How Duluth’s Aerial Lift Bridge Works

Like its predecessor, Duluth’s Aerial Lift Bridge stood as a marvel of engineering, especially considering that the new span was constructed using much of the old bridge. The original bridge’s top span, raised forty-one feet, now provided little more than wind bracing, as the new towers carried the moveable road span and massive counterweights. As did the transfer bridge, the top span carried phone lines, electric conduits, and gas and water mains across the ship canal. The power to raise and lower the roadway span came from four ninety-five horsepower electric motors located in the upper level of the two-story pilot house attached to the road span. Together, any two of the motors could raise the span. Their power came from storage batteries located beneath the South Pier approach, kept charged by city electricity accessed by both a direct line and by tapping into trolley cables that ran along the top of the lift span. The other two generators stood by for emergencies, and the same room housed a gasoline engine should electricity completely fail.

The pilot house’s lower level contained the operator’s room, which contained levers connected to a host of devices positioned at various points on the bridge and its approaches—mechanical and electric interlocks, traffic gates, bells, signal lights, pneumatic horns, and both telephone and radio communication—to ensure safe movement by land and water. With the engines directly above the operator’s room, a bridge tender’s workday was loud.

The bridge itself worked not unlike an old-fashioned sash window, with counterweights connected by ropes to raise and lower a window sash. The bridge’s 900-ton road span was originally lifted by two 450-ton concrete counterweights, one on each end of the bridge. The span and its counterweights were connected by twelve 1 7/8-inch cables that ran over four sheaves—essentially pulleys twelve feet in diameter and weighing 14 tons each—one at each top corner of the structure. As the span went up the counterweights came down. The cables, in turn, were so heavy (about five pounds per feet) that they too had to be counterbalanced. This was accomplished by attaching large chains measuring 81 feet long (half as long as the cables) to each of the two counterweights and weighing approximately 259 pounds per foot. The chains balanced the cables. When the counterweights go down, the road span goes up, and when the concrete blocks go back up, the roadway comes back down.

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:

1. 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.)
2. 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.
3. 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.
4. 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.
5. 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.”