Okoboji Trail Bridge

Located alongside US Highway 71, this unique deck plate girder swing bridge once carried the Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) Spirit Lake Branch over a channel between East and West Okoboji Lake. As railroads reached the Okoboji and Spirit Lake area, the high quality lakes became a tourist destination. The first bridge at this location was believed to have been a timber pile trestle with no movable span, completed in 1882. Almost immediately, the bridge became subject to controversy and boats operating on the lake demanded a movable span be installed. In August 1884, a swing span of unknown design was installed to permit the operation of boats and steamers between the two lakes. It is likely that this span was some sort of short wooden pony or deck truss span. By the late 1880s, the bridge required significant upgrades, and was replaced with a new deck girder swing span in 1890. The most recent additions to the bridge came in approximately 1965, when a 30-foot steel stringer span was added on either side of the swing span, giving the bridge its present configuration.

Currently, the bridge consists of an 83-foot deck plate girder swing span, approached by a 30-foot steel stringer span on either side. On the south side, the bridge is approached by five timber pile trestle spans, which were last renewed in 1921. On the north side, the bridge is approached by eighteen timber pile trestle spans, which were last renewed in 1921 and 1934. Lassig Bridge & Iron Works fabricated the main span, while the approach beam spans were fabricated by an unknown contractor. All timber components of the bridge were constructed by railroad company forces. The main span uses an extremely unusual design, consisting of a modified center-bearing turning mechanism and an unusual tapered girder shape. Each girder is fabricated as one piece, with the center 6 feet thick and the ends 3 feet thick. The unusual taper was likely added to minimize weight and maximize clearance underneath. With the exception of the ends and center, all transverse frames consist of a typical X-bracing of varying heights. The center and ends both use a riveted plate for the transverse frames. Also unique to this span, the north half is 40 feet long, while the south half is slightly longer at 43 feet long. The turning mechanism consists of a cylinder, which is held between top and bottom casting. Both castings are connected to the top and bottoms of the center transverse frames, and the bottom casting is riveted to I-beams placed transversely on the timber pier. Additional angle plates appear to connect the center of the span to the cylinder. The top casting contains a square nut, which was used to turn the cylinder and span. The approach beam spans are heavily riveted, and arranged into two sets of two. It is unclear if these spans were installed at this location new, or if they were reused from another location.

Typical center-bearing swing spans had wheels or rollers directly set onto the center of the swing span. Center-bearing swing spans were fairly unusual for railroad use, and largely fell out of favor by the 1890s. Most swing spans used a "rim-bearing" design, where the superstructure is set onto a drum, which turns on a roller nest. Rim-bearing designs required less precision and were easier to maintain. Drawings for this span do not indicate that roller bearings were used at the ends of the span and also do not indicate a roller nest was ever installed at the center of the span. It appears that the cylinder is balanced on the lower casting, and turning the span was accomplished without a significant gear system. In addition, it appears that shims were used on the bottom of the girders instead of rollers. This may indicate that the span was largely balanced around the center cylinder. The use of a rotating cylinder to turn the span appears to be a highly unusual design. It is possible that the span was designed as an experimental design, or it is possible that there were other unknown factors requiring an unusual design. It is also possible that the short span length allowed for a cheaper, easier to maintain design. During the late 1880s and 1890s, Lassig Bridge & Iron Works was an innovator of experimental and unusual bridge designs. From the late 1880s into the 20th Century, the Milwaukee Road designed many of their own bridges in house, often designing unusual and unique spans to meet specific situations. Onward Bates served as the Superintendent of Bridges & Buildings and was in charge of designing several unusual movable bridges.

It is believed that the swing span last opened in the mid-20th Century. Despite this, the center cylinder and castings appear to have been retained. After the bridge was abandoned, the center portion of the bridge was raised by adding additional cribbing at the piers and adjacent approach bents. The bridge eventually reopened to pedestrian use in the late 1970s or early 1980s. As part of the conversion to pedestrian use, additional observation platforms were added adjacent to the main span and concrete decking and wooden railings were added to the bridge. Today, the bridge is a popular fishing spot and part of the Great Lakes Spine Trail. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. It is possible that the timber components of the bridge will need repairs in the coming years, but the steel spans appear in good shape. The author has ranked this bridge as being regionally significant, due to the unusual design and age.