Types of Arches
In the most natural type of arch, the horizontal component of each reaction, or thrust, is carried into a buttress, which also carries the vertical reaction. This type will be referred to as the true arch. The application of arch construction, however, can be greatly expanded economically by carrying the thrust through a tie, a tension member between the ends of the span. This type will be referred to as a tied arch.Either a truss or girder may be used for the arch member. Accordingly, arch bridges are classiﬁed as trussed or solid-ribbed.
Arch bridges are also classiﬁed according to the degree of articulation. A ﬁxed arch, in which the construction prevents rotation at the ends of the span, is statically indeterminate, so far as external reactions are concerned, to the third degree. If the span is articulated at the ends, it becomes two-hinged and statically indeterminate to the ﬁrst degree. In recentyears, most arch bridges have been constructed as either ﬁxed or two-hinged. Sometimes a hinge is included at the crown in addition to the end hinges. The bridge then becomes three- hinged and statically determinate.
In addition, arch bridges are classiﬁed as deck construction when the arches are entirely below the deck. This is the most usual type for the true arch. Tied arches, however, normally are constructed with the arch entirely above the deck and the tie at deck level. This type will be referred to as a through arch. Both true and tied arches, however, may be constructed with the deck at some intermediate elevation between springing and crown. These types are classiﬁed as half-through.
The arch also may be used as one element combined with another type of structure. For example, many structures have been built with a three-span continuous truss as the basic structure and with the central span arched and tied. This section is limited to structures in which the arch type is used independently.
A great variety of forms have been used for trussed or solid-ribbed arch brides. The following are some of the principal forms used.Lindenthal’s Hell Gate Bridge over the East River in New York has trusses deep at the ends and shallow at the crown. The bottom chord is a regular arch form. The top chord follows a reversed curve transitioning from the deep truss at the end to the shallow truss at the center. Accordingly, it is customary to refer to arch trusses of this form as Hell-Gate type trusses. In another form commonly used, top and bottom chords are parallel. For a two hinged arch, a crescent-shaped truss is another logical form.
For solid-ribbed arches, single-web or box girders may be used. Solid-ribbed arches usually are built with girders of constant depth. Variable-depth girders, tapering from deep sections at the springing to shallower sections at the crown, however, have been used oc- casionally for longer spans. As with trussed construction, a crescent-shaped girder is another possible form for a two-hinged arch.
Tied arches permit many variations in form to meet speciﬁc site conditions. In a true arch (without ties), the truss or solid rib must carry both thrust and moment under variable loading conditions. These stresses determine the most effective depth of truss or girder. In a tied arch, the thrust is carried by the arch truss or solid rib, but the moment for variable loading conditions is divided between arch and tie, somewhat in proportion to the respective stiffnesses of these two members. For this reason, for example, if a deep girder is used for the arch and a very shallow member for the tie, most of the moment for variable loading is carried by the arch rib. The tie acts primarily as a tension member. But if a relatively deep member is used for the tie, it carries a high proportion of the moment, and a relativelyshallow member may be used for the arch rib. In some cases, a truss has been used for the arch tie in combination with a shallow, solid rib for the arch. This combination may be particularly applicable for double-deck construction.
Rigid-framed bridges, sometimes used for grade-separation structures, are basically another form of two-hinged or ﬁxed arch. The generally accepted arch form is a continuous,smooth-curve member or a segmental arch (straight between panel points) with breaks located on a smooth-curve axis. For a rigid frame, however, the arch axis becomes rectangular in form. Nevertheless, the same principles of stress analysis may be used as for the smooth curve arch form.
The many different types and forms of arch construction make available to bridge engineers numerous combinations to meet variable site conditions.