In figs. 217 and 218 Cooper's system is illustrated, in which the concrete is carried upon plates of corrugated iron a a, these being supported by the wrought-iron beams b b, c c being the concrete, and the floor being finished in the way already illustrated. Although not coming under this division of our work with the strictest attention to arrangement, still, while treating of the subject of fire-proof floors, we consider it best to give an illustration of one in which no timber at all is used, but the whole structure made up of iron, brick, and concrete. This form, illustrated in fig. 219, is that which has been used for many years in factory work, and consists of cast-iron girders a a, supporting a brick arch b b, upon which is placed the concrete flooring c c; tie rods of wrought-iron as d d connect the cast-iron beams a a at intervals.
Where the arches join the walls e e, they butt up against the inside of cast-iron "skewbacks"f f bolted in the wall. For proportions of iron beams, see chapter on Strains on Beams-, The next form of fire-proof floors we shall describe is that known as Dennett's, and although only but recently introduced has been largely used; the principle of arrangement is much the same as in other systems, but the kind of concrete is different, and it is used in the form of arches of gentle rise as illustrated in fig. 220, these arches butting against either wrought-iron beams a a, or against the wall. In the case of buildings of comparatively narrow space the floor may either be finished as at A with timber bottoms and boarding c, or filled up to the level a a with the concrete, thus forming a solid mass. When the floors are formed as in B, the concrete is formed of pieces gradually decreasing in size, till at the floor surface they can be worked and finished off with a trowel. Floors thus formed are said to be peculiarly well adapted for bed-rooms, as they are "cleanly, non-absorbent, free from vibration, and are therefore comparatively noiseless." The ceilings may be finished off flat by using a series of light joists; the curved soffits or under sides of the arches may be left exposed and may be coloured or decorated as required. We have said that the concrete used by Mr. Dennett is different from the kind ordinarily employed, in which lime is used. These, when subjected to the action of heat and having water poured upon them, are exceedingly apt to crack and give way, swelling out also to twice their original bulk, and thus exercising a destructive influence upon the walls; so that floors formed of them are by no means so strong and secure as has been supposed.
Mr. Dennett uses gypsum calcined, the coarser qualities being used; and these are generally found to be mixed with clay, a fortunate mixture, as the clay is the very material which, when burnt, is afterwards used artificially in mixing up the concrete. The gypsum is employed along with masses of material possessing great porosity, such as broken brick, oolitic stones, furnace dross, and the like. The materials are placed upon temporary arches or platforms, the upper sides of which assume the curve of the required arch; and they are pressed down with considerable force, and the whole is allowed to set or harden, which process is completed in from two to six days, leaving the mass a cement very much harder than that of plaster of Paris, and capable of sustaining heavy weights and great pressure. The last form of fire-proof floors we shall illustrate is that known as Homan's system, in which the iron beams employed are of the kind shown at a a in fig. 221, and more generally known as "Phillip's double flanged girder." Floors constructed with these do not require any cross bars to support the concrete, and are much lighter and stronger than other forms.
14. Flooring Boards are of three kinds, "folding floors," " straight joint floors," and " dowelled floors. " In the first of these systems, the boards are laid four or five close together; thus, suppose the board a a, in fig. 222,* to be the last laid down, the fourth board, b b, is then nailed to the rafters, so as to make the space between it and a a a little less than the space required by the three intervening boards, c d and e, these being forced into the space between a a and b b, and when flat, secured by nails to the joists f f f f g g, the "heading joints" g g, which should be arranged so as to meet in the centre, or, at least, above the edges or solid face of rafters. In "straight-joint floors," the boards are laid across the joists a a a a, fig. 223, with the vertical, or side joints, in one continuous line, one board being laid down and secured to the joists at a time, and the next forced up close in contact with it, so as to make the joint good, this being done with an instrument known as a flooring clamp. In "dowelled" floors, the boards are laid straight, joints edge to edge, but are kept together by dowels, or pieces of oak or beech set into the edges of the boards, as shown in fig. 224, in which the dowels are inserted, as shown at a a a a, two dowels being given to the space between the two joints, b b, b b; or the dowel may be placed so that it will be above the joists as at c, the other as d in the centre of the space between the two joists b b.
* Where figures, as " 1½ " in fig. 222, are placed at the under part of a diagram, they denote the scale to which the drawing in the figure is drawn, thus fig. 4 is drawn to a scale of " 1½ inches to the foot, " or "1½."