Year constructed: 1895
Alternate name: Des Moines River Bridge
Bridge type: Pin-Connected Pratt/Warren Through Truss
National Register of Historic Places
status:
Listed
Length: 80 feet
Width: 16.2 feet
Spans: 1
FHWA: 148950
Jurisdiction: Emmet County
Location: 130th Street over the east fork of the Des Moines River, 7 miles northwest of Armstrong, Section 24, T100N-R32W (Lincoln Township)
Details
Located in northeastern Emmet County, this short-span through truss extends north-south across the East Fork of the Des Moines River. The structure consists of a single pin-connected truss, supported by steel cylinder piers, with timber retaining walls. The five-panel truss displays elements of both Pratt and Warren configurations. The end posts and outside panels are typically Pratt-like, but the interior three panels feature a triangular, Warren-like configuration, with diagonals acting alternating in compression and tension. Even in this, the truss's web differs from the typical Warren configuration, however, because the inverted V of the center panel diagonals does not parallel the diagonals in the adjacent panels. Known locally as the Ellsworth Ranch Bridge, this structure dates to 1895. In January of that year the Emmet County Board of Supervisors received a petition for a bridge across the Des Moines River in Lincoln Township. The board laid the matter over until April, whereupon it approved construction of the bridge and solicited competitive bids for its fabrication and erection. When the proposals were submitted the following month, the board adopted the plans of King Iron Bridge Company as "the most suitable," but rejected all of the bids as too high. After further discussion, the supervisors rewrote the specifications for the proposed bridge, reducing its length from 100 feet to 80 feet in the hope that the reduction in materials would result in a lower cost. This strategy was apparently successful. By the day's end, the supervisors had awarded a contract to the King Iron Bridge Company for the construction of three bridges - including this truss--for $3,400. The Ellsworth Ranch Bridge was completed later that year. Other than a minor reconstruction in 1937, it remains unaltered today.
The Pratt and Warren truss configuration were both developed in the 1840s, but it was the Pratt that received the most widespread use in the late 19th century. The reasons for this probably relate to the versatility of the pin-connected Pratt for different span lengths and its easier erection using timber falseworks. The Ellsworth Ranch Bridge represents an unusual hybridization of Warren and Pratt technologies. It is identical in size and detail with two such trusses found in South Dakota, both built in the mid-1890s by the King Bridge Company, as represented by King's Minneapolis representative, M.A. Adams. This esoteric truss does not appear in company literature of the time, suggesting that its use was experimental and short-lived. The only one of its type identified in Iowa, the Ellsworth Ranch Bridge is thus a rare surviving example of structural experimentation by one of the country's most prolific 19th-century bridge manufacturers [adapted from Fraser and Lauber 1992].