The actual sectional area of steel, by Formula (12), Chapter II, is Two 3/4-in square bars, with a total section-area of 1.12 sq in, are satisfactory. Neglecting the compression in the ribs, by Formula (9), Chapter V, fc bt (kd-12t)jd (25 × 2.5) × (3.329 — 1.25) × 10 The maximum vertical shear adjacent to the support is, in which 6 is the average width of web in inches, and At the continuous support the straight steel is cut off at the center line of the girder, and at the non-continuous support it is hooked, and the slab has a bearing of 4 in on the brickwork or masonwork. One 4-in rod is raised over the support adjacent to each end, anchored by a hook at the non-continuous termination, and carried to the one-quarter point of the adjoining span beyond the continuous support. As no partition-weights are included in the designloads, these must be taken care of by means of double ribs or other adequate framing. 6. Metal Domes Alternating with Concrete Ribs. This system is entirely similar to the two-way terra-cotta tile and concrete-rib construction, except that rectangular dome-shaped steel forms take the place of the tile units. They are made of No. 16-gauge metal, and are intended for repeated use. The dome is square in plan, the side of the square being 20 in, and it is ordinarily used with a 5-in concrete rib in each direction. Metal lath may be attached to the soffit of the slab to furnish a flat ceiling, as in the case of the metal-tile construction. As this system has not, up to the present time, been very extensively used, the design-data are not included here, but may be obtained from the manufacturers.* 7. Concrete Slabs Used with Structural-Steel Framing. Although this type of construction may, under certain conditions, be economically executed in stone concrete, its greatest use is with cinder concrete, and for short-span conditions where the center-to-center spacing of the panels is seldom over 8 ft. For all steel-skeleton buildings which can be designed with short-span systems, the use of cinder concrete slabs, reinforced with some type of wire fabric, or metal lath, favors rapid erection and is generally the most economical for multi*Truscon Steel Company. story buildings when a suitable quality of cinders is easily obtainable. (See Fig. 18.) If, however, flat ceilings are required over large areas, the additional cost of suspended ceilings may equal or exceed that of the various RIBBED CONSTRUCTIONS previously described. The forms are hung from the structuralsteel beams which are enclosed in concrete except where some other method of fire-protection is employed. Three types of this construction, employing expanded-metal lath as reinforcement, are shown in Fig. 18. (a) This is the most common type of expanded-metal floor, the beams fireproofed with 2 in of concrete and the slab extended about 2 in above the beams. The expanded metal is raised over the beams to resist the negative stresses. (b) This type is very similar to type (a), the only difference being the shape of the fireproofed beams. The forms are a little more costly than in the preceding type, but on account of appearances this type is often preferred. (c) This type of floor is often used where the beams are to be protected by a suspended ceiling of metal lath and plaster. The forms are built flush with the tops of the steel beams, and the slabs, resting entirely on the beams, are absolutely continuous. The design-procedure for stone concrete slabs used with structural-steel framing is the same as in concrete framing. In computing the depth of cinder concrete slabs by the usual equations for stone concrete, the general formulas should be used and the limiting value of fe taken at not over 300 lb per sq in. The Underwriters' Building Code requires that a 1:25 mixture of cinder concrete develop an ultimate compressive strength of 800 lb per sq in when tested at 28 days in accordance with the method prescribed for stone concrete, and that the ratio of the moduli of elasticity of 1:25 cinder concrete and steel be taken as 1:30. Some building codes, however, in imposing limiting thicknesses of slabs for various spans and loads, base their rulings upon empirical data and permit a more liberal design for short spans. For example, the New York City Code allows a 4-in (total thickness), 1:25 cinder concrete slab, to be used for spans up to 8 ft. The usual construction is that shown in (a) and (b), Fig. 18, where the upper surface of the floor slab is sufficiently above the top of the floor beams to Fig. 18. Typical Steel-Beam and Concrete-Slab Floor-Construction. ExpandedMetal Reinforcements. General Fireproofing Company permit continuity of the reinforcement. For this reinforcement some type of wire fabric is usually the most satisfactory, especially for the large floor-areas necessary in the construction of offices and loft-buildings, in which the majority of the panels can be planned for the same requirements of reinforcement. The cost, also, of rolling the bundled fabric over the forms is relatively low. (See Fig. 19.) The weight of cinder concrete is ordinarily assumed to be 108 lb per cu ft. The sectional area of the reinforcement is computed as in concrete framing, and determines the proper size and spacing of the bars, or the type of steel fabric required from the corresponding section-areas per foot of width, given in the manufacturers' catalogues. In this connection it should be remembered that local codes may permit considerably higher stresses in steel fabric than in round or square bars of structural grade, or even in hard-grade steel. For example, the New York City Code permits the following design: "Strength of concrete slabs. In determining the safe carrying capacity of concrete slab floor fillings the gross load in pounds per square foot of floor surface shall not exceed the product of the depth in inches of the reinforcement below the top of the slab, by the cross-sectional area in square inches per foot of width of the tensional steel, divided by the square of the span in feet, all multiplied by the following co-efficients when cinder concrete is used: 14 000 if the reinforcement is not continuous over the supports, 18 000 if the reinforcement consists of rods or other shapes securely hooked over or attached to the sup Fig. 19. Steel-Beam and Concrete-Slab Floor-Construction. Triangle-Mesh Reinforcements. American Steel and Wire Company ports, and 26 000 if the reinforcement consists of steel fabric continuous over the supports, and, when stone concrete is used, 16 000, 20 000 and 30 000 respectively."* In certain types of construction, especially for roofs, it is desirable to avoid the use of forms under the slabs, and for this purpose a closely-meshed lath has been devised, the application of which to various designs is shown in Fig. 20. There are four types of this construction. *Quoted without editing. |