DETAILS OF CONSTRUCTION.
Within the space here available it is not possible to give a comprehensive discussion of this branch of the subject; and hence only a few general principles will be considered.
Since reinforced concrete construction usually consists of slabs, beams, columns, or comparatively thin walls, the cost of the forms is at best a large part of the total cost of the struc ture, not infrequently running as high as 50 per cent; and hence the design, erection, and removal of the forms are important matters. In the main the statements that were made in § 320-30 concerning forms for mass concrete apply also to the forms for reinforced con crete; but with the latter there are a number of additional matters that require particular attention.
Since the forms constitute such a large part of the cost of a reinforced concrete structure, and since there is usually opportunity to use a form for any particular member a number of times, it is highly important that the forms shall be so designed as to permit rapid erection and easy removal at an early date without undue destruction of the forms or with out damage to the concrete. It is desirable to remove the column forms without disturbing the sup ports of the beams and girders bearing on the columns, since then any defect in may be detected and remedied before any considerable load comes upon it. The bottoms of the forms for beams and girders must be left in place and be supported until the beam has gained strength enough to be self-supporting, but the sides may be removed as soon as the concrete has taken an initial set; and therefore the forms for beams and girders should be so designed that the sides may be removed without disturbing the bottoms.
The supports for the forms for beams and girders should be designed so that they may be removed one at a time—beginning at the ends.
Fig. 31, 32, and 33 are forms used by a prominent construction company, and are designed to meet the require ments stated above. In Fig. 31 the sides of the columns are made in panels by nailing the planks to cleats; but the panels are held together by bolts instead of by nails or screws. The panel parallel to the bolt is held in place by spacing pieces, a, a, which rest against hard-wood wedges be tween the bolt and the form, and which are placed as near as possi ble to the end of the bolt. Cleats, plain or moulded, are nailed to one set of panels so as to give a chamfered or a fluted corner to the column. Some constructors also place a similar beveled strip in the corners of the beam and girder forms, chiefly to prevent the breaking off of the sharp corner in removing the forms. Notice the narrow strips on two opposite sides of Fig. 31, which are to facilitate the change of this dimension of the column from floor to floor. Some constructors secure adjustability of column forms by building the mould in eight pieces,—four corner pieces and four intermediate sides composed of plain plank, different widths of plank being used for different sized columns.
Fig. 32 is the form for an interior column, which is made octagonal partly for architectural appearance and partly to save concrete when circumferential reinforcement is used. Many constructors use round columns instead of octagonal ones, but the forms for the latter are the cheaper. It is claimed that the extra cost of octagonal over rectangular forms is just about balanced by the saving of concrete.
In Fig. 33, C represents the column, G the girder, and F the floor slab. The special feature in Fig. 33 is the method of supporting (1) the floor form upon the beam forms, (2) the beam form upon the bottom of the girder form, and (3) the girder form upon the horizontal clamps of the column form. It is claimed for this method that all the forms for a floor may be erected before any of the posts to support the green concrete are put into place. (These posts are not shown in Fig. 33.) The form for the bottom of the floor slab is a panel which overlaps the beam forms, and therefore requires less accurate fitting than the usual box-shaped type, since any slight inaccuracies of dimensions are taken up at the junction between the floor slab and the beam where the error is inconspicuous. The edges of these panels are beveled to facilitate their removal in taking down the forms. Notice the wedge-shaped pieces, w, w, shown in the Section D I), which are inserted to facilitate the removal of the forms.
In taking down these forms the column forms are removed first. Next, the posts under the girders are taken down, when the girder bottom drops and is removed, and then the posts are replaced against the concrete to support it for a time longer. The nails are next drawn from the wedge-like keys, w, w, these keys are knocked out, the posts are removed from under the beams, and the beam form comes down in one picee. The girder sides, being beveled at the end, are easy to remove; and the bottom of the form for the floor slab, being beveled at all four edges, also comes out readily.
The above is a brief description of some of the requirements to be met, and also an explanation of one method of meeting them; but different constructors give different weight to the various re quirements, and differ as to the best methods of meeting them. For detailed descriptions of the different methods employed in practice, see the books mentioned in § 510.
One of the recent innovations in form construction deserves special mention. In one case a reinforced concrete shell was used for forms for columns. Shells 11 inches thick, moulded in short sections, were set vertically end on end to the proper height, and were then filled with concrete. A similar plan was followed in conduit construction, short sections of reinforced concrete shells being used as both lining and centering for the concrete conduit. This method decreases the cost of forms and facilitates their erection; but such construction is lacking in transverse strength—one of the most important advantages of good reinforced concrete columns and conduits.