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Isthmus Slough Bridge

Newport Lane Bridge

Isthmus Slough Bridge

Primary Photographer(s): Nathan Holth

Bridge Documented: September 1, 2018

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Key Facts

Facility Carried / Feature Intersected
Coos River Highway (Newport Lane) Over Isthmus Slough
Coos Bay: Coos County, Oregon: United States
Structure Type
Metal Rivet-Connected Pratt Deck Truss, Movable: Double Leaf Bascule (Fixed Trunnion) and Approach Spans: Metal 10 Panel Rivet-Connected Pratt Deck Truss, Fixed
Construction Date and Builder / Engineer
1931 By Builder/Contractor: Liesch and Tofte of Coos Bay, Oregon and Engineer/Design: Conde Balcom McCullough (Oregon State Highway Commission)

Technical Facts

Rehabilitation Date
Main Span Length
165 Feet (50.3 Meters)
Structure Length
1,758 Feet (535.8 Meters)
Roadway Width
27 Feet (8.23 Meters)
1 Main Span(s) and 35 Approach Span(s)
NBI Number

Historic Significance Rating (HSR)
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Bridge Documentation

View Archived National Bridge Inventory Report - Has Additional Details and Evaluation

This bridge is a double-leaf bascule bridge that also includes a fixed deck truss approach span next to the bascule span. In addition there are many concrete approach spans. When this bridge was built, contractors were asked to bid two designs, one with concrete approach spans, and another with timber approach spans. The concrete alternate was chosen. The bascule span is a deck truss superstructure, and the bascule design is a Chicago type fixed trunnion with below-deck counterweight. The bridge provides a 140 clear channel for navigation, which is what the National Bridge Inventory decided to report as the span length. The actual span of trusses from face of abutment to face of abutment (16 panels) is 144 feet. The center-to-center span between trunnions (the typical measurement points for the structural span length of a bascule) is 165 feet.

When this bridge was built, newspapers reported the contractor as Liesch and Tofte of Marshfield, Oregon which refers to Coos Bay, Oregon. In 1944, citizens voted to change the name of the city from Marshfield to Coos Bay. The original overall length of the bridge is reported as 1,757.8 feet. The National Bridge Inventory reports a length of 1,691 feet.

The trusses were being repainted and on top of this the sun position was not optimal at the time this bridge was visited. Photos are limited for this bridge.

Above: Plans for this bridge included this drawing of the previous bridge at this location, which was a swing bridge.

Above: A portion of an original plan sheet for this bridge's trusses showing the fixed trunnion design. Counterweight is attached to the left of the trusses in the above drawing. The trunnion is visible in the center, and to the left the rack and pinion operating system is shown.


Photo Galleries and Videos: Isthmus Slough Bridge

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