Part 7

The description just given of the gallery girders will apply to all the cast-iron girders throughout the building, of which there are 2,150; the only difference between them being, that those for the roofs or other internal portions, where no gallery is to be supported, are cast with a less amount of metal. The form of girder just described, which is unusual, was the result of several experiments performed under the superintendence of Messrs. W. Cubitt, C. H. Wild, C. Fox, and other gentlemen, previous to the commencement of the building; and the thickness of metal for the different parts of these, as well as for all the other cast-iron work in the building, was minutely calculated and determined by Mr. C. H. Wild and Mr. C. Fox, under the supervision of Mr. Cubitt, the President of the Institution of Civil Engineers, to whom the Royal Commission had intrusted the responsible duty of the chief superintendence of the whole of the work.

The Galleries.

To proceed to the gallery itself, supported by the girders just described. The timbers supporting the floor are so arranged that the weight of each bay of twenty-four feet square is distributed equally to the four girders inclosing it, and in such a manner as to bear upon them at the points immediately over the vertical standards.

In the transverse direction of the building two pairs of joists, eight feet apart in each bay, are formed into trusses by tie-rods, 1-3/8 inches diameter, passed through a cast-iron shoe at each end, and pressing up two "struts," which are made to bear against the under-side of binding-timbers running longitudinally, or crossing the joists, and immediately under them. The cast-iron shoes for the trusses are bolted down to the girders, and serve at the same time to receive the standard supports of the gallery railing. The ends of the binding-timbers are secured by bolts and oak suspension-pieces to the other two girders inclosing the square. Joists about two feet six inches apart bear from girder to girder parallel to the trusses, and resting on the binding-timbers. On these is laid the floor, 1¼ inches thick, grooved and iron-tongued. A light cast-iron railing, forming a kind of trellis-work, is fixed between the columns, and is capped with a round mahogany hand-rail. From the view at page 60 the arrangement of the galleries will be readily understood.

Testing the Cast-iron Girders.

From the very important office which the girders perform throughout the building, but more particularly those supporting the galleries, it was of the utmost importance that, previously to their being fixed in their places, the soundness of the casting should be proved; for it could hardly be expected that so large a number of girders could be produced without some of them being defective. The ordinary means of testing girders, by loading them with weights, would have occupied far too much time; and therefore an ingenious apparatus was devised by Mr. C. H. Wild for this purpose, by the use of which the testing of a girder occupied but a few minutes.

It consisted of a very strong cast-iron frame rather longer than the girder, the bottom of which was formed by two fixed beams placed eight inches apart, and supported a few inches above the ground. At each end of these a cast-iron standard was firmly bolted between them and rose to a height rather greater than the depth of the girder to be tested; on the inner faces of these standards two "shoulders" were formed, which received the projections cast on the ends of the girder, as before mentioned. Between the fixed beams below, at two points dividing the whole length into three equal parts, were placed strong cylinders, with rising pistons connected with a forcing-pump, together with which they formed a Bramah's hydraulic press. A girder being placed in this frame, in an inverted position, the force applied by means of the pistons rising from the cylinders acted upon it precisely at those points, and in the same manner, as the load from the gallery or the roof would do when afterwards fixed in its place.

The essential parts of the Bramah's press may be thus briefly described. It consists of two cylinders, the diameter of one being considerably larger than that of the other. The smaller cylinder is fitted with a solid plunger or piston, by means of which water may be forced from it into the larger; this being also fitted with a rising piston, the force is communicated by it to the weight which it is desired to raise. The power obtained by means of this apparatus arises from the distributive power of fluids and the practical incompressibility of water, and it is proportioned to the difference of the diameters of the two cylinders; so that if a pressure of one pound per square inch be applied on the surface of the piston in the smaller cylinder, and the piston in the larger cylinder present a surface ten times greater, the power is multiplied by that number; whilst, in addition, the lever power used in applying the pressure to the smaller piston is obtained. The cylinders are fitted with valves, so arranged as to prevent the return of the water from the larger to the smaller, while the apparatus is in action, and thus the power is accumulated in the former.

In the instance before us, the two 3-inch cylinders already alluded to in the proving-frame took the place of the larger cylinder of the ordinary apparatus; and they were connected with the forcing-pump by a strong metal tube. When a girder had been fixed in the frame for proving, the force-pump was worked till the pistons underneath the girder carried it off its lower bearings and pressed it upwards against the "shoulders," by which it was firmly held, and the pressure was then continued until the amount previously fixed upon as necessary for proof had been obtained. This was ascertained by means of a self-adjusting apparatus attached to the hydraulic press.

An iron cylinder 1½ inches diameter was placed in communication with the pipe connecting the pump and the press, so that the pressure obtained in it was, in proportion to its diameter, the same as that in the large cylinder; and it was fitted with a piston-rod, working in a vertical direction. This piston-rod was connected with a lever, from the end of which a scale-pan was suspended, at a distance from the fulcrum ten times greater than that of the point of attachment of the piston from the same. The weight of the scale-pan and lever were balanced by a large mass of iron at the other end. In the scale-pan a certain weight was placed, proportioned to the proof desired to be obtained; and the action of the pump was continued until the water, rising in the iron cylinder just described, forced up the lever, and with it the weight attached; and thus indicated that the pressure to which it was desired to subject the girder had been reached. The weight to be placed in the scale-pan was thus determined: the diameter of the lever cylinder being 1½ inches, and that of each of those in the proving-frame three inches, the pistons or "rams" in the latter presented together eight times the surface of that in the lever cylinder; which being multiplied by the difference of length of the two parts of the lever, determines the weight for the scale-pan to be one-eightieth of that to which it was desired to prove the girder.

The ordinary gallery girders were tested with a pressure equivalent to a weight of fifteen tons; but it was calculated that, when fixed, the greatest weight they would have to sustain would be seven-and-a-half tons. In one instance, for the sake of experiment, the pressure was continued beyond the proof weight of fifteen tons, to see what amount of strain the girders would bear without fracture, and it was found that a strain of thirty tons produced no injurious effect; but the girder broke with an additional weight of half a ton.

Roof of Transept.

We will now return to describe that portion of the roof which varies in form and arrangement from the rest, namely, the semicircular covering of the transept. This is supported by arched ribs, placed twenty-four feet apart, and constructed of Memel timber, in three thicknesses; the centre-piece four inches thick, with a 2-inch piece on each side of it. They are formed in lengths of about nine feet, placed so as to break joint; that is, the joints of the outer pieces fall upon the centre of the inner one. The thicknesses are fastened together by bolts passing through them about two feet six inches apart, besides being nailed at other points. On the inner circumference of the rib thus constructed there is then placed a piece of timber moulded to correspond with the form of the columns; and on the outer circumference two boards, each one inch thick, are bent round and attached to the rib with strong nails. On both the outer and inner circumference a flat bar of iron is secured by bolts passing through the whole depth of the rib, which, thus finished, measures eighteen inches in depth by eight inches in thickness. The ends of the ribs are fitted into sockets, formed by the upward continuation of the columns, to which they are attached by iron straps.

The ribs, which are supported by the trusses over the main avenue, have their ends bolted down upon a piece of timber secured on the upper portion of the truss; and they are further fixed in their places by oak brackets, forming a spreading foot on each side upon the same piece of timber.

Between these large ribs horizontal timbers, called "purlins," are fixed about nine feet apart, by means of cast-iron shoes, bolted both to them and to the ribs. These serve to support the minor or intermediate ribs, occurring at distances of eight feet apart; which consist of a single square piece of timber, having the two thicknesses of 1-inch board bent round their outer circumference, as on the main ribs. The boards form the gutters or furrows between which rise the ridges, in the same manner as in that portion of the roof which is horizontal.

The ridges, in this case, instead of being cut out of solid pieces, are formed in three thicknesses, bent round to the requisite curve, and so retained by small bolts tying them down to the "purlins." The sash-bars which receive the glass form, as elsewhere, the sloping rafters or supports of the ridge.

The space below the first "purlin" or plate at the springing of the arch, down to the level of the lead-flat beneath it, is fitted with louvre-frames for ventilation. The diagonal bracing between the main ribs has been already alluded to. Each set consists of four wrought-iron rods three quarters of an inch in diameter, having eyes at one end, by means of which they are secured with bolts, passing through the thickness of the ribs; in the centre they meet in a cast-iron ring, on the inner side of which the ends are screwed up with nuts.

The semicircular ends of the transept are filled in with tracery, formed by radiating timbers, strutted apart with short pieces placed in concentric rings. The circular heads of the openings are formed by iron castings screwed into their places, and the eye from which the radiating lines of the tracery proceed is also formed by solid iron castings bolted together. On the outer face the ribs of the tracery are moulded, and on the inner side glazed sashes are fixed, filling in the openings.

The lead-flat, twenty-four feet wide, extending the whole length of the transept, on either side of the semicircular roof, is constructed in a similar manner to the floor of the galleries, by under-trussing two pairs of joists in each bay. In the width of the lead-flat roof a horizontal truss is formed by flat bars of iron fixed in the direction of the diagonal of the 24-feet square bays, to resist any possible thrust or tendency of the ends of the ribs to open outwards at the springing.

The Facework.

The external inclosures of the building, on the levels of the different storeys, require but little description in detail beyond that already given. The sash-bars dividing the sashes of the upper tiers are grooved for glass similarly to those used in the roof, and were cut out by the same machinery. The glass was put in after they were framed together, so that it was necessary to arrange the ends of the bars that it could be slipped in at one end. As the bars of these sashes were of slight dimensions and considerable length, they were strengthened by wrought-iron rods passed through the sash-frame and the bars, and screwed up at the ends, causing the whole to work together. The sashes are held in their position by small cast-iron clips, which are bolted on to the columns; and as the surface presented to the wind by the upright sides of the building is of such considerable extent, wooden bridges are fixed against the sashes on the inside, by small cast-iron shoes bolted to the columns; and at the internal angles, where the wind would exert its greatest force, these bridges are further strengthened by wrought-iron rods half an inch in diameter, pressing against the back of them, which is grooved for the purpose, and screwed up at each end in the cast-iron shoes. In this manner a connected chain of resistance to any external pressure is established round the whole circuit of the building.

The louvre-frames, which form part of the face-work in all the different storeys, consist of a deal frame in which bent louvre-blades are hung on pivots at each end. These blades are of galvanised iron of an [S section] form. On the back of each blade is fixed a loop of thin iron, to which a rack is fitted; and by these means all the blades in each frame are moved simultaneously. A considerable number of these racks may also be connected, so that a large area of ventilation may be regulated at once.

The Diagonal Bracing.

From the total absence in this building of any internal division-walls, which in ordinary structures considerably add to their stability, it was thought desirable to introduce into the construction something to compensate for this deficiency. At several points in the length of the building, where a continuous connexion could be established transversely, the squares formed by the columns and girders on the different storeys have their four corners connected by diagonal rods, seven-eighths of an inch in diameter, having eyes at the ends, by which they are secured to the bolts connecting the different parts of the columns. In the centre of the square the four rods meet in a cast-iron ring, and are screwed up with nuts; ornamental faces are fitted into the rings, so that this addition to the construction is by no means detrimental to the general effect.

In a similar manner this diagonal bracing is introduced in a horizontal direction immediately under the floor of some portions of the galleries; of these there are twenty-two sets, and of those placed vertically there are, altogether, 220 sets in the building, and the manner of their introduction will be readily understood from the views of the interior.

The Staircases.

The double staircases, of which it has been mentioned there are eight in the building, consist each of four flights, about eight feet wide; two parallel ones, leading from the ground-floor to a landing, at the half-height, and the other two branching in opposite directions from the landing to the two galleries. The treads of the steps are made of a species of mahogany called sabicu, which is much harder than oak, and therefore peculiarly suited to the purpose for which it is here employed. The risers, or faces of the steps, are of deal. The stairs are supported by cast-iron girders, following the slope, the lower ones being fixed at the foot to stout timbers under the flooring, and the upper ends bolted to the cast-iron columns which support the landing. These columns are of the same pattern as the rest throughout the building, but only five inches in diameter. They are supported on concrete, and eight of them are required for each staircase. The floor of the landing is carried by lesser cast-iron girders, with flooring-joists.

The girders carrying the upper flights spring from the landing girders, and have their upper ends bolted on to the main girders supporting the galleries, which are varied in pattern for this purpose. The railing of the staircase is formed in separate cast-iron standards, one to each step, which are bolted on to the top flange of the girders; and the foot of the standard is so continued that the ends of the treads are fitted into it, and are thus supported. The pattern of these standards is assimilated to that of the gallery railing.

The hand-rail is formed of Honduras mahogany, with carved ends. On each side of the upper flight, which occupies the centre of a 24-feet space, connecting-galleries about eight feet wide are carried, establishing a communication between the two lines of gallery without descending to the level of the landing and then re-ascending. The landing is sufficiently high above the ground-floor to give ample headway for passing underneath it; so that the space occupied by the staircases on the ground-floor is but small.

The Floor and Foundations.

It now only remains to mention briefly the construction of the floor of the building, and the foundations for the base-pieces. The substratum of the site consists of gravel of an excellent quality, and sufficiently dense to have sustained, perhaps without any preparation, the load brought upon it by the bases of the columns. A thickness of concrete, proportioned in all cases to the amount of the weight to be borne by the superincumbent columns, and of such a size as to be two feet in each direction larger than the bed-plates, was placed upon the gravel, and the upper surface was finished with a bed of fine mortar to receive the bed-plates. In this manner it was calculated that in no case would a greater weight than two-and-a-half tons be borne by each foot superficial of the gravel--previous experiments having shown that a considerably larger weight could be placed upon it without any injurious effect.

The timbers supporting the joists for the floor are also placed upon small blocks of concrete, about one foot cube, at a distance of eight feet apart. On these are fixed the flooring-joists, and a deal floor an inch and a half thick is laid on them, as has been already mentioned, with intervals of about half an inch between the boards.

In order to carry off the water brought down from the roof by every alternate longitudinal row of columns, 6-inch cast-iron pipes are fitted into the sockets described in the base-pieces, and are carried in the lines of those columns through the whole length of the building, with discharges into the larger drains at the centre and at each end; the natural slope of the ground gives a sufficient fall to the pipes.

Having thus described in detail all the different portions of the construction of the building, we must proceed to give some account of its actual erection, which will enable us to mention many very ingenious mechanical contrivances which were employed in the course of its progress.

The First Operations on the Ground.

From the great extent of the area required for the building, it was not to be expected that any site would be found of the necessary size, perfectly level. On the ground occupied by the building there is a difference of level between the two extreme ends of about eight feet. In consequence of this fall of the natural surface from west to east, and in order to avoid having a considerable flight of steps at one end of the building to compensate for it, it was determined to arrange the floor with an inclination following nearly that of the ground, such fall being at the rate of one inch in twenty-four feet. All the lines of the building which would be called horizontal in fact follow this line of the floor, and those which are supposed to be upright are placed at right angles to the floor, and therefore slightly inclined from the perpendicular towards the east. The deviation, however, is so exceedingly small as to be perfectly imperceptible even to those who are aware of the fact; and no one who was not previously informed of it would be able to detect it.

It has been mentioned that Messrs. Fox and Henderson's tender for the building was verbally accepted on the 16th of July, 1850, and on the 30th of that month they obtained possession of the site from the Commissioners of Woods and Forests.

The first proceeding was to inclose the whole area (including a considerable space at each end more than would be covered by the building) with a hoarding about eight feet high, put together in a very simple manner, so that the boards were afterwards available for the flooring. The supports for the hoarding consisted of pieces of timber fixed in the ground in pairs, at intervals of the length of the boards, leaving a narrow space between them, into which the boards were dropped, and thus held in their place without any nails. Temporary offices were then erected in a convenient portion of the site, and were covered with a roofing which was a specimen of that to be used in the building itself. Considerable ranges of carpenters' sheds were also put up, and even stables for twenty or thirty horses, which were required in the progress of the works.

Setting out the Ground.

The first thing to be done towards the building itself was to set out accurately all the points where the columns would stand, as well as the general outline of the building. It will be readily understood that this was an exceedingly important part of the work, as upon its accuracy depended the fitting together of the various parts that had afterwards to be put in place.

This part of the work was executed with great precision by Mr. W. G. Brounger. He commenced by determining the four extreme angles of the building, and the centre lines of the main avenues. These formed fixed points from which were determined the whole of the centres for the columns.

Our readers will recollect that the dimension of twenty-four feet occurs horizontally throughout the building, either in multiples or sub-multiples. In order to measure off the different distances, rods of American pine were made, into which, near the ends, pieces of metal were fixed, having corresponding notches at the exact distance of twenty-four feet apart. By these means the lengths were measured off with great accuracy, as the wood used is not liable to alteration in the length of its fibre; and by means of the metal notches the rods were sure to be placed correctly together. It was necessary to make these sockets or notches of metal, from the great amount of work the rods had to perform.

In determining the length of the rods, the standard of the Astronomical Society was used; and this was referred to in all important measurements for the castings and other parts of the building, to insure their precise eventual agreement in length. This will hardly be considered to have been unnecessary when it is remembered that, from the great length of the building, a very minute error in any of the parts would have been so multiplied as sensibly to throw out the ends.

To those who are unacquainted with the fact, it may be well to mention that the standard of length referred to is obtained from a pendulum, which oscillates seconds, in the latitude of London, in a vacuum, at the level of the sea, at a certain fixed temperature. The length of this pendulum is then divided into a certain registered number of feet and inches.