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 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 undoubtedly 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, probably, to a great degree explains the more general use of terra cotta in office buildings.
As noted previously, an important feature in buildings not having heavy masonry walls is lateral stiffness. This lateral stiffness 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 blocks.
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 calculate 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.
It cannot be said to be conservative practice in any of these systems, much to exceed eight feet in the span of the arches. The uncertainty of the quality of the concrete when cinders are used, and the uncertainty of set in the deeper slabs, together with numerous other circumstances likely to affect the uniformity of the product, make it important to keep within this limit.
As will be seen from the illustrations, nearly all the concrete systems require furring down to give level ceiling.
Tests of Floor and Roof Arches. The most severe test of all forms of floor arch is their exposure to fire and water when underload. As above stated, one of the functions - and a very important one - of all fireproof materials is to protect the steel; for, if the covering falls off, leaving the steel members exposed to fire, the steel frame will soon fail. None of the materials used - terracotta or concrete in its various forms - are of themselves combustible. Failure, when it occurs, is generally due to expansion and contraction caused respectively by the intense heat and by the chilling effect of the stream of water, and to the force of the stream knocking off pieces that become loosened. All of the systems in general use have been subjected to very severe tests of this character without collapse, before being accepted by the different Building Departments; and it is probable that when failure occurs in actual building fires it is due to constructive defects, there having been less careful construction than was used in the tests.
Types of Ransome Floor Construction
If only a small portion of the covering becomes detached, the whole adjacent construction is seriously endangered. It will be seen from the above that failure is more likely to start from detachment of the covering of beams, girders and columns, than in the body of the arch, and such covering should be as substantial as possible. For this reason, haunches or a solid filling protecting the beams and girders are preferable to wire lath wrapping the same.
Tests by the New York City Building Department on floors having suspended ceilings of wire lath and plaster, resulted in these ceilings being entirely destroyed. Tests of different floor systems having rolled shapes, such as T bars or special-shaped bars, imbedded in the concrete slabs, showed less deflection under loading than when a mesh of wire rods was used.
The method of testing floor arches is as follows: A brick furnace is built, having a large combustion chamber, the top being of the floor construction to be tested. This arch is loaded with a load generally four times that specified. Measurements of deflections due to the stress are taken before and after exposure to the fire. During this exposure, which generally lasts several hours, a temperature of from 2,000° to 2,500° is constantly maintained. After some time a stream of water from a fire nozzle is played on the arch, thus reproducing as nearly as practicable actual conditions.
After the test, the load is removed to see how great the permanent deflection is. It is important in all loading tests to have the load applied over a definite area, so that the exact load per square foot can be determined, and to avoid all possibility of any portion of the load bearing on the beams instead of on the arch.