There are 6 radiating struts in each rib, varying in length from 6 feet to 12 feet, the length increasing from the springing to the centre. They are similar in section to the principals, but are only 7 inches in depth, being attached to them, and to the tie-rods, by means of wrought-iron linking plates. The top of the strut is made to touch the under side of the principal, in which position it is clasped by linking plates, and secured by a bolt 1-1- inch in diameter.
The tie-rods in each rib are composed of three lines of rods between the two extreme radiating struts, and from these struts to the extremities of the principals they are in two lines, the sectional area in each case being the same, viz. 61 square inches. The ends of the tie-rods, which are pre pared with eyes to receive the bolts, are placed side by side between the linking plates, attached to the struts, and a bolt is then passed through them : it will thus be evident, that if any elongation takes place in the ties, the struts will be acted upon.
The diagonal braces extend from the bottom of each strut to the top of one next towards the springing ; they hold the struts tight up against the principal, and at the same time assist the tie-rods in their duty. These braces are formed of round-iron, 11 inch in diameter, secured at top by bolts passing through the linking•plates, and at bottom by wedges, so as to give an opportunity of tightening them up if required.
Each compartment of principals is thus separately trussed and tied, and the whole made fast at the extremities by passing a strap or stirrup iron round the back of a metal chair, in which each end ot: the girder rests, and to which it is bolted at the side ; the jaws of this stirrup-iron are attached to the extremities of the rods by wedges.
The ends of the principals are fixed in east-iron chains resting on one side of a metal pillar, and on the other on the wall, or upon the box-beam ; those upon the pillars are east upon the upper cap, and those upon the wall and box-girder rest upon two rollers, which can travel a space of 3 inches upon a metal plate, so as to admit of expansion and contraction.
The purlins are each formed by a combination of three T irons, the centre T iron running straight from principal to principal, and those at the sides branching off at 5 feet from each end, so as to strut the girder in three points. The purlins are secured to the deck-beam by angle plates, fixed on both sides, one limb being fixed to the blade of the purlin, and the other to the deck-beam.
In addition to the lateral trussing which the ribs receive from these purl ins, diagonal braces are fixed between each two corresponding struts, connected at the top with the pur lins, and at bottom with the linking-plates, bars of their own scantling ; thus the ribs are all braet-zi together, and a firm and rigid mass of framing, formed to carry the covering.
The box-beam, upon which a portion of one side of the roof is supported, is 63 feet 4 inches long ; 3 feet 2 inches deep at the ends; and 2 feet 6 inches at the centre, the versed sine being 8. inches. The upper chamber is 20 inches wide, and n inches deep, and the body is I inches wide, by 1 foot 10 inches deep. The bottom, inches wide, is formed of two rows of plates, Adis inch thick in the mid dle, and at the ends. ',the thickness of all the other plates is inch. On the opposite side of the root, 17 east iron columns, 21 feet 6 inches apart, serve to support the ribs. The columns are 19 feet high from the base to the cap, and 4 feet 3 inches from the cap to the metal chair. At the bottom they are fastened into stones of 5 tons weight each, at 3 feet below the base. The roof is covered with galva nized corrugated wrought-iron and rough plate-glass. The total cost was £15,000.
A patent was taken out recently by Mr. Nasmyth, for improvements in the construction of fire-proof flooring and roofing.
These improvements consist in constructing floors and roofs of iron plates, which are bent into the form of a segment of a circle, or into a conical, polygonal, or other shape, by the ordinary plate-bending machinery, or by any other suitable means. These bent plates are supported on chord plates, or tension-bars which have their ends bent upwards, whereby the plates are retained in their curved position, when sub jected to pressure. The ends of the chords rest upon flanges of cast or wrought-iron girders, above which are cast or riveted knee-pieces, which prevent the bent ends of the chords from springing ; or, instead of iron-plates, angle, or T iron, bent into the required shape, and supported upon chords resting upon the flanges of girders, may be employed. Over these curved ribs, iron-plates are bent, with their ends placed underneath the bent-up ends of the chords. The spaces above the iron-plates are filled up, to form the floor ing with Portland cement, mixed with broken bricks and other suitable materials. The improved girders arc formed by bolting iron plates to the sides and tops of stone arches and chords combined as before. The side plates are made with flanges to support the arches and chords which form the joists, and have also knee-pieces bolted to them to pre vent the chords from springing, when the arch is subjected to pressure. The arches and chords may be made of one piece each, or may be made of several pieces, and bolted or riveted together.