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Rainbow Bridge

La Conner Bridge

Rainbow Bridge

Primary Photographer(s): Nathan Holth

Bridge Documented: August 26, 2014

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Key Facts

Facility Carried / Feature Intersected
Pioneer Parkway Road (Maple Avenue) Over Swinomish Slough
La Conner: Skagit County, Washington: United States
Structure Type
Metal 21 Panel Hingeless Solid Ribbed Through Arch, Fixed and Approach Spans: Concrete T-Beam, Fixed
Construction Date and Builder / Engineer
1957 By Builder/Contractor: Neukirch Brothers of Seattle, Washington and Engineer/Design: Harry R. Powell and Associates

Technical Facts

Rehabilitation Date
Not Available or Not Applicable
Main Span Length
550 Feet (167.64 Meters)
Structure Length
791.5 Feet (241.25 Meters)
Roadway Width
24 Feet (7.32 Meters)
1 Main Span(s) and 6 Approach Span(s)
NBI Number

Historic Significance Rating (HSR)

Bridge Documentation

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

View Historic Structure Reports For This Bridge

This bridge is an uncommon example of a riveted steel through arch bridge. Its apparent lack of hinges (first example in Washington), solid rib design, and lack of spandrel bracing give the bridge an uncommon and distinctive appearance. In 1958 the bridge received honorable mention during in the annual prize bridges literature produced by the American Institute of Steel Construction.

Without bridge inspection access and partly due to wire mesh covering the hole, the details of the hanger connection to the arch rib, which is concealed within the arch rib itself, are difficult for a visitor to observe. An inspection report for the bridge reveals that after passing through the bottom cover plate of the rib, a pin connects the hangers to the arch rib. The pin passes through the hanger and a pin plate which is riveted to the arch rib.

United Concrete Pipe Products Company in Auburn, Washington was listed as the fabricator of the bridge, while Neukirch Brothers of Seattle, Washington was the general contractor.

The bridge retains historic integrity with the only alteration of note being the alteration of the portal bracing to increase vertical clearance.

This bridge was found eligible for the National Register of Historic Places before it was 50 years old because of its pioneering design. Different, stronger types of steel were used in strategic places on the bridge to provide the best possible balance between strength and economy. To this day, the bridge is the longest span of its type in the state of Washington.


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