Such constructions are still in use for mills, warehouses, sugar factories, and other buildings, where great weights have to be stored.
A tension rod, t t, unites the girders, and prevents their yielding under the thrust of the arches. The nearer the tension rod is to the springing of the arch the better, but it is frequently kept high up within the arch in order that it may not be visible.
The tension rod is often used only for the outer arches of a series ; these, being thus prevented from yielding, form an abutment for the others.
Sir W. Fairbairn recommends that the rise of such arches should be 1/10 the span for floors of mills, and 1/8 the span for warehouse floors to carry heavy weights.
Fig. 235 shows an arch of about 10 feet span, carried by wrought-iron plate girders, with angle iron flanges. These girders run at right angles to the arches, their ends rest upon the heads of columns, and the girders are laterally tied together by flat iron bars, t t, secure.d to their upper flanges.
To prevent this, the late Mr. Whichcord imbedded them in fireclay blocks, which protect them from fire, and, at the same time, form skewbacks for the arches.
Fig. 236 shows in section a rolled girder with protecting fire blocks, BB. These are made in lengths of 9 inches, and with a minimum thickness at any point of 1½ inch of fireclay, which has been found to resist the greatest heat to which such a structure is likely to be subjected.
Where ceiling joists are used, they may be supported on the lower ledges of the fire blocks.
In this the blocks or voussoirs of the arch are of hollow fireclay blocks, which are stated to be 1/3 lighter than bricks or concrete. The under sides of these are dovetail grooved so as to form a key for the plastering.
1 Used at the National Safe Deposit Co.'s Warehouse.
This flooring is capable of sustaining great weights ; the girders are well protected; the arches being light may be of considerable span, though large spans increase the depth and cost of the floor, and, moreover, they do exert a certain amount of thrust upon the walls.
This flooring has been used at Whiteleys, The London Pavilion, National Provincial Bank, and in several warehouses.
Fig. 237. Doulton-Peto's System. Section of 8-inch Flooring.
Northcroft's System 1 consists of flat arches of specially moulded firebricks, resting upon fire-brick skewbacks supported by I iron girders.
These girders rest at the ends upon turned rollers, and are thus permitted to expand when heated.
Fig. 238. Northcroft's System.
In Fig. 238, a a a are the flat arches 6 inches deep, with a filling between of 6 inches of concrete. S S are the skewbacks, which are bedded in asphalte upon the I girders. The soffit is plastered to form a ceiling, and the surfaces of the floor finished off with parquet, cement, asphalte, or in any way that may be desired.
Dennett's Fireproof Floor consists of concrete arches supported where they abut upon the walls by projecting courses, and at intermediate points by rolled or riveted iron girders, as shown in Pig. 239.
The arches should have a minimum rise of 1 inch to every foot of width up to spans of 10 or 12 feet, and are sustained by centering until they are thoroughly set.
The concrete used has sulphate of lime (gypsum) for its matrix. It has been proved that this substance does not lose its cohesive power even when it is raised to a white heat and then drenched with cold water.
1 From Our Factories, Workshops, and Warehouses, by R. H. Thwaite, C. E.
Fig. 239. Dennett's System.
The floor above the arch may be formed by simply bringing the concrete itself to a smooth surface. Joists may be nailed to fillets laid upon the concrete, in a similar manner to that shown in Fig. 240, or the surface may be paved as in Fig. 239.
Fig. 240. Dennett's System.
The soffit of the arch may be finished at once with the setting coat of plastering;1 or, if a flat ceiling is necessary, joists must be fixed to the lower flanges of the girders to carry the lath and plaster. The laths are not shown in Fig. 240.
Figs. 239, 240 are taken from Messrs. Dennett's pamphlet.
"Wilkinson's System is very like Dennett's in form, but the arches are of concrete granite and the ceilings formed with fibrous plaster slabs.
This system has been used at Edinburgh University, several stations on North-Eastern Railway, by the War Department, and in many warehouses.2
Lindsay's Arch System consists of "Pumice concrete"3 arches resting upon girders which are stiffened by truss rods, and whose lower flange is protected from fire by an iron trough filled in with pumice concrete.
1 Concrete arches are often laid upon a soffit of corrugated iron, which supports the concrete while it is being laid and protects it afterwards.
2 Lawford, Transactions, Society of Engineers, 1889. 3 See p. 144.
Fig. 241 gives the section of a floor that was used for the office of the Board of Works in London, and which will resist fire to a considerable extent.
The joists are cut diagonally of triangular section, see Fig. 242, and are placed about 18 inches apart, so as to form skew-backs to concrete arches filled in between them.
Bunnett's System1 consists of hollow bricks of a peculiar shape laid in a flat arch from wall to wall, resting on angle irons held together by a tension rod passing through the bricks. The bricks are so arranged and formed laterally that each receives the support of six adjoining bricks. The under sides of the bricks are dovetail grooved to afford a key for the plastering.
Tile Floors consist of arches formed with flat tiles resting upon girders, in the same way as the brick arches above described. When the centering is fixed, the first course of tiles is laid dry, and then covered with cement; upon this a second, third, and fourth course are laid in the same manner. The spandrils are filled in with concrete, and the floor finished with joists and boarding, pavement, or in any way that may be desired.
Arched floors, especially when composed of voussoir blocks, are complicated and heavy. The arches depend greatly upon one another, and if one gives way it may lead to the failure of the whole floor. In any case they exert a thrust upon the walls.
These disadvantages have led to the adoption of simpler forms of fireproof flooring.