This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The strength of this system is considered to be simply that of the concrete arch, the wire center being intended merely for the support of the concrete until it has set, and for a permanent center upon which plastering may be applied directly if a level ceiling is not desired. This construction, Type 2, is shown in Fig. 61. It is further claimed for this wire centering, that it facilitates the more rapid drying out of the concrete on account of exposing both surfaces to the air and allowing the surplus water to drip through.
Fig. 62 shows System B, Type 1. This is a flat arch construction in which the steel members are bars spaced generally about sixteen inches center to center, the concrete slab being usually 3 1/2 inches thick. The bars are tied transversely by wire rods spaced about 24 inches on centers and serving to keep the bars in place.
Types of Roebling System of Floor Construction
For System A. A = top floor. B = under floor. C = wood screeds or sleepers. D = cinder concrete arch. E = steel rod (7/16" or 9/16") woven into wire lathing. J = tie rods. H= main floor beams. S = plaster ceiling. R= supporting wire. K= clamp supporting ceiling. L = steel rods woven into wire lathing. Note: - Items R.K.L apply to Type 1 only.
Note: - Items R.K.O apply to Types only. Item L applies to Types 1 and 4 only.
Fig. 63, Type 2, shows the construction when the suspended ceiling is omitted. This suspended ceiling does not always have the bars shown by Fig. 62, but for short spans has simply the wire cloth stiffened by rods woven into it.
Fig. 64 shows System B, Type 4, in which the floor slab rests on the lower flanges, and the cinder filling is flush with the top of floor beams. This system makes some saving in depth, but is open to certain objections, one being the disadvantage from the standpoint of strength of resting the slabs on the bottom flanges, and another the absence of all protection or covering for the top flanges of beams.
The practice of the company controlling the patents is to deposit the concrete without any tamping such as is ordinarily done in the other systems. The claim is made that this method insures lightness and preserves its porosity, being thus rendered less subject to the effects of changes of temperature, either of the outer air or under exposure to fire and water.
As will be noted later, Professor Norton advocates tamping of concrete to eliminate the possibility of voids, which he shows to be always productive of corrosion of the steel.
No continuous effect such as is had in some of the other systems exists in this construction, except as the whole construction of girders and their casing may be considered as acting together. The connection of the bars to the floor beams, and the concrete being finished flush with tops of beams, make the slab, considered by itself, discontinuous.
In the systems previously described, cinder concrete is almost invariably employed. In this system, however, the use of stone concrete is the prevailing practice.
The different types vary only in the size and spacing of the imbedded bars. (and consequently in the thickness of the concrete slab) and in the connection of these bars to the beams. This connection is made either by means of small angles bolted to the webs of floor beams similarly to regular beam framing, or by means of hangers resting on the top flanges of beams. The former construction is used only when special stiffness of the frame is required, as in high building construction.
The thickness of slab is generally 1 1/4 inches more than depth of bar. The spacing of bars and of beams varies with the required loads. The different cuts shown (Figs. 65, 66, and 67) give reasonable limits. In any case of special loading, however, or of spans exceeding 8 feet, tests should be made in accordance with the required conditions.
The explanations given on the plate, in connection with the above, should make the construction clear. It is the practice, in using this system, to have slots in the brick walls at the level of the floor slabs, and the bars and concrete slabs are then imbedded in these slots. This gives a good tie for the walls, and obviates the necessity of channels against the walls to take the floor construction.
Figs. 69 and 70 show the Ransome system of floor construction. This is one of the oldest forms of concrete-steel construction, and is used in various modified forms to suit different conditions. It consists of steel rods imbedded in the tension side of the concrete; these rods run transversely to the beams, and are tied longitudinally by other rods. In some forms of this construction, steel girders and beams are replaced by deep concrete beams with heavy rods imbedded therein, and tied at intervals by U-shaped rods. The use of rods in the concrete makes possible many varied forms of construction, but special knowledge of the subject is required to design such forms properly.
The use of concrete and concrete-steel arches cannot as yet be considered to be very general. They are of comparatively recent introduction; and although, in the aggregate, they may now be said to be extensively used, there is as yet no one form recognized as standard.