SKEW-BRIDGE is a bridge in which the passages over and under the arch intersect each other obliquely. Where space and neat ness do not require to be considered, an oblique arch may be avoided, either by building the bridge square with the upper passage, and making the span so wide as to allow the stream to pass under it without being diverted ; or by building the arch square with the stream, and of sufficient length to allow the upper passage to take an oblique course over it ; hut either of these is a clumsy expedient, although well adapted for some situations.
The skew-bridges built by George Stephen son, on the Liverpool and Manchester Railway were the first in which the scientific principles of the method were properly applied. On that railway, out. of rather more than sixty bridges, about one-fourth were built on the spew; one, built of stone, conducting the turnpike road across the line at Rainhill, being at an angle of only 34 degrees, by which' the width of span is increased from 30 feet, the width a the railway from wall to wall, to 51 feet, the width on the oblique face of the arch. In the skew-bridge at Boxmoor, the angle is the square span 21 feet, and the oblique span 39 feet. The difficulty of build
ing skew-bridges increases with the obliquity of the angle from 90° to which is supposed to be the most hazardous angle for a semi circular arch; but beyond that point, instead of increasing, it rather diminishes to about which appears to be about the natural limit for a semi-cylindrical arch. Elliptical oblique arches are deficient in stability, more difficult lo execute, and more expensive, than semicircular or segmental arches.
The construction of skew-bridges of iron or timber is comparatively simple, the ribs or girders of which such bridges are composed being of the usual construction, laid parallel with each other, but the end of each being in advance of that next preceding it. The extra ordinary iron bridge by which the Manchester and Birmingham Railway is conducted over Fairfield Street, Manchester, at an angle of only is a fine example of this kind of skew-bridge. It consists of six ribs, of rather more than 128 feet span, although the of the street is only 48 feet, resting upon very massive abutments of masonry.