A treatise on mountain roads; live loads, and bridges - Softcover

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1879 Excerpt: ...of the timber bridges which were erected in many parts of the country one or two generations back, bridges constructed solely of timber are not in much favour. Many of the old timber bridges have been renewed in stone, brick or iron. Timber bridges of any great span are not now in vogue, and where that material is employed alone, moderate spans on the raking-strut principle are preferred. Teak-wood bridges of this character will last 20 to 30 years. TRUSSED-GIRDER BRIDGES. For bridges of wood and iron combined, the trussedgirder principle is excellent. These girders are of very simple character. Timber scantlings not exceeding 14 feet in length, 12 by 16 inches square, and 2 inch rod iron, will make trussed-girders for a span of 48 feet to carry any road load. No castings are needed; and as the rod, or bar, iron carries the weight principally, and the timber only occupies a secondary position, has no framed joints, and is readily renewed, these bridges are to be classed more as semi-permanent, than as temporary, structures. Serious damage to these bridges arising from decay or faulty material is readily to be observed by an inspector; for a depression in the roadway, or a broken tie rod cannot well pass notice. The iron work is also in a sheltered position, and very accessible to the usual means of preservation of that material. Timber superstructures, or superstructures of timber and iron, should always, when possible, be carried on masonry substructures. Timber substructures are unsatisfactory owing to the expansion of that material in wet seasons in the direction of the fibre, and to the variations in exposure to air and water to which they are subjected. The disadvantage of the employment of the trussedgirder is, that in case excessive floods should rea...