The appearance of concrete arches must often receive attention and at times they are faced with cut stone. However, by the use of selected aggregates, or by tooling and setting of tiling, ornamental appearances are secured, and are preserved, when the arch is properly waterproofed. It is of importance, however, that arch lines and surfaces be kept simple and accentuated, and not lost in inappropriate and meaningless applications of ornament. The use of pedestals, massive lighting standards is satis factory. The design of overhangs, off-sets, pilasters gives relief from the flatness.
In the United States, arch rings are usually built continuous with the piers. Rarely in this country, but more often in Europe, the arch ring is built in two parts with hinges at the crown and at the abutments, with the object of fixing the action line of the loads, thus lessening the flexure on the ring, and the effects of change of temperature.
Cantilever Concrete Bridges have the ap pearance of arches but are composed of bal anced cantilevers. These are suited to spans too long for girders; and, since they exert only a vertical pressure, they will fit foundation conditions too yielding to withstand the thrust of a real arch. The cantilever arms are ribs supporting a slab and beam deck Examples of such highway structures are Hopple Street, Gindnnati, Ohio, 70-foot span; Wayne County, Mich., 100-foot 6-inch span; Runnymede Ave nue, Cincinnati, Ohio, 72-foot span; Washington Street Bridge, Norfolk, Conn., 50-foot span.
Bowstring and Open-web Cased Girders are common in Europe and increasing in the United States in cases where arch construction through limitations of heading or foundation conditions do not permit an arch.
The Benson Street Bridge over Mill Creek, at LocIcland, Ohio, is an example of an arch with a suspended floor consisting of slabs and girders to which suspenders are attached.
Slab Bridges, used up to 25-foot span, con sist of a simple slab span to which the parapets are attached, resting on abutments. A highway bridge of 22-foot span, for instance, has a slab thickness of 12% inches, reinforced with rods, spaced 5% inches c.c.; and parapets 41% inches above the slab; %-inch rods spaced 12 inches c.c. run transversely to the bridge. Expansion slid ing joints of tarred paper or galvanized iron are placed on top of the abutments. Vertical drain pipes drain the roadway.
Girder Bridges are used up to 50 feet span and may be preferably of dedc type, where there is headroom, or of the through type. The two or more girders receive the deck slab. A span of 30 feet, for instance, has three girders 17 indhes veide and 2 feet 9 inches deep below the deck slab, whioh is 10 inches thicic. For addi tional information see BRIDGE.
Railroad Trestles are built by placing pre viously- molded slabs on concrete bents, spaced 15 to 20 feet, composed of concrete of vertical piles and a concrete cap. They are also built in place.