|
The arch is a rather uncommon type of structure in America, because the conditions which make it economical are unusual. For deep gorges with rocky sides, or for shallow streams with rock bottom and natural abutments, arches are eminently proper and economical. But when a steel bottom chord is needed to take up the thrust between springing points, all the economy of the arch vanishes.
The advantages of the arch are a possible economy of metal and an aesthetic appearance, while its disadvantages are a lack of rigidity and, for most types, an uncertainty concerning the maximum stresses in the members.
Arches are sometimes used for large train-sheds, in which their architectural effect is certainly very fine, but they require about twice as much metal as do cantilevered trusses supported on columns; consequently they can be adopted only when appearance is an extremely important factor in the design.
When bridge foundations have to be built on piles or on any other material that is liable to slight settlement, or when the abutments could possibly move laterally even a mere trifle, it is not proper to adopt an arch superstructure; for any settlement or any motion whatsoever in either piers or abutments would upset the conditions assumed for the computations, and thus cause to be increased to an uncertain amount some of the stresses for which the superstructure was proportioned. This criticism does not apply to the three-hinged arch, but even this design requires good, solid abutments and firm foundations for piers.
Arches can be erected on false work, by cantilevering, or by
79
|