ADVANTAGES OF THE REINFORCED ARCH. If the loads upon the arch were all fixed, it would be possible to design an arch ring so that the resultant pressure would pass through the center of each cross section, and consequently the entire arch ring would be in compression; but if part of the load is a moving one, or if the line of pressure does not pass exactly through the center of each cross section of the arch ring, there will be developed bending stresses as well as direct compression. When the arch ring is subjected to bending, the greatest economy is likely to be obtained by the use of reinforced instead of plain concrete. However, the gain in economy through the use of steel reinforcement in an arch is not very great. If the line of pressure does not depart from the middle third of the arch ring, no tension is developed; and therefore the steel reinforces only in compression, and steel is not as economical a material to resist compression as concrete. On the other hand, if the line of pressure departs from the middle third of the arch ring, the probabilities are that owing to the comparatively heavy direct compressive stress the resulting tension will be quite small, and hence the unit tensile stress in the steel will be very low. Of course,
the unit stress in the steel could be increased by using a smaller per cent of it; but if the percentage of steel is quite small, to secure a proper distribution of it would necessitate the use of impossibly small cross sections.
Even though the economy of the use of steel in arches is not great, the reinforcement is practically of great value. Concrete is much more reliable in compression than in tension, and hence the use of steel to carry the tension adds to the reliability of the structure as a whole. Further, the steel is an economical insurance against uncertainties in the data, errors in the computations, shrinkage stresses, unequal settlement of the foundations, defective materials, and careless workmanship.