How Duluth’s Aerial Lift Bridge Works
The bridge’s motors drive winching drums, which wind (and unwind) the cables. Uphaul cables raise the bridge’s lift span (roadway), which in turn lowers the counterweights and chains to keep the bridge in balance. Downhaul cables do just the opposite, lowering the lift span, which raises the counterweights and chains. So the bridge must be in proper balance at all times. The crew of the Kansas City Bridge Company initially balanced it so well, the counterweights had to be adjusted to compensate for the weight of its Essex Green paint job.
Until 1986 the entire lifting or lowering process was set into motion when an operator turned the handle pictured above.
The Bridge’s 1986 Conversion
The bridge’s biggest renovation project, a refitting of the its mechanical systems, came in 1986. Automobiles had become heavier over the years and traffic had escalated dramatically, thereby increasing the load—and stress—on the bridge. To reduce structural stress, the operating machinery within the pilot house’s top level was moved to new machinery houses at each end of the lift span (see diagrams below); a new pilot’s house was installed as well. New counterweight cables took the place of those that had worn away. The old battery system was retired, supplanted by standard electricity. Nuts and bolts replaced rivets, and the bridge got a fresh coat of paint.Finally, the single handle that had set the bridge in motion since 1930 was replaced with a computer system, which then bridge supervisor Steve Douville called “a real challenge.” He explained that “Its ability to control two separate machinery spaces roughly a city block apart was amazing. Raising, lowering, starting, stopping, acceleration, and deceleration at various speeds and under various load conditions was truly too good to be true—but the malfunctions ranged from the mysterious to the serious.” Thanks to a quirky computer card, the bridge frequently acted up, and at least once it shut down when the span was fifteen feet from fully raised. Whenever the problem occurred, operators were forced to raise the bridge manually
The Lifting Procedure
The process of raising the aerial bridge’s road span for marine traffic begins when a vessel contacts the bridge’s pilot house and requests a lift. Originally this was done by a horn signal, but for decades vessels have used marine radio. When a vessel is about one and a half miles from the bridge inbound or just starting its turn to lineup with the bridge outbound, its captain calls the bridge and requests the bridge to lift. A bridge operator’s goal is raise the span for no more than twelve minutes. Sine 1990, when operators start the lifting process, they set into motion a sequence of events:
- A recorded message is played, informing tourists and pedestrians of a pending lift. (The voice on the recording is late bridge operator Al Kennedy, who ironically lost his voice before retiring.)
- The bridge’s traffic lights cycle to yellow and then to red, gongs sound, and the “Warning: Restricted Area” lights on both ends of the bridge start to flash.
- When the traffic lights turn red, an interlock releases, allowing the operator to lower traffic gates. When all gates are down, another interlock releases, illuminating a blue light on the control console, indicating the span is ready to be raised.
- While the operator at the controls checks the security cameras, another operator visually checks both of the span’s sidewalks and the roadway to ensure they are clear of all persons.
- When both operators are satisfied that the bridge is clear, the “raise” button is pushed.
Duluth Aerial Lift Bridge operators and vessel captains passing through the canal have retained one signal left over from the days before radio: The Captain’s Salute, one long horn blast followed by two short blasts exchanged between the vessel’s captain and the bridge tenders. It originally meant “Ahoy There!” but in Duluth’s ship canal it is essentially an exchange of “Thank you,” and “You’re welcome.”