MINNESOTA ARCHITECTURE - HISTORY INVENTORY FORM Project: Local Historic Bridge Study - Phase II Duluth, St. Louis County, Minnesota Identification SHPO Inventory Number SL-DUL-3143 Historic Name DM&N/DM&IR Ore Dock 6 Approach Review and Compliance Number Current Name DM&N/DM&IR Ore Dock 6 Approach Form (New or Updated) New Field # Description Address Ore Dock 6 Approach over Multiple Streets Linear Feature? Yes Linear Length: 1.1 miles City/Twp Duluth HPC Status: Unknown County St. Louis Resource Type Structure Architect/Engineer DM&N Legal Desc. Twp 49 Range 14 Sec 5 QQ NESE Style No Style USGS Quad Duluth Heights Construction Date 1917-1918 UTM Zone 15N Datum NAD83 Easting 565753 Northing 5177951 Original Use Transportation Property ID (PIN) Current Use Transportation Description The Oliver Bridge, known as Bridge 6544 in Minnesota and as Bridge B-16-755 in Wisconsin, spans the St. Louis River between the Gary-New Duluth neighborhood of Duluth, St. Louis County, Minnesota and the Village of Oliver, Douglas County, Wisconsin. The double-deck steel bridge is of riveted steel construction using built-up members. It is comprised of a balanced center swing span and steel approach viaducts on each side. It has an overall structure length of 1,889.75 feet (ft.) and an overall width of 25 ft. (DM&IR 1999, MnDOT 2013). Aligned on an east-west orientation, the structure carries railroad tracks on the upper deck and a two-lane vehicular roadway on the lower deck. The railroad carried by the bridge is the Canadian National Railway (CN) (see history section for names of predecessor lines). From the west, the railroad tracks approach the bridge following a gentle curve from a northwest to east alignment, then following a straight west-to-east alignment across the bridge and into Wisconsin. The bridge carries Minnesota State Highway (SH) 39 on the Minnesota side, becoming Wisconsin Highway (WH) 105 at the state line, located at the midpoint of the river. From the west, SH 39 parallels the CN right-of-way (ROW) on the south side of the tracks. As the two-lane highway approaches the bridge, it follows an “S- curve” alignment and approaches the lower deck, ducking under the span of the upper deck to access the lower deck. In Wisconsin, WH 105 parallels the railroad tracks about a half block to the north. As the road approaches the bridge, it follows an “S-curve” alignment, ducking under the end span of the upper deck, to access the lower deck. As a result, the lower deck of the viaduct has one less span at each end compared to the upper deck The substructure of the Oliver Bridge includes abutments, piers, and steel bents. At the ends of the bridge, there are reinforced- concrete abutments with wingwalls, one for each deck. Fill and sheet pile are behind the wingwalls on the Minnesota side. At the interior ends of the first and last spans of the upper deck, there are rectangular, reinforced-concrete piers, which is where the approaches to the lower deck transition to the steel viaducts. The swing span rests on a center-bearing pivot mechanism anchored atop a circular, reinforced-concrete center pier (Pier 2). At each end of the swing span, there are rectangular, reinforced-concrete piers (Piers 1 and 3) with breakwaters that carry the ends of the approaches. The western pier (Pier 1) is surrounded by sheet pile. The bridge no longer operates as a swing span; the last time the bridge functioned in this capacity was in 1947. The substructure of approach viaducts are comprised of four-legged, rigid frame steel bents resting on steel base plates set atop concrete footings. The bents are set perpendicular to the span and are two panels tall, with the lower bridge deck passing through the upper panel of each bent. The bents consist of latticed legs that support plate girder bent caps under each deck, with latticed strut and angle iron sway bracing that is attached to the legs of the bents by gusset plates. The approaches to the lower deck of the bridge, between the abutments and the steel viaducts, were replaced in 2001. These spans now rest on steel bents SL-DUL-3143 MINNESOTA ARCHITECTURE - HISTORY INVENTORY FORM Project: Local Historic Bridge Study - Phase II Duluth, St. Louis County, Minnesota with concrete pier caps. The superstructure of the Oliver Bridge consists of a balanced center, swinging, Warren through/deck-truss main span with steel deck plate girder and steel stringer approach viaducts on each side. The main span is located over the main channel of the St. Louis River and rests on a center-bearing pivot mechanism anchored atop the center pier. The rigid connected Warren truss is 306 ft. in length and, when rotated, accommodates two 125 ft. wide channels. The built-up truss is comprised of 12-panels, six on each side of the pivot pier. Each panel is comprised of latticed steel diagonals and verticals. In addition, there are vertical end posts at each end of the swing span. The Warren truss serves as a deck truss for the railroad line and as a through truss for the roadway. Under the truss, there are angled brackets to help transfer the weight of the span to the center bearing. At the center bearing there are four steel trucks at 90-degree intervals, each with two rollers that balance the span and carry part of its weight. The trucks are affixed to the span and rotate with it, following a steel track mounted atop the pier. Just outside the trucks is a cast steel bull gear. The gear mechanisms, which engage the bull gear, are mounted within the struts under the span. The approach viaducts are comprised of 31 upper deck spans, 27 on the Minnesota side of the swing span and four on the Wisconsin side. The upper deck extends the entire length of the approaches and the lower deck is one span shorter on each end. There is one less span at each end of the lower deck as this is where the highway approaches duck under the end spans of the upper deck to access the lower deck of the bridge. Individual spans range from 25 to 90 ft. in length, typically, with 30 ft. spans over the four-legged bents and 70 ft. spans between the bents. Within each span there is sway bracing between the girders. The superstructure consists of steel beams that carry the lower deck and two parallel deck girder spans. There is sway bracing between these paired spans. Originally, the upper deck carries to sets of tracks (double track mainline). Each of the two parallel spans of the upper deck carried a single track. However, one set of tracks have been removed. The set of track on the northern side of the structure remain in place and are active; while the set of tracks on the southern side of the structure have been removed. The remaining set of tracks consists of steel rails laid on bridge ties resting on the girders. The upper deck includes a metal railing on each side of the deck. The current lower deck, which carries the vehicular roadway, dates to 2001. It is cast-in-place concrete with a Type “F” railing, and sits atop steel stringers. This deck has a width of 22.8 feet and a vertical clearance of 14.1 ft (MnDOT 2012; MnDOT 2013). A bridge plate is located on the north side of the bridge; at the first Wisconsin approach span, and reads “BR 6544 1910, 2001.” EVALUATION AND ANALYSIS Historical Context Railroads in Minnesota, 1862-1956 Minnesota's Iron Ore Industry, 1880s-1945 Mining Development on the Iron Range, 1892-1954 Historical Narrative Significance The DM&N / DM&IR Ore Dock 6 Approach is a part of the DM&N / DM&IR Mountain Iron to Mesabi Junction (Duluth) railroad corridor and was constructed as part of Ore Dock 6. The DM&N / DM&IR Mountain Iron to Mesabi Junction (Duluth) railroad corridor was previously determined eligible for the National Register of Historic Places (NRHP) under Criterion A, in the area of history, for its association with the historic mining transportation system in Duluth (MnDOT Cultural Resources Unit Correspondence with Mead & Hunt, 5 April, 2004). The DM&N / DM&IR Ore Dock 6 Approach has significance as a contributing resource to the DM&N / DM&IR Mountain Iron to Mesabi Junction (Duluth) line. This railroad line has local and statewide significance under NRHP Criterion A within SL-DUL-3143 MINNESOTA ARCHITECTURE - HISTORY INVENTORY FORM Project: Local Historic Bridge Study - Phase II Duluth, St. Louis County, Minnesota the historical context that was established in the “Railroads in Minnesota, 1862-1956 Multiple Property Documentation Form (MPDF)” (Schmidt et al. 2007:F-194-196). Within the “Railroads in Minnesota, 1862-1956 MPDF” the railroad corridor meets Registration Requirement 2, as a railroad corridor historic district, because it provided transportation between a significant class of resources (iron ore) and an important transfer point for this product (ore docks on Lake Superior at Duluth). Moreover, as the first railroad line to reach the Mesabi Range, it established an important railroad connection that did not previously exist, thereby enabling the profitability, and corresponding significant expansion of iron mining on the Range. Upon its completion, the DM&N provided mine owners with a means to cost effectively ship ore to Lake Superior, where it could be transferred to ships destined for steel mills hundreds of miles away. Until the DM&N reached the Mesabi Range, its ore deposits were of little value due their distance from mills.