It has the disadvantage from the standpoint of strength, that the load all conies on the lower flanges of beams, and further, that all continuous effect of slabs is lost.
Fig. 59 shows System No 7, really a modification of System No. 3, in which, in order to save depth, the floor slab is flush with the top of the floor beams.
This system also has the disadvantage of loss of continuous effect. In all the above systems, the more common spans are from 5 feet to 8 feet. The company controlling the patents, how ever, claim to be able with safety to adapt the construction to longer spans, even under heavy loads.
In these systems, as well as in all others where a cinder filling is used on top of the floor slab, the filling should contain some cement, as otherwise the unneutralized cinders are likely to cause corrosion of the steel.
The depth of floor slab varies with the load and the span, but is ordinarily 3 inches or 4 inches for loads under 200 lbs. and spans of about 5 feet.
Plate IV illustrates types of the Roebling system of fireproof floors. Fig. 60 shows System A, Type 1, which consists in general of a wire center sprung between the bottom flanges of floor beams, and upon which is deposited cinder concrete in the form of a seg mental arch whose top is flush with the top of floor beams.
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 con struction in which the steel members are bars spaced generally about sixteen inches center to center, the concrete slab being usually 31 inches thick. The bars are tied transversely by wire rods spaced about 24 inches on centers and serving to keep the bars in place.
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.
Plate V shows types of the Columbian system of fireproof floors. This is a flat arch system, in which the action of the floor slab is that of a concrete beam with imbedded steel bars.
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 con nection 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 construc tion is used only when special stiffness of the frame is required, as in high building construction.
The thickness of slab is generally 11 inches more than depth of bar. The spacing of bars and of beams varies with the required