Page:Final Report - The Columbia River Interstate Bridge.pdf/29

 is suspended from each of its four corners by sixteen plow steel wire ropes, 2 ins. diameter, which pass up and over cast steel sheaves 12 ft. in diameter, and are attached by a group of equalizing levers to concrete counterweights built around steel frames. These counterweights are of the same weight as the span, as nearly as possible exactly to balance it, so that the work to be done in operation consists of overcoming the friction and inertia, and of lifting the unbalanced portion of the ropes. The span is guided in its ascent and descent by cast steel jaws which engage upon tracks on the towers; at one end of the lift



MANUFACTURING THE STEEL.

span these jaws guide both laterally and longitudinally, but at the other end the span is guided only laterally, thus allowing for expansion and contraction of the span and for slight variation in position of the towers. Operation is effected by means of operating ropes, of which there are two pairs at each corner of the span. The operating machinery. which is placed upon the span at its center, just above the roadway clearance. consists of