Concrete While definite knowl edge derived from experience in the endurance of concrete under the demands of bridge service was lacking, engineers were slow to adopt it in place of the well established stone and iron construction. But more and more of late years confidence has grown in the relia bility of concrete as bridge material, and now concrete bridges of every type are to be found in all parts of the world. Such failures as have been recorded have been due to faulty handling of the material. The actual proof of the de pendability of concrete was contributed by an experiment upon a German bridge (at Dussel dorf) about to be replaced by a larger one. This bridge was but a few years old and had a span of 92 feet. It was loaded with 18 times the load for which it had been designed, and then showed only a few small cracks. The ex periment ended at that point and the bridge was demolished with dynamite. The concrete in this bridge had been mixed in the proportion of 1 part cement, 4 parts sand and 4 parts sharp chipped stone.
The considerable expense formerly incurred in building timber centres for concrete arches has been largely done away with by the use of light steel truss supports hinged at the centre of the arch. These need to be strong enough only to support the material used in construct ing the arch rib, as this in turn will support all the structure above it. Other advantages of concrete construction for bridges are that the work is more speedily accomplished and that the plant required is simpler and that the ulti mate cost is less — with small bridges very much less.
The simplest form of concrete bridge, in use only for short spans, is the girder type. This is sometimes constructed upon a skeleton I beam, but more commonly the reinforcement is of substantial rods mainly along the tension side of the beam, arranged in the conventional lines of stress. Such a bridge may be sup ported on piers or on trestle bents, also of concrete. A notable example of the concrete girder structure is the double-deck viaduct, 1,700 feet long, at Kansas City, crossing the tracks of 32 railroads which use the Union Station in the Kaw Valley. A peculiar feature of this viaduct is that the upper deck is on a grade of 5.527 per cent and the lower deck is of three different grades beginning at 2.34 per cent and rising to 3.47 per cent. The 12th Street Viaduct in Saint Louis is another exam ple. It is 1,481 feet long and 79 feet wide, built in 36 slab and girder spans. It is in two separate sections longitudinally, one inch apart, each section being carried on a double row of piers.
A reinforced concrete through truss bridge has recently been erected near Merthyr, Wales. It is of two spans of 94 feet each. The trusses are of the Warren type, 10 feet deep and in 11-foot panels. The pier and abutments are of
skeleton structure, remarkably light in weight. The lower chord has a cross section 10 by 18 inches and the upper chord is 18 by 18 inches. The web members are 10 by 10 inches. The reinforcement of the lower chord is of 12 1% inch rods in two sets of six, overlapping be yond the panels to which they are apportioned and terminating in hooks.
A notable example of the concrete canti lever truss type is the bridge carrying the Vic toria (Australia) sewer across the Barwon River and its flats. It is 2,424 feet long, in 13 spans of 176 feet and one of 136 feet. The suspended spans are 40-foot girders, free to move at one end.
The prevailing type of concrete structure, however, is the arch. While in many instances the forms of the arch usually seen in masonry have been closely followed, a peculiar flat form has been developed as the typical concrete arch. A striking example of this type is the Risorgi mento arch over the Tiber at Rome. It is the longest concrete arch yet (1916) constructed a span of 328 feet 1 inch. The rise is 32 feet. The structure is cellular, of seven arched ribs 6 feet 6 inches wide at the key, expanding to 10 feet at the piers. The ribs are 4 feet deep at the key and 10 feet at the piers. Another remarkable bridge of the same slender type is the Pont Mativa over the river Ourthe, at Lilge, Belgium. It is a span of 180 feet with a rise of but 12 feet and only 14 inches deep at the centre. This flat arch type is used princi pally for low bridges. For high-level bridges a high arch is generally chosen, sometimes reaching the deck with its centre, but often carrying the deck on a series of spandrel arches footed on the main arch. The open spandrel arch saves much in material and weight, besides lending itself to most pleasing architectural designs. The latest development of this type of bridge is a main arch composed of separate ribs, above which rise the spandrel arches on pillars, instead of the solid through and through walls of earlier constructions. Among notable long-span arches must be men tioned the 3I5-foot span of the Langwies Via duct on the Chur-Arosa electric railway (Swit zerland), rising 134 feet above the spring of the arch and 203 feet above the valley; the Auckland (New Zealand) arch, a clear span of 320 feet with a rise of 84 feet; the Larimer avenue (Pittsburgh) arch, a clear span of 300 feet 5 inches, the longest in the United States; the Stein-Teufen arch (Switzerland), a span of 259 feet; and the Walnut Lane (Philadel phia) arch, a clear span of 232 feet. The bridge at Luxembourg has a concrete span of 250 feet, but is sprung from masonry abut ments.