Vertical Lift Bridges.— The vertical lift or direct-lift bridges do not exhibit the wide variety in design seen in the bascule bridges. The sali ent feature of construction in the vertical lift bridge is the skeleton tower at each end of the lift span with its suspended counterweight. In some bridges the tops of these towers are con nected across the waterway by trussed girders. The chains or cables which carry the counter weights pass over single or double sheaves at the top of the tower structure. The counter weights balance the lift span when it is closed. As the span is lifted the weight of the sus pending chains is added to the counterweight, and as the span approaches the top of the lift it is more and more overbalanced. To obviate this difficulty two devices are in common use: the counterweight is made in sections which are automatically and consecutively caught by brackets on the tower as the weight descends; or counterbalancing chains are hung in loops on the channel side of the tower, and picked up loop by loop as the span ascends.
The Strauss direct-lift bridge is distinct from all others in its employment of the paral lelogram of links to raise the span, as in the Strauss bascule, thus avoiding the cables or chains of the other types of vertical lift bridges. These great trussed lever arms at the top of the triangular tower structure display an out line wholly different from all other types.
The relative economy of the vertical lift bridge may best be shown by a few compara tive figures. The largest swing bridges are about 500 feet long, and weigh about 3,000 tons. They afford a fairway of about 220 feet on each side of the centre pier. A rolling lift bascule to give the same clear waterway of 220 feet would weigh about 2,000 tons. A ver tical lift bridge affording a free waterway of 220 feet would weigh about 1,500 tons, includ ing its counterweight. One of the largest ver tical lift bridges in operation is a double deck bridge across the Missouri River at Kansas City. The lifting span is 413 feet in the clear, and rises to a height of 55 feet above the water. This bridge is peculiar in that only the lower deck is lifted. The same idea is embodied in a vertical lift bridge across the Willamette River at Portland, Ore., but in this case the upper deck may be lifted. The lower deck is raised to a clearway of 51 feet above high water, for ordinary traffic, and the entire span is raised 93 feet more — a total of 144 feet clearway— to accommodate the ocean steamers using that harbor. Consult Waddell, J. A. L., Engineering) (2 vols., New York 1916).