Interstate Bridge

View an article discussing the construction of this bridge

View an article discussing the collapse of the swing span

In the 1890s, railroad traffic between Duluth and Superior was required to use a Northern Pacific Railway bridge located at St. Louis Bay. This bridge was a single track structure, contained a sharp curve, two drawspans and was often at full capacity. Because a number of railroads interchanged at the Twin Ports, a new bridge was desired to connect the two cities. In 1894, the Duluth-Superior Bridge Company was incorporated by the St. Paul, Minneapolis & Manitoba Railway Company (StPM&M) for the purpose of constructing a new multi-use bridge across the St. Louis River to connect Rices Point in Duluth with Connors Point in Superior. This point was the shortest distance between the two cities, and provided the shortest channel. Due to its location, careful planning would be required to prevent the structure from obstructing navigation. Congress authorized the construction of a toll bridge in 1894, with final approval by the Secretary of War later that year. Work on the bridge began in 1896, with the substructures completed by November of that year. The bridge was opened to traffic in 1897. The original bridge was designed by A.P. Boller, with the stonework constructed by Alexander McGaw and the superstructure fabricated by the Pennsylvania Steel Company. The StPM&M became part of the Great Northern Railway (GN) in 1907.

As originally constructed, the bridge consisted of a 491-foot, 12-panel, pin-connected Pratt through truss rim-bearing swing span, approached by a 302-foot, 10-panel, pin-connected Pennsylvania through truss span on either end. Each end was approached by traditionally composed timber pile trestle spans. The bridge was constructed to serve two railroad tracks, placed at 13-foot centers in the center of the structure. In addition, a 9-foot roadway and 4-foot sidewalk were constructed from cantilevered floorbeams on either side of the main trusses. A single street car track was located on each roadway, providing streetcar service over the bridge. The piers of the bridge were constructed of Kettle River Sandstone, with granite copings. The piers would be founded on open caissons, which were supported by numerous timber piles, driven to bedrock. The superstructure was constructed with a combination of laced and solid members, heavy pinned connections and a traditionally composed floor. The lower chord was constructed using numerous parallel eyebars, while the upper chord used traditionally built-up members. The portal bracing of both the fixed and swing spans would use a pedimented lattice, and the portal bracing of the swing span also used a solid beam with circular cutouts. The swing span used a hybrid Pratt and Pennsylvania truss design, with the inner panels using subpanels. The center of the swing span consisted of a rectangular tower, constructed of built-up members and topped with ornamental spires.

On Saturday, August 11, 1906, the freight steamer "Troy" collided with the center span of the bridge, collapsing the span. When the ship hit the boat, it crushed the cantilevered extensions of the floorbeams outside of the trusses, and cut through the bottom chord. When the span collapsed, it crumpled straight down, with relatively little twisting. Whitney Brothers of Duluth, Minnesota was contracted to clear the channel, and a smaller navigation channel was opened by August 16th. The wreckage of the swing span was placed onto barges, and moved to an off-site location for repair. Ferry service temporarily replaced the bridge while the span was reconstructed. A $60,000 contract was awarded to the Reid Wrecking Company of Sarina, Ontario to rebuild the swing span. During the repairs, residents of Duluth and Superior were frustrated by the slow progress, and at one point in late 1907, replacing the damaged span with an entirely new span was contemplated. Work on rebuilding the swing span began in the spring of 1908, and the bridge reopened to traffic on September 27th, 1908. Reconstruction of the span involved salvaging as much of the span as possible, and replacing damaged and deformed members.

While the bridge was originally constructed to carry two railroad tracks, only a single track was installed on the bridge, and no steam train had used the bridge prior to the 1906 collapse. As part of the 1908 reconstruction, a wooden trestle was constructed for the GN on both sides of the bridge. In addition, the Minneapolis, St. Paul & Sault Ste. Marie Railway (Soo Line) and subsidiary Wisconsin Central Railway (WC) began using the bridge upon reaching the Twin Ports that year. In On November 21, 1924, the east truss span was struck by a freighter, causing the span to collapse. It would be quickly replaced with timber pile trestle spans. The replacement of the east approach provided this bridge with its final configuration. Streetcars were replaced by diesel busses by the late 1930s, and the streetcar tracks were removed from the bridge. The bridge remained a critical link for both railroad and roadway traffic through the mid 20th Century.

By the mid-20th Century, transportation in the United States was evolving to an automobile dominated system. In 1957, Interstate 535 (I-535) was authorized as a spur route of Interstate 35, part of the new Interstate Highway system. Construction on a new high level bridge for I-535 began in the late 1950s, with the John A. Blatnik bridge opening in 1971. The Blatnik Bridge was constructed immediately adjacent to this bridge, and resulted in the removal of roadway traffic from the swing bridge. GN merged with Northern Pacific Railway and the Chicago, Burlington & Quincy Railroad in 1970 to form Burlington Northern Railroad (BN). BN inherited three bridges across the St. Louis River between Duluth and Superior, and this bridge soon became excess. The swing span and east approach were demolished in 1971, and the west approach was retained as a fishing pier and pedestrian walkway, with the walkway attached to the bottom of the truss span. Today, the west truss span and trestle approach are all that remain of the structure. Overall, the remaining bridge is in fair condition, with no significant deterioration noted. The author has ranked this bridge as being regionally significant, due to the age, history, truss design and importance to the area. With an upcoming project to replace the Blatnik Bridge, it is unknown what the future holds for the remaining structure. The author hopes that the remaining span will continue to be maintained for pedestrian use.