AN ENGINEERING WONDER.

AN ENGINEERING WONDER.

Philadelphia is erecting a bridge of reinforced concrete on the line of Walnut lane across the Wissahickon creek and connecting Germantown and Roxborough. When completed, it will be not only the longest concrete arch in existence, but, also, one of the most wonderful pieces of engineering the world has even seen. The central span is 233 ft. between the piers. Its underside is *33 ft. above the creek and 101 ft. higher than the surface of the Wissahickon drive. The springing point of the arch at the face of the piers is 34 ft. above the drive. The falsework under the main arch has just been removed, and the mass of concrete and steel is now supporting itself. The general plan of the structure is that of twin supporting arch-ribs, floored across on the top; and this scheme is also followed throughout the live approach-arches, thus combining strength and lightness. Over each main arch-rib the floor is carried on eight pairs of spandrel or relieving arches, which carry the four longitudinal walls upon which the floor will rest. The whole will he hollow underneath, thus requiring no earth filling. The structure is of concrete and steel, with a surface-finish resembling stone. Such steel as is used is deeply imbedded in the concrete, un exposed to view or to the elements, and no painting will ever be required. The parapets along the sidewalks will be concrete balustrades of classic design. The driveway will be 40 ft wide, with ample space for double trolley tracks and the roadway. The two sidewalks are each 8 ft. wide, and the deck of the bridge will he 147 ft. above the creek. A few years ago the five approachspans, although small in comparison to the main airh, would have been considered quite large Tney are each 53 ft. in the clear, between the piers, and those next to the main span rise 70 ft. above the ground. The total length of the bridge is 585 ft. Chief Engineer George S. Webster, of the bureau of surveys, and his assistant, Henry 11. Quimhy, arc the designers of the structure.

Concrete Arch Over Walnut Street, Philadelphia, Pa.

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