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Types of Concrete Arches

arch, stresses, loads, steel, system, ring and solid

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TYPES OF CONCRETE ARCHES Arrangement of Spandrels.—In ordinary bridges of short span, solid arches with filled spandrels are commonly employed, as shown in the example of the last article. In such arches, spandrel walls are used to retain the filling above the arch ring. These are usually light reinforced walls and must be designed to resist the side pressure of the filling with its live load. When the depth of filling is considerable, a thin wall with counterforts is often employed.

For larger bridges, and where heavy filling would be required, open spandrels are often used. In these, the floor is usually carried by slabs and the loads are brought vertically upon the arch ring by cross walls. In such arches, the dead loads with their lines of action are definitely known, and the use of influence lines gives an accurate method of determining the effect of moving loads at any point of the road surface.

In a large open spandrel arch, the arch ring, instead of being solid, is frequently composed of two or more longitudinal ribs. The bridge floor is supported by beams and slabs, and the load transferred to the ribs by a series of columns. The determination of stresses is made in the same manner as for the solid arch, the whole section of the arch rib being used to carry loads brought by the columns, instead of determining loads and sections for a 1-foot slice of the arch ring. The loads must be brought upon the ribs axially, so as to produce no horizontal bending moment, and the width of the rib must be suffi cient to enable it to act as a column between points of support. The width may increase from the crown to the support so as to maintain a proper relation between width and depth.

171. Methods of are several methods of arranging the reinforcement in concrete arches. Numerous patented systems are more or less in use, while many designers place reinforcing bars in any way that seem to best meet their needs without following any particular system.

The Monier system was the earliest type invented, and consists in placing wire netting near the surfaces of the arch at both intrados and extrados. This system has been quite largely used in Europe.

The Melon arch has steel ribs, consisting of bent I-beams, or of built-up lattice girders, spaced 2 or 3 feet apart, extending from abut ment to abutment, they are self-supporting, and may sometimes be used to carry the forms in placing the concrete for the arch ring.

Many Melan arches have been constructed in the United States, most of those built previous to 1900 being of this type.

In the Thacher system, steel bars are used in pairs, one immediately above the other, near the extrados and intrados, the bars being inde pendent of each other. Several modifications of the Thacher system have been patented, in which the rods alternate in position or are connected in some way.

In other systems, attempts are made to use single tension bars, bent to pass from the extrados at certain points to the intrados at others as the occurrence of tensions may require.

When the stresses upon the concrete in an arch are kept within proper limits, the unit stresses upon the steel are very small, and more steel must be used than would be necessary to carry the tensions if reasonable unit stresses for the steel could be allowed. The steel is not therefore economically used in carrying stresses. It is rather intended to give added security against unforeseen contingencies, preventing cracks in the concrete, and guarding against distortions due to slight settlement of foundations or structural defects.

172. Hinged Arches.—Hinges are frequently used in arches for the purpose of making the stresses more nearly determinate, they give definite points through which the line of pressure must pass.

Three-hinged Arches.—Three hinges are usually employed and have the advantage of fixing the line of pressure so as to make it statically determinate. It is assumed that the hinge acts without friction and the line of pressure passes through the center of the hinges. Making this assumption, the horizontal and vertical com ponents of the thrusts at the supports may be determined by means of moments about the hinge centers. In large arches the hinges have the advantage of eliminating the temperature stresses. Slight settle ment of the foundations may occur in hinged arches without sensibly changing the stresses, while the accuracy of the computed stresses in a solid arch is dependent upon the rigidity of the supports. Hinges are usually expensive to construct, and the form of the arch, if eco nomically designed, is not so graceful as that of a solid arch.

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