CONCRETE BRIDGES. These include arch, girder, slab, truss and trestle forms. The location of piers, length of span and thicknesses of main members will be determined by the character of foundations and loading to be carried; the width by the traffic, and the height by high water mark, and adjoining property, and the length of flood discharge, and channel width. The character of finish, ornamentation and other msthetic features will be determined by the situation. Since concrete bridges are permanent, they should provide for increased loading of future. On important highways, motor trucks of 30 tons must be carried; and on interurban bridges, from electric cars of from 75 tons to electric locomotives of 100 tons.
Reinforced Concrete Arches are either of closed-spandrel or of open-spandrel type. The first type is similar in form to a stone masonry ardi, with thinner sections. The reinforcement should be about 1 per cent of the crown sec tion in both extrados and intrados, and should continue throughout the arch ring, bonding into the piers. The two lines of reinforce ment should be connected with substantial wiring, sufficient to transmit stresses. Re inforcement should run transversely to the arch ring and be tied in the spandrel walls. Reinforcement may be preferably rods, or a light built-up framework of struc tural shapes (MeIan System). W. J. Douglas ((American Civil Engineer's Pocket Book)) gives a formula for the cost of a reinforced concrete bridge with ordinary foundations as follows: Cost in dollars =--0.8b1Vil where b =- width in feet, 1 =length in feet and d=--- aver age depth in feet of bed of stream below road way level (1916).
Arch bridges are developing into the open spandrel type, becoming more articulated and skeletonized; similar to the development from the Romanesque to the Gothic cathedral. In the open spandrel type, the loads are carried by a slab deck to transverse walls or columns, and in turn by these to the arch ring. A further development is to divide the arch ring into two or more separate ribs upon which columns rest to carry the load of the deck. The ex treme articulation of the Arroyo Seco Bridge is notable.
By such skeletonization, the designer can more certainly arrange the shape of the arch ring to the load, span and rise, and more economically carry the loads and reduce the weight upon the foundation. The subsequent
life of the structure due to expansion can be better provided for. Waterproofing is also more simple.
For high rise bridges of usual width the open-spandrel type is more economical. Closed spandrel, earth-filled arches, when of low rise and under 100 foot in span, are usually more economical than open-spandrel arches.
In both open- and closed-spandrel types side walks may project beyond the faces of the structure and so reduce the length of the pier4 Form of Arch Rings.— When circumstances of waterway and head room permit, the shape of the arch ring may be chosen from the standpoint of economy, and will then vary with the character of loading. For instance when the load is uniform per horizontal unit of length, as in open-spandrel structures, the parabolic curve is chosen (see Arroyo-Seco Bridge) . For closed-spandrel, earth-filled arches with a ratio of rise to span greater than one-fourth, an ellipse is suitable, and when this ratio is less the curve should be between a segment of a circle and an ellipse. In this case the curve is approximated to by a three or five centered curve. An arch springing down low upon the piers imposes less transverse action on the pier.
Expansion and contractions due to change of temperature must be provided for in con crete bridges. Heavy concrete sections demand provision for a range of 35°C. or 40°C. only. Sliding joints provide for movement of floor system and must often be waterproofed.
Thickness of Arch Rings.— Empirical for mulas for determining the thickness of arch rings at crown are generally used. The thick ness of an arch ring should increase toward the spring, except for hinged arches. The radical thiclthess at any point may be taken as the crown thickness, multiplied by the secant of the angle which the radial section makes with the vertical. For segmental and three centred arches the thickness at the spring is from two to three times the crown thickness. However these dimensions, useful for pre liminary plans, should always be checked by an analysis, using the method of the elastic theory.