Steel arches are divided into three classes, based on the number of hinges each truss or rib contains. The simplest form has three hinges, one at each support and one at the crown. When two hinges only are employed they are placed at the skewbacks or supports. The third class of arches has no hinges and ill I . .Rarer Fro. 27.— Spandrel-Braced Arch.
its ends are usually fixed rigidly to the abut ments or piers.
The form of three-hinged arch most fre quently adopted for bridges is a horizontal upper chord and a bent lower chord united by vertical and diagonal web members as indicated in Fig. 27. Such an arch is designated as spandrel braced. The hinge at the crown is generally placed in the lower chord and only infrequently in the upper chord or midway between them, although in either of the latter positions the stresses throughout the structure are somewhat less. The explanation of the above lies in the fact that the placing of the hinge in any posi tion other than in the lower chord seems to detract from the good appearance of the struc ture, due to the necessary arrangement of the web members.
In another form of three-hinged arch both upper and lower chords are curved, being con nected together by web members like a truss in some arches and by a solid web plate in others. (See Fig. 28). An arch of this form is referred to as an arch rib, and two or more ribs constitute the bride structure. Where the solid web plate is use the section of the rib gi is similar to a section o a plate girder, and the chords are often called flanges. With the solid section the rib is generally made a constant depth throughout, although with this section, as well as where web members are used, the depth may vary. The theoretical outline of the rib would be narrow at the springing and crown, with greater depth at the haunches. Such an outline is not graceful, however, and is seldom used. In some of the more recent and heavy structures the ribs gradually increase in depth from the crown toward each springing or skew back, having the maximum depth at the latter ave‘e .474.
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points. Such an arch presents a graceful ap pearance, and the loss in economy is not great. The floor of the bridge is supported either on columns carrying the load down to the ribs or on suspenders hung from the ribs.
Two-hinged arches are somewhat stiffer in the vicinity of the crown than the three-hinged type, and on this account are preferred by some engineers for heavy traffic. In a three hinged structure the crown is free to rise and fall due to the expansion or contraction caused by temperature changes without producing any bending stresses in the members, but with the crown hinge omitted, such bending stresses may be large. The additional material required to resist these stresses may be considerable, so that generally the three-hinged arch is the more economical. In form the same general out lines are employed for both the two and three hinged types.
The no-hinged arch is seldom used in America. It is much stiffer than any other type, but this fact in itself is a disadvantage on account of the very large bending stresses caused by changes in temperature, particularly at the supports. Another disadvantage is the difficulty in constructing absolutely unyielding supports, for any movement of these produces additional bending stresses throughout the arch. Consequently arches of this type are not adapted for spans of any great length, and there are few in existence.
Concrete Arches.— For the shorter spans where the supports can economically be made unyielding, the concrete arch presents a struc ture not only very pleasing to the eye but at the same time of more permanent character than one of steel. Earth-filled arch bridges (see Fig. 29) are suitable for short spans or for long spans of relatively low rise, where the dead load weight of the earth fill is not excessive. For the longer spans of higher rise the open spandrel construction (see Fig. 30) is usually adopted. Here a reinforced concrete structure, similar to those shown in Figs. 18 and 19, is supported on columns which carry the load to the arch itself. While more complicated in construction the saving in dead load weight may be considerable, and the opportunity afforded for artistic treatment is increased. In some instances the arch ring extends the full width of the bridge, while in others the struc ture consists of two or more ribs, each carry ing its proportion of the floor load, and braced together so as to act as one structure. See CONCRETE CONSTRUCTION.