A Bridge to the Past

A Bridge to the Past. . In 1919, Hennepin County hired the Illinois Steel Bridge Company to replace a deteriorating timber trestle built across Long Meadow Lake in 1891. The new 860-foot bridge was the longest Camelback through-truss bridge in the state when it opened in 1920. Five steel spans rested on concrete piers in the lake and concrete abutments at the shore. By 1923, the piers were leaning and an abutment had cracked, the timber pilings supporting them were unstable in the floodplain's poor soil.

After the county repaired these problems, the bridge required only routine maintenance for decades. Over time, though, larger vehicles and more traffic damaged railings and truss members. The concrete deck wore out and was replaced with a wood deck in 1961.

The bridge was no longer needed after the Minnesota Department of Transportation opened a new six-land bridge just downriver in 1980. The next year, MnDOT demolished an 1981 swing span bridge, a companion to the Long Meadow Lake Bridge that carried traffic over the Minnesota River. Ownership of the old truss bridge passed to the City of Bloomington. The deteriorating structure was closed to vehicles in 1993 and to all users in 2002.

The city planned to demolish the bridge but bikers, hikers, bird-watchers, and preservationists successfully rallied to save it. The bridge was listed in the National Register of Historic Places in 2013 and was rehabilitated for pedestrian and bicycle use between 2015 and 2017.

Truss Terms
Trusses are made of smaller parts, like beams and angles, that are fastened together by rivets, bolts, and other connections. Special vocabulary describes the relationship between a bridge truss and the deck. A through truss has structural elements above and on the sides of the roadway, travelers pass through it. A pony truss runs on the sides, but not above, the roadway. A deck truss carries the road on top of it.

A Camelback truss can be either a through truss or a pony. It is a variation of the once common Pratt truss, patented in 1844, and the Parker truss, patented in 1870. The top "chord" is straight on a Pratt truss and curved on a Parker, while the top chord of a Camelback has five slopes.

Photo Caption: In 1919, Hennepin County engineers prepared plans for the Long Meadow Lake Bridge, also known as the Old Cedar Avenue Bridge and Bridge No. 55. Timber piles supported the piers and abutments.
(General layout of Bridge No. 55, prepared by Hennepin County, 1919 with later revisions)

Caption: Unstable ground caused the abutment to move, producing large cracks in the concrete by 1923.
(Lee Brothers, photographer; from 1923 report prepared by Hennepin County, at Minnesota Department of Transportation)

Caption: After the bridge was closed in 2002, its fate was uncertain for years.
(Charlene Roise, photographer, 2014)

Caption: While the piers and abutments were rebuilt, the trusses had to be supported in another way. The truss structure would collapse if lifted by the bottom chord, so beams were inserted below the top chord and raised by hoists on each side of the truss. Beyond the upstream hoise, the contractor built a temporary bridge (right) for workers and equipment. Temporary coffer dams around each pier kept work areas dry.
(Charlene Roise, photographer, 2016)