Arched Floors are much used in America - either brick arches supported by iron girders with "porous terra cotta,"3 protecting blocks forming skewbacks, or arches of hollow blocks like those in the Doulton-Peto system (Fig. 237), or concrete arches like those of Dennett, but supported upon corrugated iron soffits.
A section of one of these is shown in Fig. 255. It consists of wooden joists on which 2" x 1" strips support a course of bricks whose upper surface is covered with a layer of concrete, and upon which is a tiled or boarded floor. The ceiling is of terra-cotta tiles fixed to the joists by iron clips - jointed, and plastered below.
Slow-burning constriction 4 is a term applied to the kind of floor generally used for mills and warehouses. These consist of solid beams or beams bolted together and 8 or 10 feet between centres, upon these are laid floor planks 3 inches to 3½ inches thick, over which is spread a layer of mortar ¾ inch thick, and over this again is laid a grooved and tongued floor of hard wood 1¼ inch thick. Sometimes two thicknesses of rosin-sized sheathing paper are substituted for the layer of mortar.
1 Transactions, Society of Engineers, 1888, p. 58.
2 Transactions, R.I.B.A., 1886, p. 129, Mr. Gass's paper.
3 Porous terra cotta is composed of clay mixed with combustible material, such as sawdust, cut straw, charcoal, etc. "When baked the combustible material is consumed, leaving the terra cotta full of holes. It is fireproof, light, will hold nails, and gives a good surface for plastering (sec Part III.)
4Woodley's Fire Protection of Mills, New York.
Ironwork is always protected by terra-cotta blocks (Figs. 255-2571), plaster, etc. Ceilings are of terra-cotta tiles, or plastered on wire-cloth netting with -| inch squares. Exposed woodwork is protected by terra cotta or sheets of tin. Partitions are of hollow tiles.
Fireproof Roofing is not much in fashion in this country, though Lindsay's strained wire system has been used for part of the roof of the National Liberal Club, Branch Bank of England,2 etc. In America it is sometimes constructed with porous terra-cotta blocks resting on T irons supported by I beams, or roof trusses are encased in terra cotta.
Wrought - iron girders were adopted for fireproof floors in Paris some time before they were known in England, and any notice of the different systems of fireproof flooring, however brief, would be incomplete without a reference to some of the plans originated in France.
Though these are not commonly adopted in this country, some description of them may be useful in suggesting ideas for new systems, which may be arranged and modified in accordance with more recent experience.
Thuasne's System consists of I-shaped wrought-iron girders slightly arched, with a rise of about 1/200, and placed at about 3 feet 3 inches central intervals. At right angles to them, and also 3 feet 3 inches apart, are laid flat iron bars or interties, bbb, whose ends pass through slits in wrought-iron bands, i i i, which embrace the girder at intervals of 3 feet 3 inches. The ends of the interties are secured by a pin passing through them on the farther side of the iron band.
1 From Transactions, R.I.B.A., 1886, P1. XVIII.
2 Cunuington, Building News, 15th March 1889.
Crossing the interties at right angles are light iron rods called "fautons," ///; these are generally about ½ inch square (not flat as shown in Fig. 256), placed about 9 inches apart, and bound to the interties with wire.
A flat centering or boarding is placed under this network, and coarse plaster of Paris is poured in upon it to a thickness of about 3 inches. This soon becomes hard, and serves not only to stiffen the floor but to form the ceiling, a fine coat being required on the under side as a finish to superior work.
Fig. 258. Thuasne's System.
The girders are tied into the walls at each end by iron straps secured to vertical bolts in the wall.
Small square wooden joists are laid over the girders, and boarded in the usual way.
In some cases cast-iron chairs are used instead of the wrought-iron straps to carry the ends of the interties.
Fer Tubulaire is the French name for a girder of peculiar form invented by M. Zores and shown in Fig. 259.
Fig. 259. Cross Section Fer Tubulaire.
Fig. 259a. Longitudinal Sectional Elevation.
Floors of this kind were exhibited in the Paris Exhibition of 1857, and have been used for warehouses in this country.
They are thus described in the official report on civil construction.
"The 'fer tubulaire' may be described as being in section of the form of a capital A without the small triangular top. Those exhibited are said to be for a bearing of 20 feet, and are of the following dimensions - viz. 4¾ inches high, 2 3/8 inches wide at top, 4 inches wide at bottom exclusive of a small flange of ¾-inch projection on each side. The sides of the girder are 3 /10ths of an inch in thickness, and the top and flanges 7/20ths. These girders are placed at a distance apart of 2 feet 8 inches from centre to centre, and are tied together at intervals of three feet by flat bar-iron ties of ¾ inch by 3/10 inch bolted to the bottom of the flanges, and the flooring finished according to one of the following methods :
Flat arches of hollow brick between the girders, with joists of ' fer a coulisse' (hereafter described) or of wood and wooden flooring, or for passages with the spandril filled in with plaster, and floored over with tiles ceiled underneath to soffit of flat arch.
The spaces between the girders filled in with hollow blocks of plaster 4 inches deep. Flooring and ceiling as in No. 1.
Wooden flooring as in No. 1, with ceiling on small iron laths hollow between floor and ceiling.
Wooden flooring without ceiling."
The girders of this (\ section (G G in Fig. 259) are said to possess the following advantages over those of the I form, commonly used :
"Second, A floor constructed with these girders costs some 20 per cent less than one similar in all respects but constructed with girders of I section.
"Third, This form of joists requires no strutting, while the I girder requires lateral pressure to such an extent that it is said not to be employed to the best advantage unless absolutely filled in with either hollow brick arches or plaster, more than half its strength being dependent upon its lateral rigidity."
This is a form of iron joist resembling the I section, but with three flanges, the second longer than the upper flange and close below it.
Fig. 260 shows at f this form of girder in use as a common joist.
The main girders of fireproof floors have also been made in France of this section.
The girders are placed about 20 inches from centre to centre, and the space between them filled in with hollow bricks, or a hollow block of plaster resting on the lower flanges. The advantages claimed for these girders are, that the second flange assists the top flange in affording the necessary resistance to compression when the girder is loaded, and that it also stiffens the girder in a lateral direction ; and further, that it affords a convenient arrangement for laying the boarding at once without the intervention of joists, and without employing nails.