EXAMPLES OF PLAIN CONCRETE ARCHES. We will close the consideration of plain concrete arches by giving a few details of some of the larger arches that have been built.
Table. 99 gives the principal dimensions of some of the larger plain concrete arches having fixed ends. A comparatively few structures were omitted for the lack of complete data. Table 99 is valuable as showing the dimensions of arches that have stood successfully for a number of years, and are useful as a guide in assuming trial dimensions for a new structure.
Fig. 235, page 705, shows the cross section of a 280-ft. concrete arch, the longest yet put under construction. For comparative data, see Table 99. This arch, now (1909) in process of construction, is the central span of a concrete bridge to carry Detroit Avenue, Cleveland, Ohio, over Rocky River. The central arch is flanked on one side by one 59-ft. and two 50-ft. arch spans, and on the other side by one 59-ft. and one 50-ft. arch spans. The bridge has a 40-ft. roadway and two 8-ft. sidewalks. The main arch consists of two parallel ribs, each 22 ft. wide and 11 ft. thick at the springing, and 18 ft. wide and 6 ft. thick at the crown. The space between the ribs at the crown is 16 ft., and is spanned by a reinforced slab (the only reinforced concrete in the bridge). The parallel twin construction is oarried all through the spandrel arches, the piers, and the approach spans. The twin method of construction was first employed in the Luxemburg bridge—see No. 2, Table 90, page 648, and also § 1285. The advantages of this form of construc tion are: (1) it saves considerable load on the foundations; (2) lessens the amount of spandrel filling required; and (3) permits the same center to be used for the two parallel arches.
For a graphic solution of the stresses in the main arch and a tabular statement of the stresses for the dead load, for two positions of the live load, for the wind, and for temperature changes, see Engineering-Contracting, March 10, 1909, page 184-87. The main arch ribs are built of a 1 : 2 : 4 portland-cement concrete with slabs of stone embedded therein in a radial position quite close together; and most of the remainder of the bridge is built of a 1 : 3 : 5 rubble concrete. The rubble concrete of the arch rings was assumed to weigh 160 lb. per cu. ft., and the remainder of the concrete 150 lb. per cu. ft. The safe compressive strength of the concrete in the arch rings was assumed to be 600 lb. per sq. in. The center for the
main ribs consists of two three-hinged steel trusses 23 ft. apart. The arch will be concreted in transverse sections (¢ 1343).
One of the first large plain-concrete railway arches was the double-track bridge on the Illinois Central Railroad over the Big Muddy River, near Grand Tower, Ill.* The bridge consists of three elliptical arches, each of 140 ft. clear span and 30 ft. rise. For the dimensions of the main arches, see No. 5, Table 99, page 705. Each main arch is surmounted by ten trans verse arches whose abutment walls are pierced by longitudinal arches. Only the spandrel arches were reinforced.
Fig. 236, opposite, shows the longitudinal section of one of the main arches. The arch ring was built in voussoirs approximately 8 ft. long on the intrados; and the numbers on the voussoirs in Fig. 236 show the order of placing the concrete. Each voussoir keys into the next one by two 4-inch by 12-inch projections on each side, made by timbers built into the block first completed. The face of the arch ring was divided into false voussoirs 4 ft. on the intrados by nailing triangular strips on the forms. Fig. 237, page 706, shows the centers employed in erecting the side spans. For the central span, to provide for possible drift, the middle portion of the center was supported upon 60-ft. plate girders which rested upon piles.
Observations were made from January 20
May 23, 1903, with gages reading to thousandths of a foot, to determine the amount of expansion. The extreme movement in that time was 0.012 ft., which was equivalent to a temperature change of 16° F. if we assume the coefficient of expansion to be 0.000,005,4, or to 12° F. if we assume the coefficient to be 0.000,006,5 (see 1334). No deflection of the arches or movement at the expansion joints could be detected during the passage of heavy trains.
Fig. 238, page 707, shows some of the details of the arch ring and of the center of one of the five 60 foot arches of the Pennypacker bridge on the Philadelphia and Read ing Railroad.f The arch was constructed as a monolith. A 1 : 3 : 6 concrete was used below the springing line, and a 1 : 2 : 4 above. The upper surface of the arch pitches 6 inches toward the axis of the bridge, and is drained by 4-inch pipes built into the concrete of each pier. Tongue and groove vertical transverse expansion joints are provided in the spandrel wall, two over each pier, extending from the haunches through the coping.