The Viaduct Works' Handbook

40 iron upon a thick layer of concrete. Piles 3 feet diameter and up- wards, are sunk by weighting, and dredging the soil from the inside; they are usually cast in short lengths, with inside flanges and bolts’ and each length is also parted into about four segments, with vertical flanges and bolts, so that the whole are sent out from this country in the form of small segmental plates, that are of convenient size and form for stowing in a vessel, and for transport on arrival in the colony. In designing a bridge of considerable length, in the most eco- nomical manner, it will be obvious that the length of its spans must, to a certain extent, be governed by the cost of the necessary piers to support them. It is found in practice that there is a certain propor- tion to be retained between the height of piers and the spans them- selves, which is arrived at by roughly estimating the gross cost* of the bridge at any assumed spans, and again estimating it at other spans, and comparing the results. As, for example, a bridge is to be erected over a ravine, 1600 feet wide, the bottom of which is, for the greatest part, nearly level, and composed of solid rock, and the depth at deepest part 200 feet from under-side of the intended level of girders; it will at once be seen that piers are requisite,—we may assume that spans similar to those of Tagus Bridge, already described, are used,—we should then have for the superstructure complete 16 spans, or 1600 feet at, for double line of way, £10 8s. per foot = £16,640 And 15 piers, which if made of the Crumlin Viaduct class, each 200 feet high, will require 185 tons of cast-iron, at, say, £8 per ton = £1480 And 41 tons wrought-iron, at, say, £16 = 656 £2136 for 200 feet Or, at the rate of £10 14s. per foot, each will cost £2140 = 32,100 Total cost . £48,740 Again estimating the cost, assuming that 60 feet spans of the same class are used, we should have for the superstructure 27 spans, which would require in each span, for double line, 28 tons of iron, at, say, £16 per ton, as before, or £448 for 60 feet, or £7 10s. Der foot:—