These forms are usually built in lengths of 16 feet, with one center at each end, and with three to five (depending on the size of the sewer or conduit) intermediate centers in the lengths of 15 feet.
The segmental ribs are bolted together. The plank for these forms are made of 2 by 4-inch material, surfaced on the outer side, with the edge beveled to the radius of the conduit. The seg mental ribs are bolted together, and are held in place by wooden ties 2 by 4 inches or 2 by 6 inches.
359. Forms for Torresdale Filters. In constructing the Tor resdale filters for supplying Phila delphia with water, several large sewers and conduits were built of concrete and reinforced with ex panded metal. In section the sewers were round and the conduits were horseshoc-shaped, with a comparatively flat bottom. The sewers were 6 feet and S feet 6 inches in diameter, and the forms were constructed similarly to the forms shown in Fig. 165, except that at the bottom the lower side ribs were connected to the bottom rib by a horizontal joint, and the spacing of the ribs was 2 feet 6 inches, center to center. Fig. 166 shows the form for the 7-foot 6-inch conduit. The centering for the 9-foot and 10-foot conduits was con structed similarly to the 7-foot 6-inch conduit, except that the ribs were divided into 7 parts instead of 5 parts as shown in Fig. 166. The spacing of the braces depended on the thickness of the lagging. For lagging 1 inch by 2-1 inches, the braces were spaced IS inches, center to center; and for 2 by 3-inch lagging, the spacing of the bracing was 2 feet 6 inches.
These forms were constructed in lengths of 8 feet. The lagging for the smaller sizes of the conduits was 1 inch by .2",, inches, and for the larger sizes 2 inches by 3 inches, all of which was made of dressed lumber and covered with No. 27 galvanized sheet iron. The bracing of the forms was arranged to permit the centering being taken apart and brought forward through the sections set in front of it. Three sets of these forms were required for each conduit. The specifications re quired that the cen tering be left in place for at least 60 hours after the con crete had been placed. It was also required that this work should be con structed in monoli thic sections—that is, the contractor could build as long a section as he could finish in a day; and that the sections should be securely keyed to gether.
360. The Blaw Steel Forms. The Blow collapsible s feel forms,a s shown in Fig. 167, appear to be the only suc cessful steel forms so far in general use.
There have been many attempts to devise steel centering for column, girder, and slab construction, but no available system has yet been invented. The main trouble of those used is their liability to leak, tendency to rust, and liability to injury by dents in removing.
The Blaw collapsible steel centering is in general use for sewer and conduit construction. This centering consists of one or more steel plates about inch thick and bent to the shape required by the interior of the sewer to be constructed. The steel plates are held in shape by angle irons. When set in position, the sections are held rigid by means of turnbuckles, which also facilitate the col lapsing of the sections. The adjacent sections are held to gether by staples and wedges, the former being riveted to the plates. The sections are usu ally made five feet long, and in any desired shape or size re quired for sewer or conduit work. When these forms are used to construct concrete sewers or conduits, the surface of the forms must • be well coated with grease or soap to prevent the concrete from ad hering to the steel.
361. Forms for Walls. The forms for concrete walls should be built strong enough so that they will retain their correct position while the con crete is being placed and ram med. In high, thin walls, a great deal of care is required to keep the forms in place so that the wall will be true and straight.
Fig. 16S shows a very common method of constructing these forms. The plank against which the concrete is placed is seldom less than inches thick, and is usually 2 inches thick. One-inch plank is sometimes used for very thin walls; but even then, the supports must be placed close. The planks are generally surfaced on the side against which the concrete is placed. The vertical timbers that hold the plank in place will vary in size from 2 inches by 4 inches to 4 inches by 6 inches, or even larger, depending on the thickness of the wall, spacing of these vertical timbers, etc. The vertical timbers are always placed in pairs, and are held in place usually by means of bolts, except for thin walls, when heavy wire is often used. If the bolts are greased before the concrete is placed, there is usually not much trouble experienced in removing them. Some contractors place the bolts in short pieces of pipe, the diameter of the pipe being about 5 inch greater than that of the bolt, and the length equal to the thickness of the wall. When the bolts are removed, the holes are filled with mortar.