In order to obtain the requisite depth of girder without unnecessarily increasing its weight or diminishing its thickness, it was necessary to cast it open; and for this purpose various ornamental forms were proposed, but the trellis form was decided on as being the strongest. It consists of a top and bottom rail or flange, A and D, united by four perpendicular struts, BB, CC, and the diagonals EFG, the load being supported over the struts BB. "If we dissect this girder and consider first the top rail A, the struts BB, the diagonals EE, and the central portions of the bottom rail D, we have a simple trussed beam, the rail a being exposed to compression, and prevented from deflecting by the struts BB, which discharge the weight on the parts ED E, which in effect form a tie suspended from the two ends of the rail A, which keeps it extended.

The struts cc rest on the columns and support the whole. If we now disregard these parts and consider merely the bottom rail D, the diagonals FF, and the central part of the top rail a, we have an arch FAF confined by the tie D. If, however, a weight is placed on only one of the struts B, (which may happen by crowds moving along the galleries,) there would be a tendency to distort the form of the arch. This is effectually prevented by the introduction of the diagonals G G. If we now view the girder as a whole, we find that it combines both the trussed girder and the arch, and that the central part of the top and bottom flanges is active in both cases. This part therefore is exposed to a greater strain than the other parts, and is accordingly strengthened by widening the flanges. The whole of the metal is thus distributed exactly in the parts in which it is required for supporting the load. At the same time the whole forms a stiff frame, admirably adapted for connecting the columns and preventing their deviating from the perpendicular." The depth of this girder is 3 feet, and its length 23 feet 3 inches.

No less than nine varieties of girders or trusses are used, varying in strength, construction, and material, according to their position in the building and the duty they have to perform. There are three strengths of 24-feet cast-iron girders, 24-feet wooden trusses, 24-feet wroughtiron trusses, 48-feet wrought-iron trusses; and three strengths of 72-feet wrought-iron trusses. One of the 24-feet wooden girders is shown in Fig. XXII. The top flange A, consists of a piece of oak 3 inches deep by 24 inches wide, surmounted by a deal board 1 inch thick and

Fig. XXII. ELEVATION OF 24-FLET WOODEN TRUSS.

6 inches wide, and parallel between the vertical struts B B, and tapered off to 3 inches wide at the ends. cc are vertical oak struts at the ends 4 in. x 24 in., and B B are oak struts 4 in. wide and 14 thick. The length of the girder is only 23 ft. 3 in., a space of 43 in. being left at each end to afford space for the column. The distance from centre to centre of the struts BB is 8 feet. The bottom flange D, is formed of two flitches of deal, 4 in. deep by in. thick, placed side by side and fixed by wood screws to a deal board in. thick and 8 in. wide between the struts B, and tapered to 3 in. at the ends. The two flitches are connected at the ends by pieces of oak 2 ft. long, 4 in. wide, and in. thick, placed between them. A wrought-iron strap к,

tiens, Sections and Details laid down to a large Scale from the Working Drawings of the Contractors. By Charles Downes, Esq. Architect, and Charles Cowper, Esq. Assoc. Inst. C. E. &c." London: 1831,

K

2 in. wide, in. thick, and 2 ft. long, is passed round the upright o, to which it is secured by ain. bolt. Two similar bolts secure the strap to the flitches of the bottom flange. The diagonals EE, FF, GG, are 4 in. x in.: EE being exposed to tensile strain are secured at each end by 3 in. bolts passing wrought-iron straps HH, II, 2 in. wide and in. thick. The straps нн are 10 in. long, and one is placed on each side. The straps II are 2 ft. long, and pass round the upright c. The diagonals EE are of oak. The diagonals FF being exposed to compression. are merely notched into the top and bottom flanges, as shown by the dotted lines. The diagonals GG are notched into the top flange and rest upon the in. board of the bottom flange, being secured by a pin passing through the two flitches. The struts B B are secured at bottom by a pin in. diameter passing through the two flitches, and are notched into the top flange and secured by a wooden pin.

Each of the roof trusses over the 48-feet avenues, Fig. XXIII., is mostly of wrought-iron, the parts which are of cast-iron acting under compression only. The top rail A is formed with two pieces of angle-iron placed back to back an inch apart, as shown in Figs. XXIV. and XXV., their total sectional area being 3 inches, and the bottom rail, B, with two flat bars of wroughtiron also an inch apart, and increasing in sectional area up to 3.38 inches as they approach the centre of the bearing. At the ends, these bars are riveted on to cast-iron standards, and the intermediate distance is divided into 8 feet lengths by other cast-iron standards, c, to which bars are also riveted, thus forming a framework of rectangles. The rivets pass through the angle-irons and bars, the standards and the ties forming a rigid top table under compression and a lower suspension truss; the standards or struts are also so pressed up, as to raise the centre of the upper table to a camber of 4 inches. Each of the rectangles is divided in the direction of one of the diagonals by flat bars, FED, passing between and riveted to the bars, which form the top and bottom rails. The other diagonal is fitted with a flat bar of wood, G, in order to give the girder a uniform appearance. One of these trusses weighs about 13 cwt., and being loaded with a dead weight of 10 tons, there was a deflection of 3 inches, but it perfectly recovered its form when the weight was removed.

Fig. XXIV.

Fig XXV.

The 72-feet trusses, Fig. XXVI., for spanning the nave, required to be increased in strength, in order to support the larger weight of roof due to the increased area: their depth was not allowed to exceed that of the connecting pieces used throughout the building, namely 3 feet. This larger truss was constructed on the same principle as the 48-feet trusses, the only differences

I

Fig. XXVI. ELEVATION OF A PORTION OF THE 72-FEET TRUSSES.

being a great increase in the scantling of the angle-irons and bars, and the length of 72 feet being divided into nine 8-feet lengths instead of six. The weight of one of these trusses is about 35 cwt.; the sectional area of the two angle-irons Figs. XXIV. XXV., being 5.71 inches, that of the two bottom bars at their maximum 6.75 inches, and that of the principal diagonal ties 3.38 inches. A load of 16 tons caused a deflection of 6 inches, but the truss perfectly recovered its form when the load was removed. These trusses were cambered or bent upwards about 10 inches.

The great extra weight thrown by the transept roof upon the last 72-feet trusses of the nave, where it intersects the transept, rendered it necessary to construct trusses of double depth, with extra lattice-work and increased scantling. The top rail is composed of two pieces of angleiron placed back to back, and further connected on the top by a flat piece thus T. The lower rail is formed by two flat bars placed upright and riveted at the ends to standards of cast-iron. Also in the centre are two slots or sinkings, in which the ends of two of the diagonal bars are riveted. The whole length is divided into three equal parts, each 24 feet long, by strong cast-iron standards, the ends of which are riveted between the rails, and these spaces are again subdivided into three 8-feet lengths by wrought-iron standards. The top of each standard is next connected with the foot of the next but one to it by diagonal flat bars, which together with the short pieces fastened into the slots complete the trellis-work.

The wrought-iron trusses were put together at the Works. They were riveted together on horizontal supports on which the proper curve for the camber had been marked out. The bars were previously cut to the required lengths, and punched and drilled with holes for the rivets. They were laid out on stages in their proper forms, with the cast-iron standards temporarily kept in place by bolts passed through some of the rivet-holes. A row of temporary forges by the side of the platforms supplied red-hot rivets, which were passed through the holes and hammered into form by the workmen. Several sets of men being employed, as many as 16 trusses were completed in one day. Each of the 48-feet trusses were held together by more than 50 rivets, requiring twice that number of holes to be made in the bars of iron. About 25,000 rivets were required for the whole work.

By means of drilling, punching, and cutting-machines, worked by a steam-engine of 6-horse power, the pieces of bar-iron were adjusted to their requisite lengths: the holes for riveting

having been marked upon them with templates, were punched out, and any larger perforations required for rivets of extra size were drilled. The punching-machine is represented in Fig. XXVII., the upper part of which is furnished with shears for cutting off the ends of the bars. The stand containing the punch and shears is formed by a heavy mass of cast-iron containing two indentations, in the lower of which the punching is performed, and in the upper the cutting. Motion is given to each of these by a cogged wheel at the back; both the punch and the shears working slowly up and down. The punching of a hole occupied two or three seconds, and the iron became quite hot from the effect of the pressure. All that the workmen had to do was to place the bar on a solid rest in the proper position, and the descending punch perforated the hole. The action of the machinery is then reversed, and the punch ascends, during which time the bar being readjusted, or another bar put in its place, the operation is repeated continually.

The drilling-machine, Fig. XXVII., differed somewhat in its detail from that described in the article DRILLING (p. 526). The bar to be drilled was placed flat upon the table of the machine, and the drilling-point, working vertically, was suspended at one end of a lever with a counterpoise at the other. This lever was connected by a rod and crank with another near the ground, one end of which was formed by a tread to be worked by the foot. By pressing down this tread the counterpoise end of the upper lever was raised, and the drill brought down upon the iron. Three men were required at this machine, as well as at the punching-machine.

The roof trusses over the 48-feet avenues were raised much in the same way as the 24-feet

trusses. In all cases horses were employed to run out the end of the fall rope, which was passed through a pulley or catch-block at the foot of the shear-legs, in order to change its direction from vertical to horizontal.

The roof trusses over the 72-feet or main avenue were fixed in the following manner :-" A single mast or derrick, more than 70 feet high, was placed in the centre of the avenue, and steadied in an upright position by guide-ropes spreading from the top in various directions. Near its summit the hoisting tackle was firmly lashed on. The trusses to be hoisted were brought from the places where they had

been put together, and placed across the main avenue at the points where they were to be fixed. Two ends of a stout chain were passed round the upper portion of the truss, at points dividing its length into about three equal parts. To this chain the hoisting tackle was attached, guide-ropes being further fastened to each end of the truss, to steady it in its ascent. In order to stiffen the truss horizontally, struts were attached at the centre, projecting on each side, and held in their place by tie-rods attached to the upper part of the truss, and forming a triangle on each side. Before the truss, therefore, could bend in a horizontal direction, the attachment of these tie-rods must have given way. Six horses drew out the end of the fall-rope, and in the course of a very few minutes the truss was hoisted to its giddy height, and each end slipped in between the projections made in the connecting-pieces to receive it.

One of the tall masts used in raising the 72-feet trusses was worked on each side of the transept from the centre to the ends of the building; and it was constantly maintained in an upright position while traversing from point to point by alternately slackening and hauling up the ropes which steadied it. It was moved forward by means of crowbars and levers applied to its foot, which rested on stout planks placed on the ground. As many as seven of these trusses were hoisted in one day by each derrick, which had therefore to be moved forward a distance of 7 x 24 = 168 feet.

In fixing the girders provision was made for the expansion and contraction of the iron under varying changes of temperature. Between the projections on the connecting-pieces and those on the ends of the girders a space was left for the insertion of oak keys, which being driven in, the girder was fixed in its place, while the compressibility of the wood left sufficient play for the expansion of the metal. In the upper and lower flat flanges of the girders, small sinkings were cast near the ends: corresponding with these sinkings, a notch was left in the projection which came out from the connecting-piece; and it has been already stated that when the girder was put into its place, iron wedges were driven in between the notch and the sinking, by which means any lateral motion of the girder was prevented.

The wrought-iron trusses were held by the connecting-pieces in a similar manner to the castiron girders; but, as an additional security, bolts were passed through holes provided in the standards at the ends, and through the connecting-pieces, where they were screwed up with

nuts.

The extra strong roof trusses crossing the main avenue near the side of the transept were the first that were fixed across the central avenue: the heaviest weighed no less than 8 tons each,

and about 150 men were engaged in hoisting each. They were secured to the columns by four strong bolts, passing through the end standards. Extra columns were also introduced beneath these heavy girders: they were built up in stories, and connected at the level of the girders by bolts and straps. A cast-iron shoe on the top of the columns furnished a bearing for the ends of the truss. These additional columns project slightly into the main avenue from the line of the other columns, and this is the only instance in the building of columns occurring at a less distance than 24 feet apart.

By a repetition of operations such as those above described, the whole framework of this vast structure, with the exception of the transept, was completed.

The absence of internal division-walls, which add greatly to the strength of ordinary

Fig. XXIX. DIAGONAL BRACING.

buildings, led to the introduction of a diagonal
bracing at several points in the length of the
structure. The squares formed by the columns
and girders on the different stories have their
four corners connected by diagonal rods of iron
ths inch in diameter, with eyes at the ends
by which they are secured to the bolts connect-
ing the different parts of the columns.
four rods meet in the centre in a cast-iron ring,
and are screwed up with nuts; ornamental
faces being afterwards fitted into the rings. See
Fig. XXIX. Diagonal bracing is also introduced
in a horizontal direction under the floor of some
portions of the galleries.

The

The timber supporting the joists for the ground floor is placed upon small blocks of concrete, 8 feet apart, and each about 1 foot cube. On these the flooring-joists are fixed, and a deal floor 1 inch thick is laid on them at an interval of half an inch between the planks. The gallery floor, which is supported by the 24-feet girders, consists of cross-beams so undertrussed with iron rods, shoes, and struts as to distribute the whole weight which it has to bear pretty equally upon the eight points at which the ends of the beams rest upon the girders. Joists stretching from the iron girders to the beams, and from one of the beams to the other, form the supports for a floor which is not more than 1 inch thick, but is strengthened and rendered impervious to the passage of dust by the insertion in a groove cut in the edge of each floorboard of iron hooping forming a tongue. The gallery is furnished with a railing designed by Mr. Owen Jones. See Fig. XXX.

The double staircases, of which there are eight, consist each of four flights about 8 feet wide,

viz. two parallel ones leading from the ground floor to a landing, and the other two branching in opposite directions from the landing to the two galleries. The treads of the steps are of a very hard kind of mahogany named sabicu: 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. There are eight of these columns for each staircase: they are each 5 inches in diameter, and are supported on concrete: the floor of the landing is carried by cast-iron girders with flooring-joists. The girders carrying the upper flights spring from the landing-girders, and their upper ends are bolted on to the main girders which support the

big. XXX. DOUBLE STAIRCASE.