Main Channel | South Branch | North Branch Bridges
Scientific American, May 11, 1895
THE VAN BUREN STREET DRAWBRIDGE OF THE METROPOLITAN
WEST SIDE ELEVATED RAILROAD OF CHICAGO.
We recently illustrated the operations of construction of the Metropolitan West Side Elevated Railroad of Chicago, the most impressive example of an elevated railroad system that has as yet been constructed. In our present issue we illustrate the Van Buren Street bridge, an element of the same system, which crosses the Chicago River, supporting two tracks of the Elevated Railroad. The bridge is of the bascule type, opening in the center for vessels to pass through, the trusses rising backward and upward, each one moving in a vertical plane. Its construction is peculiar in the omission of trunnions, the great girders rolling on a segment of a circle, one of which is formed on the backward prolongation of each of them.
THE VAN BUREN STREET BRIDGE, CHICAGO
DlAGRAM OF CONSTRUCTION.
On reference to the cut, two small diagrams will be seen illustrating the bridge, open and closed. The rocking operation of the bridge is made clear in these cuts. They indicate two additional features. The working strllts, as they are called, by which the bridge is opened and shut, constitute one of these features. One such strut for each half of the bridge is connected to the point representing the center, from which the circular segments are struck, and runs back horizontally or nearly so to the machine for opening. In the two diagrams this working strut is indicated by a single line extending from such center back along the fixed roadbed. The cuts also show how part of the deck carried by the trusses, as they rock backward, descends beneath the level of the fixed decks on either side of the river.
Referring to the general view of the bridge, it will be seen that each section of the bridge comprises three parallel trusses, each with a circular segment. For the circular segments to roll on, there are provided steel ways on which are projections or teeth of steel, which enter pockets in the faces of the segments. This insures alignment of the three trusses in their operation. On the right hand of the picture is seen the portion of roadway extending back of the center of curvature, which portion goes down when the bridge rises; it is cut off obliquely at its rear end, and a latch or link swings over this end, operating as a lock to prevent the bridge from being opened. There are also latches at the center of the bridge.
THE VAN BUREN STREET BRIDGE, CHICAGO
OPENING AND CLOSING MECHANISM.
The mechanism for opening and closing the bridge operates by means of the working strut. This is a bar carrying on its rear portion a rack and running nearly horizontally back from the center of curvature of the circular segment into the power houses. In the powerhouses are four 50 horse power electric motors, operating a train of gear wheels, the last of which wheels engage with the racks. When a rack is drawn backward, the section of the bridge rises; when a rack is projected forward, the section descends. Within the backward extension of the roadway just alluded to ballast is placed to bring the center of gravity into proper position. The effect of this is that the bridge naturally rests partly open, and if it is stirred from this position, tends to rock back and forth. The working struts are attached by pin connections to the central trusses; one strut operates each half of the bridge.
Assuming the bridge now to be closed, if it is desired to open it, the machinery is started so as to draw the racks backwara. As each rack moves, its first effect is to revolve a cam by which pin latches at the center of the span are withdrawn and the latches at the heels of the trusses, one of which is shown in the general view of the bridge, are swung backward, leaving the trusses free to move. As the motion continues, the bridge opens, twenty seconds sufficing for the entire operation. In closing, the reverse succession of operations takes place. The pin latches at the center are designed to prevent lateral movement and to insure the ends of the rails abutting in line ; the other latches hold the trusses closed Each truss may be treated as a cantilever, the tail girder representing the anchoring span.
We have referred to the counterpoise weights. They are placed within the tail girders and between them, beneath the railway floor. As these weights are sufficient to prevent the bridge from naturally coming to a horizontal position, in the closing operation force has to be applied to bring the end down. To work each half of the draw span, two of the fifty horsepower electric motors are provided, which are wired to operate together or alone. If by any accident the current is cut off, compressed air brakes are automatically applied, which instantly bring the bridge to rest.
In the closing process, before the ends come together the sections are automatically brought to a full stop, so that the final closing has to be performed with special care. A powerful emergency brake is supplied to guard against accidents, which brake can be made instantly to act upon the structure. It is believed that all these precautions and structural features make an accident impossible. Owing to the height of the bridge, it will have to be raised for comparatively few vessels, as most can freely pass under it.
One of the cuts shows the lifting mechanism placed beneath the road way, the under surface of the roadway or deck forming the ceiling, The general relation of the trusses to the abutments is shown in the larger diagram, giving a view partly in section of the structure, the open position being indicated by dotted lines. The small illustration gives a view of the deck of the bridge. The distant section in this cut is shown partly raised, a further descent of about three feet being required to complete the closing.
THE VA N BUREN STREET BRIDGE CHICAGO