72 STEEL CONSTRUCTION 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 con struction.
In all calculations of the weight of dead loads where this system is used, the difference in weight between cinder concrete and stone concrete must be noted.
Figs. 69 and 70 show the Ransome system of floor construc tion. This is one of the oldest forms of concrete-steel construc tion, 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 construc tion, 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.
The Building Departments of all cities have required special and severe tests of full-sized arches to be made before allowing any of the types to be used in construction. Their use is un doubtedly growing, and perhaps more especially in warehouses and buildings of heavy construction. There are certain features not possessed by any of the concrete systems ; and this fact, prob ably, to a great degree explains the more general use of terra cotta in office buildings.
80 As noted previously, an important feature in buildings not having heavy masonry walls is lateral stiffness. This lateral stiff ness is secured to a considerable degree by the floor construction, which serves to tie together all parts of the framing at each floor level, and also to distribute the lateral strain throughout the whole.
A floor construction which fills the whole depth of the beams is therefore better calculated to perform this function than one that is comparatively thin, as are nearly all the concrete systems. Another important consideration concerns uniformity of material. Porous terra cotta, like brick, is easily inspected, and a nearly uniform product can thus be secured. The strength of concrete and of concrete steel, however, depends very largely upon the use of proper materials and their proper mixing and laying in place. Much greater variation is here likely to occur, and consequently a greater or less uncertainty as regards uniformity of results must exist. Another point to be considered is the necessity of having the concrete or concrete-steel system installed by the company controlling it, this resulting from the patents covering each form of construction. A still further advantage is the flush ceiling given by the terra cotta blceks.
There are, however, numerous points to be cited in favor of many of these systems. The general trend of investigation and discussion is toward a better understanding of the possibilities of concrete steel in general, and this will not unlikely result in the future in its more extensive use.
It is not the general practice of individual designers to calcu late the required depth of slab in the above systems, except in the case of unusual loads and spans; but, as in the case of the terra cotta systems, tests have largely determined the limits of spans for various depths and loads. As concrete arches are used for heavy as well as light loads, however, there is need of more exact data than is at present available to determine their capacities under different conditions.