Examples or

EXAMPLES or Vovssoxa ARCHES. A few illustrations of actual arches will be given to show some of the details of construction of arches and of centers. Space will not permit a full presentation of all important details, but a few examples will be given to illustrate the general principles discussed in the preceding portions of this chapter.

Plauen Arch.

Fig. 216, page 661, shows a half section of the largest masonry arch in the world—see Table 90, pages 648.

The total span is 295.3 ft. and the total rise is 59.5 ft.; but the span of the arch proper is 213.3 ft., and the rise 21.2 ft. The largest of the transverse arches through the haunch is for the passage of a street. Between the crown and the transverse arches are longi tudinal arches.* Luxemburg Arch. Fig. 217 shows a half cross section of the second largest voussoir arch in the world—see Table 90, pages 648. Nominally the span is 277.7 ft., and the rise 101.7 ft.; but really the span of the arch, counting from the top of the curved abutment, is only 233 ft., and the rise only 53 ft. The bridge carries a 32-ft. roadway and two 10-ft. sidewalks. The arch consists of two parallel ribs each approximately 18 ft. wide, set approximately 18 ft. apart, with a reinforced concrete floor slab spanning the dis tance between them. This is an original and truly noteworthy con ception. The advantages of this feature are: 1. The amount of masonry, and consequently the cost, is reduced nearly one third. 2. By dividing the arch it was possible to complete an arch ring in a single working season. 3. The centers for the first arch ring can be moved over and be used again for the second.

The design has been criticized adversely for the following reasons: 1. The full arch is not visible, or rather the skewbacg is invisible, which gives an inartistic effect. 2. The curved abutment looks like a column that is bending under its load, and tends to give an impres sion of instability.* Cabin John Arch. Fig. 218,f page 663, shows the eleva tion of the Cabin John voussoir arch, near Washington, D. C. It was completed in 1859. The arch is a circular arc of and carries a conduit (clear diameter 9 feet) and a carriage-way (width 20 feet). The top of the roadway is 101 feet above the bottom of the ravine. The voussoirs are Quincy (Mass.) granite, and are 2 feet thick, and 4 feet deep at the crown and 6 feet at the springing. The spandrel filling is composed of Seneca sandstone, which, for a distance above the arch of 4 feet at the crown and 15 feet at the springing, is laid in regular courses with joints radial to the intrados; and hence the effective thickness of the arch is about 8 feet at the crown and about 21 feet at the springing.

For more than forty years this was the largest masonry arch in the world; and at present it is the largest voussoir arch in this coun try. . It is also the largest masonry arch in this country, except two

concrete arches —see Nos. 1 and 2 of Table 99, page 703.

Bellefield Arch.

Fig. 219 shows a half section and par tial plan of the main arch of Bellefield bridge at the entrance to Schenley Park, Pittsburg, Pa. For the main dimensions of the arch, see No. 20 of Table 90, page 648. Fig. 219 is given to show the method of securing empty spaces in the spandrel filling. The spandrel walls parallel to the roadway are built of rubble masonry, and are connected at their top by brick arches; and the transverse spandrel walls, also built of rubble, are stopped on a level with the springing line of the brick arches. Notice the concrete backing near the crown between the extrados and the roadway.* Arch. Fig. 220 shows a half section of the Pont-y-Prydd bridge—see No. 25 of Table 90, page 648. This is a remarkable bridge. It was built by an "uneducated" mason in 1750; and although a very rude construction, is still in perfect condition. A former bridge of the same general design at the same place fell, on striking the centers, by the weight of the haunches forcing up the crown; and hence in building the present structure the load on the haunches of the arch was lightened by leaving horizontal cylindrical openings through the spandrel filling. The cylindrical arches extend from the face of one parapet wall to that of the other. In addition, the filling immediately over the arch and around the cylinders was charcoal. This is among the first applications of this method of lessening the load on the haunches. Between the surface of the roadway and the extrados is rubble mas onry laid with horizontal joints. The outer, or showing, arch stones are 2.5 feet deep, and that depth is made up of two stones; and the inner arch stones are only 1.5 feet deep, and but from 6 to 9 inches thick. The stone quarried with tolerably fair natural beds, and received little or no dressing. It is a wagon-road bridge, and has almost no spandrel filling, the roadway being very steep. A stress sheet of the arch shows that the line of resistance remains very near the center of the arch ring. The maximum pressure is about 1,025 pounds per square inch. It is an example of very creditable en gineering.

Pennsylvania R. R. Bridge.

Fig. 221 shows one of the spans of Little Juniata Bridge No. 12 on the Pennsylvania Rail road; and is given mainly to show (1) the method of draining an arch and (2) the amount of backing that is ordinarily employed.* Formerly the backing was usually rubble masonry, but now it is generally concrete; and in either case, the amount is ordinarily enough to add materially to the strength of the structure, although the backing is not usually considered in computing the strength of the arch.