The durability of concrete, and the perfect protection that it affords to the reinforcing steel which is buried in it, give a great advantage to these materials in the construction of arch ribs. Although the theoretical economy is not so great as might be expected, there are some very practical features which render the method economical. It is always found that, before any considerable transverse stresses can be developed in a reinforced-concrete arch bridge, the concrete will be compressed to the maximum safe limit while the unit-stress in the steel is still comparatively low. Since a variation in the dead load often changes the line of pressure from one side of the arch rib to the other, and thus changes the direction of the transverse bending, it becomes necessary to place steel near both faces of the arch rib, in order to withstand the tension which will be alternately on either side of the rib. Of course the steel which is (for the moment) on the compressive side of the rib will assist the concrete in withstanding compression, but this is not an economical use of the steel. There is, however, the practical economy and advantage, that the reinforcement of the concrete makes it far more reliable, even from the compression standpoint. It prevents cracks in the concrete, and it also permits the use of a much higher unit-pressure than would be considered good practice in the use of plain concrete. This advantage becomes especially great in the construction of arches of long span, since in such a case the dead load is generally several times as great as the live load. Therefore the maximum variation in the line of pressure produced by any possible change in loading is not very great; and any method which will permit the use of a higher unit-pressure in the concrete is fully justified by the use of such an amount of steel as is required in this case.