This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Fig. 42 shows the Kahn system of reinforced hollow tile floor. This floor is light, produces an even surface on the under side, and gives improved resistance to the transmission of sound. The construction is extremely simple, for the secondary beams, which take the form of joists at 10 to 16 inches between centres, are formed between hollow tiles, which are simply laid upon a flat centering. The tiles are made from 4 to 12 inches deep. Floors of this description may be constructed up to 25 or even 30 feet span, while primary beams are formed of the usual construction.
The Wells system, of which the floor slabs were illustrated in Fig. 26, consists in its essence upon the arrangement of the main reinforcements of the beams. The rods, as shown in the general drawing (Fig. 43) and the detail (Fig. 44), are twin, each pair connected by a very short web, and so somewhat like a pair of dumb-bells in section. These lie either side by side or above one another at the bottom of the girder along the centre of the span; but the web is cut back to the point of contraflexure, and one of the twin rods bent up to pass over intermediate supports. The two rods thus separated are again connected by strap hangers of small bar section, indented so as to cling to the concrete, these straps being twisted round each rod. The rigidity of the Kahn system is thus obtained, as the steelwork of each girder can be built up before being lifted into place and the concrete inserted, with the theoretical rodding of the Hennebique, and without any difficulty being experienced in keeping the bonders or stirrups in place as the concrete is filled in or while it sets; any additional vertical bonders being driven into the concrete immediately before setting commences, and keeping their places from the outset owing to their indented form.
For illustration and description of this, see page 185, Vol. IV. This material is of great use in the construction of armoured concrete, particularly for the production of slabs. It produces a thoroughly even distribution of reinforcement, while it offers resistance in both directions. A further advantage in the use of this material is that the tensile stress upon the metal tends to close up its meshes, thus producing compressional stress upon the concrete at right angles to the tensile stress, the effect of this being to improve the elastic qualities of the concrete.
A hollow floor may be constructed as indicated in Fig. 45, the ceiling slab ana the beams being formed on a flat centering, while the floor slabs are moulded in advance and are set in position. The assistance of the floor slab in taking the compressional stress in the beam is lost by this construction, and to make up for this a compressional reinforcement is introduced, while the considerable advantages of a hollow floor and flat ceiling are gained.
Flat roofs are constructed on precisely the same principle as are floors, while the slight slope necessary is readily provided in this material, and there need be no question as to the efficiency of the protection of the metal against fire. By giving the roof a fall in both directions from the centre line the cross beams may be given greater depth at the centre, where the greatest bending moments are to be met.
Fig. 46 shows the construction of a roof on the Kahn hollow-tile system. The simple form of the centering is here visible.
The most economical method of constructing a building in armoured concrete follows precisely the same principle of concentrating all loads upon evenly distributed points as that described under the head of "Steel Frame Buildings" (see Vol. IV.), with the following advantages: The structural load-carrying framework need not be encased either for protection or for appearance sake, although if architectural effect is desired a stone facing may be applied here as in the case of steel frame buildings. With proper care and thorough supervision there is no fear of the failure of the building from the corrosion of the metal. The material particularly lends itself to the satisfactory protection of the metal from fire, and the difficulty of properly protecting the spandril framing is completely overcome. The rigidity of the joints between the various members is necessarily good, and needs no special provision beyond the splaying out of pillars at these junctions. Variations in the original design can be readily effected, while in the case of steel framing this is governed by the steelwork already supplied or ordered.
The construction and arrangement of the moulds call for much ingenuity, in order that they may be thoroughly rigid, that they may cause as little obstruction as possible, that the damage to the wood may be small, and that it may be possible to re-use the moulds as often as possible. Skilful arrangement will expedite erection and considerably reduce the cost of this part of the work, which forms a large item in the total cost. The rigidity of the moulds is particularly necessary, in order that work which is being carried on may not cause vibration in concrete which has recently been put in place; for it is found, as might be expected.
that vibrations in the concrete, while setting, very seriously affects the strength of the resultant material. Fig. 47 shows an arrangement of centering adopted with the Coignet system. The beam here shown has. been moulded in advance, and when set in position it supports the slab centering. Fig. 48 shows the construction of centering, also used with the Coignet system, when the beams are not moulded in advance.
The moulds about the sides of walls or pillars and on the vertical sides of beams should be left in position for not less than 48 hours after moulding. The centering under floor slabs should be left in position for at least a week, while props supporting the beams and slabs at the centres of their spans should be left in position as long as possible; but not less than one month.