P Concreting the Arch. For convenience in concreting, an arch is frequently divided into a number of strips, which are practically arches in themselves. In all cases the concreting should be carried up from the springing line toward the crown, uniformly on each side of the arch. While this concreting is in progress the action of the centres should be carefully observed. Generally as the load on the haunch increases, the crown will tend to rise. If this tendency becomes excessive, it may be overcome by loading the crown with any material that is convenient, or by placing the concrete for that Staples Sub Grade End FIG. 93.—Travelling Form for Roof Arch, New York Subway Tunnels. portion of the arch before proceeding further with the haunches. It is well, if this method of placing the concrete is used, to so divide the arch that a complete ring may be placed without intermission. Another method is to divide the arch in strips extending the full width of the arch. The strips are first placed near the springing line, then, to overcome the tendency of the crown to rise, the strip at the crown is placed and so on until the arch is completed. Backfilling.-Backfilling is usually begun after the arch has hardened but before the centres are struck. The reason for this is obvious. If the filling were placed after the removal of the centres, it would be necessary to place the filling uniformly over the arch, as filling a large weight on one side while the other is unloaded might so seriously deform an arch as to endanger its safety. On the other hand, when parapet wall and railing are built before the centres are removed, the settlement of the arch may cause these to crack badly, and while this would in no way endanger the safety of the arch, still it is unsightly and therefore to be avoided. It would therefore appear that in some cases, particularly, where, instead of earth backfill, a system of relieving arches, etc., are used, that when possible the centres should be removed and the arch allowed to settle in place before that portion of the work above the arches is begun. A properly designed and executed concrete or reinforced arch, is economical, permanent, strong, rigid, and last but not least, can easily be made a thing of beauty. Various concrete arch bridges have been built where the effect is as pleasing as the best stone arches, and at a considerable saving in cost. Also the introduction of reinforced concrete permits of a light, graceful arch being built which is not attainable in stone masonry. CHAPTER XXIV CONCRETE BEAM AND GIRDER BRIDGES Advantages of Concrete Bridges.-Kinds of Girder Bridges.-Reinforced-Concrete Advantages of Concrete in Bridge Work.-The use of concrete in bridges was, until quite recently, limited to the arch. This limitation was caused by the low tensile strength of concrete, and where, for any reasons, the arch was considered undesirable, the use of steel or timber became necessary. With the introduction of reinforced concrete, however, the limitation of concrete work ceased to exist, as by the proper placing of steel reinforcement, the concrete could be relieved of all tensile stresses, and at the same time its great compressive strength called into play. We, therefore, at the present time, find not only arches of reinforced concrete, but also various types of flat bridges, and in a few cases even trusses constructed of this reliable material. Bridges, of all engineering structures, are probably the most exposed to the action of external destructive forces, and at the same. time receive the severest load treatment. In bridges of steel or wood, constant inspection, painting, and repairing are necessary if the structure is to be kept in anything like first-class condition, and even when these are carried on, almost continually, periodic renewals will be necessary. This causes the cost of maintenance to be very high, and this cost of maintenance is a large and important factor in the final cost of the bridge. With concrete bridges this continual painting and repairing is entirely unnecessary and the cost of maintenance is therefore very small. Also as concrete increases in strength with age, and as it is in no way affected by atmospheric conditions, a well designed and constructed concrete bridge may be said to be everlasting. A concrete bridge, therefore, when once built is built for all time, and periodic renewals are entirely obviated. The initial cost of a concrete bridge is therefore practically its final cost. It would appear, moreover, that even should the first cost of a concrete bridge be considerably higher than the initial cost of a steel or timber structure, that in view of its extremely long life and very low cost of maintenance, a concrete bridge would be the most economical in the end. The initial cost of a concrete bridge, while somewhat greater than that of a timber structure, is frequently lower than the first cost of a steel bridge. In localities where suitable sand, gravel, and broken stone are easily available, necessitating the transportation of only a comparatively small amount of cement, and reinforcing steel, the initial cost of a concrete bridge will be considerably less than that of a steel bridge, and will approach very closely the first cost of a timber structure. Another advantage of concrete bridges is that the major portion of the work can be readily done by local labor, and a great portion of the material can be purchased locally. Thus a large percentage of the money spent in the construction remains in the community, and the community is therefore doubly benefited. Traffic passing over a concrete bridge makes little or no noise. The same amount of traffic passing over a steel or timber bridge would cause a noise that would be heard for a considerable distance. This is particularly true where either steam or electric cars form part of the traffic. This elimination of all noise is particularly desirable in built-up communities, such as cities and large towns. Concrete, before it has set, is extremely plastic, and can therefore be moulded into practically any shape or form desired. Thus in building a bridge of concrete, a very pleasing and artistic design may be executed, at but a small increase of cost, resulting in an efficient and beautiful bridge. With but few exceptions steel bridges are far from being things of beauty, and at their best, can in no way compare with concrete structures. Classes of Concrete Bridges.-Concrete bridges may be classified as either arch bridges or flat bridges. Arch bridges of both plain and reinforced concrete have been discussed in the preceding chapter. A flat bridge is one in which the load on the structure acts vertically on the supports. A flat bridge may consist of either a straight flat slab, or of a combination of beams and slabs or of a combination of beams, girders, and slabs. All flat bridges require reinforcement. Flat-Slab Bridges.-The simplest form of a reinforced-concrete bridge is the flat slab. This consists of a sheet of concrete of uniform thickness, supported at each end. It is designed as a slab whose span is the distance between the abutments. The main reinforcement, therefore, extends from abutment to abutment, and may be of any of the numerous forms of reinforcing bars common to reinforced concrete. Structural shapes and even old railroad rails have been used in this capacity, and have given complete satisfaction. A secondary reinforcement perpendicular to the longitudinal bars or shapes should be placed in all bridges of this type. The function of this secondary reinforcement is to aid in the distribution of stresses due to concentration, to take temperature stresses, and to prevent the formation of cracks. Generally no special provisions for shear are necessary in flat slabs. It is customary, however, to bend a portion of the main reinforcement up to the top of the slab near the point of support. Should the bridge be continuous over two or more spans, additional reinforcement should be placed at the top of the slab, over the points of support, to take the tensile stresses caused by the negative bending-moment. Bridges of this type will generally be found economical for spans up to 15 feet. For larger spans the thickness of the slab and, hence the dead load, becomes excessive, and some other type of bridge should be used. Beam-and-Slab Bridges.-Beam-and-slab bridges consist of two or more reinforced-concrete beams extending from abutments to abutments and supporting a slab on which the roadway is laid. These beams are in the majority of cases entirely below the slab, but in some instances are carried up above the slab to form the side rail.. The design of a beam-and-slab bridge is essentially the same as that of a slab and beam in ordinary floor construction. The slab is supported by the beams and carries the superimposed live and dead loads. The beams are supported by the abutments and carry the slab with its attendant live load. The beams must be carefully |