Fig. 191.—Form for Ornamental Column for Church at Oak Park, Ill.
SLAB AND GIRDER FORMS.—Slab and girder construction for roofs and floors is of three kinds: (1) Concrete slab and steel beam construction in place; (2) concrete slab and girder construction in place (3) separately molded slab and beam construction. The third method of construction is distinct from the others in respect to form work as well as other details and is considered separately in Chapter XX.
Slab and I-Beam Floors.—Centers for floor slabs between steel I-beams are made by suspending joists from the beam flanges and covering them with lagging. Frequently the joists and lagging are framed together into panels of convenient size for carrying and erecting. The construction is a simple one in either case where slabs without haunches or plain arches form the filling between beams. Figure 192 shows an arch slab center; plain hook bolts, with a nut on the lower end, passing through holes in the joists are more commonly employed. For 1-in. lagging the joist spacing is 2 ft., for 1½-in lagging, 4 ft., and for 2-in. lagging, 5 ft.
Fig. 192.—Form for Arch Slab Between I-Beams.
Fig. 193.—Form for Flat Slab Floor Between I-Beams.
A more complex centering is required where the slab has to be haunched around the I-beams. The center shown by Fig. 193 was designed by Mr. W. A. Etherton for the floor construction of the U. S. Postoffice Building erected at Huntington, W. Va., in 1905. The center consists essentially of the pieces A (2×4 ins. for spans not exceeding 6 ft.) and the 2×3-in. triggers B, which rest on the lower flanges of the floor beams and thus support the forms. The trigger is secured at one end to the piece A by a 1×3-in. cleat C and at the other end by 1×3-in. cleats D on either side of A, which serve also as supports for the batter boards E. The six-penny nail F is but partly driven and it is to be drawn before removing the forms. When the supports of the beams are not fireproofed the cleats D extend to the bottom of the trigger B, but otherwise one cleat extends lower to secure the cross-strip G. To remove the forms, draw the partly driven nail F; knock off the strip G or loosen it enough to draw the nails in B; pull the triggers on one beam, and the forms will drop. If the soffit board H is used it is necessary first to remove the strip G. For larger beams use the spacing blocks H as shown; for smaller beams omit the trigger B and extend A to rest on the flange of the beam, then to remove the form A must be cut preferably near the beam.
No complete records of the cost of these forms were obtained, but the following partial information is furnished by Mr. Etherton: Considering a panel 6 ft. span by 19 ft. long on 15-in. I-beams, the lumber consisting of 1-in. boards supported by 2×4-in. cross-pieces on 2×3-in. triggers spread 3 ft. on centers, soffit of beams not fireproofed, it required one carpenter five hours at 30 cts. per hour to complete the panel. Figuring from this alone I should say that 10 cts. per sq. yd. is a fair estimate for carpenter work. In working over the forms for another floor the 1-in. boards require more time to handle and I should say that the saving in cost of work over the first floor would be not over 2 cts. per sq. yd. Two laborers moved their scaffolding and took down the forms from three completed panels of 13 sq. yds. each in one hour. Smaller panels require a longer time per yard. Counting for the proper piling of lumber I should allow one hour for one man to take down the forms for a 13-sq. yd. panel when conditions are the best. We contracted with two laborers to remove the forms from the third floor and roof and pile them in good shape on the ground just outside of the building for an amount averaging about 4½ cts. per sq. yd., and the men made but small wages on the contract. The lumber was used on three floors and the roof, and the best of the 1-in. boards and all of the 2×4-in. and 2×3-in. stuff were used on a second job. For a safe estimate based on the data secured I should figure the cost of labor and materials for a three or four-story building about as follows: