IRON IN BRIDGE BUILDING.

Globe, Volume XXII, Issue 1946, 20 May 1880, Page 2

IRON IN BRIDGE BUILDING.

The use of iron in bridge construction has produced a boldness of conception in the present generation of engineers which casta the performances of their predecessors entirely in the shade. A half-century ago such spans as the fallen one of the St. Charles and lay bridges, for the loads that they were calculated to support, were impossible. Now they are far from being of the first magnitude. There are ten truss bridges across the Mississippi above St. Louis, which are net regarded as very wonderful structures, and yet seven of them have spans as long as those of the Tay bridge.

The bridges at Winona, La Crosse,. Dubuque, Keokuk, and Hannibal hare spans of 240, that at Bock Island of 250. and that at Louisiana of 256 feet. The span which gave way at St. Charles was 320 feet in length, yet the same bodge has two spans 406 feet long. Oyer the same river is a truss bridge at LeaTenworth, with three spans of 340 feet, and another at Glasgow with five of 315 feet. Across the Ohio there is a truss bridge at Steubenville with, a span of 320 feet, one at Parkersbnrg of 350, one at Cincinnati with a span of 515 feet, the longest truss yet built, and one at Louisville with a span of 400 feet. The truss bridge over the Kentucky River, on the Cincinnati and Southern railway, has three spans 375 feet in length, resting on iron piers 175 feet high. The proposed bridge over the Hudson at Fongbkeepsie has five spans of 500 feet, with piers 135 feet above high water. la Europe there is a truss bridge over the Vistula at Graudenz with 12 spans of 300 feet. The truss bridge of Leseart, in France, has a span of 314 feet, and was pushed across from one abutment to the other after being put together. Tbe bridge over the Rhine at Wesel has four spans of 313 feet. The Kulenburg bridge in Holland, which was. the monarch truss before the construction of the Cincinnati bridge, has a span of 492 feet. From these examples it would seem that the St. Chailes and Tay bridges, instead o£ being risky engineering ventures, are entirely within the domain of experience. But, nevertheless, the fact remains that, notwithstanding the boldness with which the engineers of the present day meet the exactions of the locomotive, they are comparatively novices in the use of iron. The first iron bridges were of cast iron, and soon proved to be too lightly proportioned. The first suspension bridges were similarly defective. Does it remain to be proved that the wrought iron work of the past twenty years betrays too great a confidence in the material ? Were the St. Charles and Tay disasters unaccountable accidents, or were they fair tests of current engineering theories ? These are ques ions which engineers would da well to discuss.—" St. Louis Globe."