to bulge against them, giving a perfect balance of forces.
The first task was to strengthen the piers by grouting, reinforced by steel bars. Holes were drilled into the piers and cement was forced into them under pressure. More than twenty tons of cement were injected into each pier, and the piers were further strengthened by the insertion of bars made of “Staybrite” steel.
As the engineers were anxious that the work should be as permanent as the wit of man could make it, special attention was paid to the steel used. Stainless steel was decided upon, and “Staybrite” steel was finally selected. For reinforcement in cement a steel bar should have adhesion, which was lacking in the ordinary smooth-surfaced steel bar. Tests carried out at the National
Physical Laboratory showed that the difficulty of adhesion could be surmounted by making the bar oval and by indenting it so that a series of flats was formed. Tests showed that, whereas a plain bar, having been set in cement, required a maximum load of only 2·85 tons for its withdrawal, an oval indented “Staybrite” steel bar required 18·85 tons. More than 120 tons of this steel were used for reinforcing the bases of the piers.
TWO ARTICULATIONS of the first steel chain which was used to strengthen the dome of St. Paul’s Cathedral, London. The completed chain is about 450 feet long and weighs 35 tons. The metal is a rustless steel with a high chromium content.
The dome of the cathedral presented great difficulty. Not only was it out of the perpendicular, but also there was a tendency for its diameter to spread. To have taken down and rebuilt the dome would have been a long and costly undertaking. The engineers decided that it could be saved and given a new lease of life by tying the inner wall of the dome to the outer wall and then binding the dome with hoops or chains. Steel tie rods of 4-in diameter and 40 feet long, were used for tying the two walls. The rods were threaded at either end and were secured by washers and nuts.
For the binding hoops a different kind of steel was used. It was essential that it should be rustless and non-corrosive, that, it should have great strength and yet be as light as possible. A rustless steel known as “Twoscore” was selected. It has a high chromium content which makes it almost immune from atmospheric corrosion, and it has excellent mechanical properties as well as great tensile strength.
Two chains of this steel were made, raised and bolted in position. The first encircles the outer wall of the dome at a point just below the Whispering Gallery. It comprises an articulated chain of the pin-and-link type, built up of a series of three full links alternating with two full and two half links, the section of a full link being 3⅜-in square. The total length of this chain is about 450 feet and the chain weighs 35 tons.
The second chain forms a 32-sided polygon and is fitted round the bases of thirty-two bastions to the outer wall at the level of the clerestory walk. This chain is also of the pin-and-link type, the links being of similar design to those in the first chain. The second is considerably longer than the first.
These chains are designed to withstand a load of 1,000 tons which, it is anticipated, will be sufficient to prevent any further increase in dome diameter. Each chain has an effective cross-sectional area of some 34¼ square inches. At maximum load the intensity of the stress in the steel is approximately 29 tons per square inch, a figure made possible by the use of a hardenable steel.
While this restoration work was being carried out it was necessary to protect the piers by temporary scaffolding. Some 217 tons of timber and 120 tons of steelwork were used for the purpose. This difficult and intricate task lasted five years; the cost was about £400,000, raised by public subscription. Because of this restoration work it is hoped that the cathedral is now sound for centuries to come.
Diver Renews Foundations
AN interesting underpinning feat was carried out at Winchester Cathedral, where there were serious subsidences in various parts of the building. The most alarming falling away, disclosed in the presbytery, amounted to nearly 2½ feet. The outer walls and their buttresses were considerably out of the perpendicular, in some places by as much as 2 feet in 44 feet and in others by as much as an inch in a foot. This sinking of the walls had caused the vaulting and arches inside the building to become distorted and cracked. Stones occasionally fell from the roof, a sign that disintegration had set in.
The architect and the engineer who were called in to save the cathedral sank a pit a little distance from the building. They found that at a depth of 10 feet the work had been stopped by water. The early builders had filled in the trench at water level with trees, principally beech. These they had placed side by side horizontally, and upon this somewhat unstable foundation they had built the massive walls of the cathedral. Here and there the soil was so loose that two layers of trees were necessary. Although some of the logs had become rotten because of the presence of water, others were perfectly sound. The sinking of the building has been entirely due to the soft ground immediately below the trees. Further excavation revealed the presence of a bed of chalky marl, in some places 6 feet thick, immediately below the logs. This, in turn, rested upon a bed of peat, some 8½ feet deep. Water had penetrated this peat and clay, issuing through the gravel below them. The water in the gravel came from the River Itchen, near which the cathedral is situated.
STRENGTHENING THE PIERS on which the weight of the dome of St. Paul’s Cathedral rests. Holes were drilled into the eight piers and more than 20 tons of cement were injected into each pier. Further strength was afforded by the insertion of reinforcing bars of special stainless steel.
It was impossible to remove the chalk and peat and to fill in the intervening space with concrete or brick in the ordinary way, for as soon as any of the clay or peat had been removed the space was filled with water. It was a question of building a foundation under an existing one in water from 12 to 14 feet deep, and it was work which could be carried out only by an expert diver. The plan adopted was to shore up the exterior walls and to attack the foundations in sections, five or six feet at a time.
A pit some five or six feet in length was sunk, the old foundations uncovered and the beech trees exposed. Water was then reached; the diver went down, removed the clay and peat and inserted the new foundations. The pits which the diver had to dig were absolutely dark, and the whole of his work had to be carried out by the sense of touch.
When the peat from each of the five-feet beds had been removed, jute bags filled with concrete ready mixed were lowered to the diver, who carefully placed them side by side until the whole area was paved with them. They were then slit open to allow the material to spread evenly over the bed of the pit.
Scaffolding 212 Feet High
FOUR layers of bags were placed in each pit, slit open and trodden down firmly and evenly. The concrete was then left for several days to set. Remaining water was next pumped out and the concreting was continued right up to the original foundations.
Meanwhile, attention had been paid to the cracks and fissures in the masonry, some of which were 11-in wide. The first step was to force in air under considerable pressure to blow out the accumulated dust of ages, also to dislodge owls, martins, rats and mice and their nests. Swarms of bees were dislodged. After air had been blown in, water was forced in to wash out the cracks, the effluent coming out black; the washing was continued, and when the effluent was clean and colourless the grouting machine was brought into play and liquid cement was forced under pressure into every interstice and cavity, thus solidifying walls, piers, and towers. The work occupied three years and the cost was £50,000.
Extensive restoration work has been carried out also at Lincoln Cathedral, necessitating considerable grouting of walls, piers and towers, the removal of timber flooring because of damage by the “death-watch” beetle, and the building of special scaffolding 212 feet high to reach damaged portions of the towers. Large cavities and cracks had to be filled in solid by grouting.
During the execution of the work a dungeon was found, which in early and medieval days had been used for confining prisoners who were shortly to be executed. It had neither door nor window, nor even a ventilator. An ancient doorway and stairway were also found.
The work of the cathedral engineer is indeed varied. He is always stumbling upon unexpected difficulties calling for considerable resource, patience and ingenuity.
STEEL GIRDERS, forming in plan a St. Andrew’s Cross, span the central tower of Liverpool Cathedral just over the vaulting above the transept crossing. The girders are 21 feet deep and have a span of 79 feet. The total weight of the finished steelwork is about 220 tons.