Fig. 243 shows one of the seven arches across the Wabash River at Peru, Ind.t There are three noteworthy features of this bridge. 1. The arrangement of the reinforcement of the arch ring, whereby a rod is near the intrados at the crown and near the extrados at the springing, one third of the rods crossing the neutral line at the mid-point of the semi-arch, one third at the upper third-point, and one third at the lower third-point. This disposition of the steel is on the supposition that the unreinforced arch has a tendency to fail by ten sion in the intrados at the crown and by tension in the extrados near the springing—see Fig. 189, page 610. 2. A new method of balancing the thrust of unequal spans. The arch shown in Fig. 243 is flanked on the right by a 100-ft. span and on the left by an 85-ft. span. For the effect on the appearance of the bridge and also to increase the waterway, it was desired to make the spans increase from the shore toward the middle of the river; and to keep the road way level and as low as possible and also to give a maximum water way, it was not possible to balance the thrust of the longer span in the usual way, i.e., by making the shorter span proportionally much flatter, as the shorter arch would be entirely submerged at highwater. The submergence of the end spans would not only decrease the waterway, but might through the effect of buoyancy cause a collapse of the adjoining spans. Further, it was not possible to make the piers heavy enough to support the unequal thrusts, without unduly reducing the waterway. These conditions were met by inclining each arch of a shorter span upwards toward the adjacent longer span, so that at any pier the shorter arch has virtually a higher springing than the longer arch, although the apparent springings are maintained at the same level by slightly distorting the curve of the shorter span at the pier. By this method a 75-ft. span is balanced by an 85-ft. span, an 85-ft. by a 95-ft., and a 95-ft. by a 100-ft., all on 6-ft. piers, although the rise of the several arches is practically constant, being 15 feet for the 100-ft. span and 13 feet for the 75-ft. The effect on the thrusts of the arches by in clining of the shorter span upward toward the longer is shown in Fig. 243, for the pier between the 95-ft. and the 100-ft. spans. The thrust T, of the 95-ft. arch meets the thrust T, of the 100-ft. span on
that side of the center of the pier toward the greater span, by reason of the elevation of this end of the shorter span. The thrust T, of the 100-ft. arch being greater than T, the resultant R, of the two thrusts is deflected across the center line of the pier, but is kept within the middle third of its base. 3. A new method of striking the centers. The centering was supported by 2- by 12-inch joists, which were too flexible to carry their load except when braced in both directions at the third points.* To strike the centers, one system of sway bracing was removed from the upright supports, which allowed them to buckle and thus relieve the centers.
Fig. 244 shows half of the 100-ft. span and half of one of the two 80-ft. spans bf the double track reinforced-concrete arch bridge on the Cleveland, Cincinnati, Chicago and St. Louis Railway, near Danville, Ill.t The road also has a similar bridge near Robinson, Ill. The bridge is 42 ft. wide outside to outside of parapet walls, and 27 ft. inside to inside. The springing line of the intrados of the 100-ft. arch is 30 ft. above the bed of the stream. Note that the crowns of all three arches are at the same elevation, the springing lines of the smaller arches being higher than those of the larger one. The base of the rail is 19.75 ft. above the extrados of the crown of the main arches, and the track rests upon a cushion of earth 5 ft. deep. The spandrel arches have a span of 8 ft., and a thickness at the crown of 2 ft., the space above the arches being filled with concrete to the level of the crown. The reinforcement is square corrugated bars. The steel in the main arches is 6 inches from the surface at the crown and 24 inches at the haunches. The series of spandrel arches are built in three sections, one over each of the main arches. Between each section of spandrel arches is an expansion joint, the details of which are shown in Fig. 244, page 719. The numbers on the arch rings show the order of the concreting.
For a detailed description of the proposed Henry Hudson memorial reinforced-concrete arch of 703 ft. clear span over Spuyten Duyvil Creek, New York City, see Engi neering News, Vol. LVII' (Nov. 21, 1907), pages 559-61; and for in teresting editorial comments on the same, see page 555.