The construction consists of several layers of hard tile one inch thick, laid breaking joints. The number of layers varies with the conditions, but generally does not exceed four. The rise of the dome is ordinarily not great ; and it rests either between walls or, in some cases, on heavy girders. The tiles are usually set in Portland cement, except that the first course is set in plaster in oraer to obtain a quick set and to dispense with a certain amount of centering.
This system is almost always installed under a guarantee from the company controlling the patents, as to its efficiency and adap tability to the conditions of the special case in hand.
Floor and Roof Arches. The types of concrete and concrete-steel arches are becoming more numerous each day, and only a few will here be discussed. They may be separated primarily into flat arches and segmental arches. In most of the systems of the flat-arch construction, the action is essentially that of a beam of concrete in which metal is embedded on the low side to increase the tensile strength, since concrete is not as strong in tension as in compression. In a few of the systems, however, when special-shaped bars are used at short intervals, the effect is more that of a simple slab of concrete supported by these bars, which act as small beams between the main floor beams.
In the segmental form of concrete construction, the metal, where used, is generally intended more as a permanent center for forming the arch and for supporting it until the concrete has fully set, when the concrete is considered as taking the load independ ently of the steel center.
Plate III shows types of Expanded Metal Floor Construction. Fig. 54 shows System No. 9, which can be adapted to long spans. It is not the general type of this form of construction, however, as the types shown below are generally considered more economical. In calculating the weight of the construction, the arch should be figured separately from the filling above, as the weights of these are different. The same remark applies to all systems of concrete construction.
Fig. 55 shows System No. 3, with a furred-down ceiling to give a level effect. This ceiling is not a necessary part of the construction, and is often omitted. The space between ceiling and floor slab is available for running of pipes, wires, etc.; and, to avoid punching of beams when such use is made of this space, the ceiling is dropped below the flanges of beams far enough to allow the passage of pipes, wires, etc.
This system is the one generally employed for long spans and heavy loads, as it gives the most substantial protection to the steel, and has certain elements of strength not possessed by the other systems, as follows : The haunches, besides protecting the webs and flanges of beams, shorten in effect the span of floor slab, and stiffen the floor beams against side deflection. The sheets of expanded metal can be made in effect continuous over all floor beams, and, because of this, the whole construction from wall to wall acts together, and has the advantage of a continuous beam over a number of supports. While it is impossible to state exactly what this advantage amounts to, on account of the uncertainty of actual conditions conforming to the theoretical assumption, it is probably safe to assume that the strains in the floor slab of a construction having this continuous feature would not be more than three-quarters as much as if the slabs were discontinuous. It should be noted in the above system, that if the furred ceiling is omitted the lower flanges of the beams are protected in a man ner similar to that shown for System No. 7.
System No. 5, illustrated by Fig. 56, differs from System No. 3 only in the method of protecting the beam. As will be seen, all the strength afforded by the haunch is lost by this construction, and, as will also be seen later from results of tests, the protection is much less fireproof.
Fig. 57 shows System No. 4. which differs from System No. 3 only in the entire omission of protection to floor beams. This system is therefore only semi-fireproof, and in event of fire in the story below would not be to any degree fireproof. It is sometimes used with a fireproof suspended ceiling, but, as will be noted further on, tests of such ceilings have shown them to be of ques tionable value as efficient fire barriers.
Fig. 58 shows System No. 8. This system is chiefly adapted to light loads on moderately long spans where the beams are in general not over 8 inches or 9 inches deep. In such cases, where a flush ceiling is desired, it is sometimes more economical than some of the other systems with suspended ceiling.