This impressive vertical lift bridge was completed in 1915, and at the time had the heaviest lift span in the country, according to the Historic American Engineering Record documentation. The bridge serves a variety of trains including Metra, Amtrak, and Norfolk Southern. The bridge was originally owned by the Pennsylvania Railroad. The bridge is a vertical lift bridge built by Waddell and Harrington, an innovative and noteworthy engineering company associated with famous bridge engineer John Alexander Low Waddell. The bridge is the only such example on the Chicago River. As the only vertical lift bridge near the downtown area, it is not only historically significant, it is further imperative to preserve since it helps enrich a Chicago tour, by ensuring that all of the most common movable bridge types are around in the Chicago area for people to investigate.
The bridge provides a 130 foot clearance in the raised position. The towers are 195 feet in height. The bridge has a significant 47 degree skew. The machinery house is located on top of the truss in the center, while the bridgetender building is located directly underneath the machinery house. This bridgetender building is no longer used, and a brick structure on the ground near the bridge is the current bridgetender building.
The bridge replaced a swing bridge structure. Like many railroad bridge replacement projects, railroad traffic continued to flow over the swing bridge while the replacement bridge was being built. The lift truss span was constructed outward from the towers with the use of special falsework that angled back into the tower so that it would not be in the river obstructing boats. The bridge was also designed so that the railroad grade could be raised by up to 25 feet in the future if needed, which was one of the reasons why the vertical lift bridge type was selected, since it was found this bridge type could accommodate such a change. As built, the bridge contained 6,941,000 pounds of structural steel and machinery. An interesting design feature of the bridge was that the northern piers of the bridge were built overly wide, so that half of these piers could support half of a second vertical lift bridge, should the railroad have wished to add more trackage to the line. The diagram to the left shows how this second bridge would have been configured. As seen today, this second bridge was never built.
Ping Tom Park is a peaceful and excellent location from which to view this bridge.
Observant visitors to this bridge and many other vertical lift bridges may note unusual, massive chains hanging from the bottom of the counterweights and connected to the lift towers. The purpose of these chains may not be readily apparent, however it is actually an extremely interesting and creative use of simple physics to provide a consistent counterweight effect throughout the operation of the bridge. This bridge's lift span and counterweight is held by a system of cables, which run through a sheave or "cable drum" at the top of the towers. As the bridge raises, the cables run through the sheaves to pull the lift span up, while the counterweight receives the slack in the cable, which is pulled down by the counterweight which itself moves downward. As the bridge is lowered, the reverse happens: the cable is returned from the counterweight side to the lift span side of the sheaves, which allows the lift span to lower and the counterweight raises. These cables actually have considerable weight to them, and when the bridge is lowered, most of the weight of the cables is added to the lift span side, while when the bridge is raised, most of the weight of the cables is added to the counterweight side. This means that without something to address this changing weight distribution, the bridge is not constantly counterbalanced throughout its operation. The chain solves this problem. The chain is connected to the tower roughly halfway up the tower, and it is also connected to the counterweight. When the bridge is lowered and the counterweight raises, transferring cable weight to the lift span, the counterweight also picks up some of the chain's length, and thus some of the weight from the chain is added to the counterweight system, making up for the lost cable weight. When the bridge is raised and the counterweight lowers, receiving additional weight of the cable system, most of the length of chain is picked up and its weight held by the fixed towers, thereby removing that weight from the counterweight system.
This ingenious yet perhaps simple approach to maintaining balance of weight in vertical lift bridges was mentioned in J. A. L. Waddell's 1893 patent for a lift bridge. Waddell's design for a lift bridge in 1893 is considered the first example of a "modern" vertical lift bridge. Indeed, the general technique of using a system of chains for maintaining proper counterweight that J. A. L. Waddell claimed in his patent were still being used in bridges of more recent times, including Chicago's youngest vertical lift bridge, built in 1974. The function of the chains, called counterchains, is outlined in the 1893 patent as follows:
The lower ends of the weights are connected to the end of the span by a counter chain, composed of cast iron links, to pass beneath the inner foot of the tower and upwardly therein, the weight of the chain being such that it will exactly
counter-balance the weight of the series of cables connected therewith, while its cost is much less than would be a series of cables.
The end of the counterchain and the several series of weights are connected by eye-bolts, screwing into the bottom weight, chains, connecting the eye-bolts with a pulley, having a loop, which is connected by an open link, with the end of the cast iron counter-chain. The several groups of weights will thus distribute their power which will exert its full force in a single line of draft, and serve to nicely balance the weights of the several cables throughout their entire movement, as the counter-weight chain will follow and displace them throughout their entire movement.
Below is a diagram demonstrating the operation of these counterchains. Click on the image below to view a larger version of the diagram. Thanks are due to Jim Barker for helping to interpret the description provided in the Waddell patent.
Information and Findings From Chicago Landmarks Designation
Address: South of 19th St., East of Lumber St. (South Branch of the Chicago River)
This Bridge Is A Designated Chicago Landmark
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