In constructing the 175th Street Arch in New York City, the forms were built so that they could be easily moved. The arch is elliptical and is built of hard-burned brick and faced with granite. The span of the arch is 66 feet; the rise is 20feet; the thickness of the arch-ring is 40 inches and 48 inches at the crown and springing line, respectively; and the arch is built on a 9-degree skew. The total length of this arch is 800 feet.

This arch is constructed in sections, the centering being supported on 11 trusses placed perpendicular to the axis of the arch and having the form and dimensions shown in Fig. 170. The trusses are placed 5 feet on centers, and are supported at the ends and middle by three lines of 12 by 12-inch yellow pine caps. The caps are supported by 12 by 12-inch posts spaced five feet center to center, and rest on timber sills on concrete foundations. The upper and lower chord members of the trusses are of long-leaf yellow pine, but the diagonals and verticals are of short-leaf yellow pine. The lagging is 2| by 6-inch long-leaf yellow pine plank. The connections of the timbers are made by means of 3/8-inch steel plates and 7/8-inch bolts arranged as shown in the illustration. As it was absolutely necessary to have the forms alike, so that they could be moved along the arch and at all times fit the brickwork, they were built on the ground from the same pattern, and hoisted to their place by two guyed derricks with 70-foot booms.

Fig. 170. Arch Centers at 175th Street, New York City.

Fig. 170. Arch Centers at 175th Street, New York City.

On the 12 by 12-inch cap was a 3 by 8-inch timber, on which the double wedges were placed. When it was necessary to move the forms, the wedges were removed, the forms rested on the rollers, and there was then a clearance of about 2 1/4 inches between the brickwork and the lagging. The timber on which the rollers ran was faced with a steel plate 1/4 inch by 4 inches. The forms were moved forward by means of the derricks. The settlement of the foms under the first section constructed was 1/4 inch; and the settlement of the arch-ring of that section after the removal of forms,was 1/4 inch.*

*Engineering Record, October 5, 1907.

368. Forms for Bridge at Canal Dover, Ohio.* The details of the centering used in erecting one of the 106-foot 8-inch spans of a reinforced-concrete bridge over the Tuscarawas River at Canal Dover, Ohio, are shown in Figs. 171a and 171b. Besides this span, the bridge consisted of two other spans of 106 feet 8 inches each, and a canal span of 70 feet. The centering for the canal span was built in six bents, each bent having seven piles. A clear waterway of 18 feet was required in the canal span by the State Canal Commissioner, and this passage was arranged under the center of the arch. The piles were driven by means of a scow. The cap for the piles was a 3 by 12-inch timber.

Fig. 171 a. Centers for Bridge at Canal Dover, Ohio.

Fig. 171 a. Centers for Bridge at Canal Dover, Ohio.

Plank 2 inches thick were sawed to the correct curvature, and nailed to the 2 by 12-inch joists, which were spaced about 12 inches apart. The lagging was one inch thick, and was nailed to the curved plank.

The wedges were made and used as shown. The centering was constantly checked; this was found important after a strong wind.

The centering for the other two of the main arches was constructed similarly to that of the arch shown.

After some difficulty had been experienced in keeping the forms in place during; the concreting of the first arch, the concrete for the other arches was placed as shown in Fig. 172, and no difficulty was encountered. Sections 1 and 1 were first placed, then 2 and 2, finishing with section 6.

Fig. 171 b. Centers for Bridge at Canal Dover, Ohio.

Fig. 171 b. Centers for Bridge at Canal Dover, Ohio.

*Engineering Record, February 9, 1907.

The concreting on the canal span was begun November 1, and finished November 12; and the forms were lowered by means of the wedges five weeks later. The deflection at the crown was 0.5 inch, and after the spandrel walls were built and the fill made, there was an additional deflection of 0.4 inch. In building the forms, an allowance of 1/800 part of the span was made, to allow for this deflection. The deflections at the crown of the other three arches were 0.6 inch, 1.45 inches, and 1.34 inches.

Fig. 172. Diagram of Order of Placing Concrete in Bridge at Canal Dover, Ohio.

Fig. 172. Diagram of Order of Placing Concrete in Bridge at Canal Dover, Ohio.