Table IV. Dimensions of Concrete and Reinforcements for Interior Square Panels. Two-Way System. New York City Building Code, 1926 Superimposed Loads 120, 125, and 150 Pounds per Square Foot

-120 lb per sq ft

CHAPTER XI

COMBINATION FLOOR SYSTEMS

1. Different Floor Systems and Their Applicability. For industrial buildings as ordinarily designed, and in fact for all concrete-frame buildings in which the unit superimposed floor-load equals or exceeds 125 lb per sq ft, solid concrete construction of either the BEAM-AND-SLAB or the GIRDERLESS type, is usually the most satisfactory. For live loads of from 40 to 75 lb per sq ft, within which limits fall the requirements for offices, school-rooms, and rooms in institutional buildings, some type of RIBBED CONSTRUCTION should be considered for those which have either concrete frames or bearing-walls, as this construction will probably show a considerable saving in cost, due largely to the lighter dead load. Steel-framed buildings are most economically designed with floor systems employing the various ribbed constructions, or with cinder or stone. concrete arches, the choice depending upon local conditions. In any case, when there is doubt in regard to the relative economy of different systems, a careful study should be made of comparative designs, as the choice often depends upon the desirability of unbroken ceiling-areas and the permissible locations of columns, as effecting the sizes and dimensions of the bays.

The principal systems, other than ordinary slab construction, which may be used with either a concrete or a structural-steel frame, are those in which concrete ribs are cast between wooden forms, or between fillers of terra-cotta or gypsum blocks, or of some type of metal tiles or TIN-PAN. The so-called CINDER-CONCRETE ARCH which, in reality, is ordinarily constructed as a slab with a one-way reinforcement, is used only with structural steel framing; and as the span of cinder-concrete slabs is usually limited to 8 ft, this type of floor is known as the SHORT-SPAN CONSTRUCTION, to distinguish it from the terracotta, gypsum, or metal-tile systems, which are particularly suited to longer

spans.

The ONE-WAY SYSTEMS, in which the entire load is carried on concrete ribs running in one direction, are appropriate for oblong panels in which the longer dimension is more than one-and-one-third times the shorter dimension. (See page 68.) Construction of this type has the advantages of simple framing and cheap centering, but on long spans requires a considerably greater depth of structural floor than where the panel dimensions favor a two-way design.

The two-way systems, that is, those with concrete ribs and reinforcements running in two directions, parallel to the sides of the panel, naturally result in a thinner structural floor, since the load may be proportioned in each rectangular direction, as explained for solid-concrete slabs on page 67; and where it is practicable to obtain support for the slabs on all four sides, and when the result

ing panel dimensions are approximately square (page 72), this method of design is the most economical. Furthermore, when using a two-way reinforcement, it would seem logical to reduce the value of the design moments. This principle has been acknowledged in the approvals given to certain patented systems by many of our largest cities, which permit the use of reduced bending moment-factors for certain types of two-way construction. For example, New York City allows the two-way slag-block floors Fig. 7 to be computed for the following bending moments.

Square panels, simply supported on four sides

M = WL/12 (instead of WL/8)

Square panels, semi-continuous spans

M = WL/15 (instead of WL/10)

Square panels, fully-continuous spans

There is no apparent reason why this same reduction should not apply to ordinary concrete-slabs having two-way reinforcements, as well as to patented systems.*

The metal tiles and metal domes, employed in the one-way and two-way systems respectively, have the advantage of furnishing a lighter dead load for the same effective concrete-section than either terra-cotta or gypsum tiles. On the other hand, the terra-cotta blocks appear to add very materially to the strength of the construction, and both furnish good surfaces for plaster, which can be applied at less cost and often more satisfactorily upon terra-cotta or gypsum than upon metal lath. As considerable trouble has been experienced from staining when plaster is applied to alternate surfaces of concrete and terra-cotta tiles, it is preferable to use terra-cotta soffits at the bottom of the concrete ribs or, in the two-way systems, a terra-cotta soffit one way and a terra-cotta channel the other (page 298), in order to furnish a continuous terracotta surface. Similarly, gypsum soffits are usually placed along the bottoms of the concrete ribs when gypsum blocks are used as fillers. (See page 303.) The advantage of using wooden forms, of approximately the same width as the metal-tile, is dependent upon the cost of lumber and the labor of construction. As the wooden pan, as ordinarily constructed, is more substantial than a lightweight metal pan, and less likely to deflect under load, it is possible, where codes permit, to use a thinner slab above the ribs. It is also much easier to attach inserts and leave openings where wooden forms are employed. The design procedure is the same for each type.

In comparing the relative economy of different systems of floor-construction, a careful evaluation should be made not only of the cost of materials and the labor of erection, but also of the ease and rapidity of construction, the effect upon contiguous work, and the dead weight of the construction as influencing the other structural members in the building. These features are peculiar to * See footnote on page 69.

each individual job and the following articles can only indicate in a general way the applicability of the different systems, and describe the methods of design to be employed after the choice has been made.

2. Terra-Cotta Tiles Alternating with Concrete Ribs. In this classification are included the ONE-WAY SYSTEM and the TWO-WAY SYSTEM.

(1) One-way system. Construction of this type consists of a series of

Structural Steel

or Concrete Beam

Section Taken in Direction of Main Reinforcement

Bar-Chairs

Section Taken Across the Blocks

Fig. 1. One-Way System. Terra-Cotta Tiles Alternating with Concrete Ribs.

Fig. 2. One-Way System. Open Centering. reinforced-concrete beams or ribs, 4 or 5 in wide, formed between lines of hollow tiles which are 12 by 12 in in plan and from 4 to 12 in thick, according to the load and span. (See Fig. 1.) A thin concrete slab, usually from 2 to 3 in thick, is laid on top of these. It should be cast monolithic with the ribs as in the case of the slabs which form the flanges in ordinary T-beam construction. The tiles are laid with open ends, closely jointed to prevent waste of concrete, when a continuous terra-cotta surface is desired for plastering, soffit-blocks of