Digitizing Oregon City's Arch Bridge


The Oregon City 'Arch Bridge' is a steel through arch type bridge spanning the Willamette River between Oregon City and West Linn, Oregon. The completed bridge was opened to the public on December 28, 1922, with the cost of construction reported as $300,000. It was added to the National Register of Historic Places in 2005 and fully renovated in 2012.

The Oregon City Bridge is just downstream from the 40 ft (12 m) tall Willamette Falls and the Willamette Falls Locks, the oldest navigational locks in the United States. (www.wikipedia.org/wiki/Oregon_City_Bridge)

aerial photo
Arch bridge_finished bridge aerial

A through arch bridge is a structure that is made from materials such as steel or reinforced concrete, in which the base of an arch structure is below the deck but the top rises above it. It can either be lower bearing or mid-bearing. Thus, the deck is within the arch, and cables or beams that are in tension suspend the central part of the deck from the arch. (www.wikipedia.org/wiki/Through_arch_bridge)

Legendary state engineer Conde Balcom McCullough (May 30, 1887 – May 6, 1946) designed the Arch Bridge early in his career. Working for the Oregon State Highway Department, McCullough custom designed hundreds of Oregon bridges, including over 30 arched spans. The Arch Bridge represents one of his first achievements.

His signature is evident in the obelisk pylons with sconced light fixtures, ornate railings and Art Deco piers. In a letter McCullough wrote to an acquaintance, Portland banker J.C. Ainsworth in 1922, McCullough said he was “foolishly proud” of the recognition he received for his work on the Willamette Arch Bridge. He noted that an engineering trade journal described the challenges of the location as “ten to one more difficult than any problem presented by the Portland bridges. (www.wikipedia.org/wiki/Conde_McCullough)

The Oregon State Highway Commission Biennial Report of 1922 called it “the most artistic appearing large bridge in the State.”


The bridge is 745 ft in length and 28 ft wide, with a 360 ft main span that provides 49 ft of vertical clearance at low river levels.

3D Model

Since the bridge is symmetrical in one direction, but not in the other, it was modeled as a 'half' component and duplicated along its longitudinal axis. The bridge is sloped from the higher West Linn elevation, down towards the Oregon City elevation, which makes it asymmetrical in elevation.

Welded Steel structure

Though the 755-foot-long bridge appears to be a concrete structure, it is actually made of structural steel covered with shotcrete (cast-in-place concrete) and other coatings.

Arch bridge_welding the arch
Arch bridge_man atop steel 2
Arch bridge_steel structure
Arch bridge_initial steel arch
Gunite (shotcrete)

The Arch bridge is the only Oregon bridge to be encased in gunite, which protects it from corrosive sulfur dioxide emissions from paper mills south of the bridge. "The work required 40,000 square feet of 2 inch guniting on the steel ribs; 1,200 square feet of 6 inch gunite for the web on the underside of the arch; 800 square feet of 4 inches thick; 1,200 square feet of 3 inches thick, and 2,800 square feet varying from 6 inches down to 2." W.A. Scott, Engineering World (December 1922)

Arch bridge_spraying gunite 2

Contractors spraying gunite onto the structural steel

Arch bridge_spraying gunite
Arch bridge_structural framework

Arch bridge scaffolding & formwork

Art Deco Egyptian revival & Classical features

The Arch bridge was a fairly modern structure for its time, both in the simplicity of its shape and the expression of its various architectural details. The bridge's main feature arch is simply expressed and the gunite and concrete finishes reinforced the abstracted aesthetic. There are however a few classical touches in the detailed expression of its guardrail balustrades and newels, skewed corbel 'knees' on the outside face of the bridge, as well as ornamental light fixtures and obelisks.

There is a clear reference to Art Deco Egyptian revival architecture, which was prevalent at the time. In pure Neo-classical fashion, It was popular to blend Ancient Greek, Roman and Egyptian ornamental styles, in the pursuit of a powerful and dominant expression of form. (www.wikipedia.org/wiki/Egyptian_Revival_architecture)


Curvilinear cast concrete balustrade


concrete baluster details

Replacement strategy

The Arch bridge was built to replace a pedestrian cable-suspension bridge completed in 1888. The existing bridge was utilized continuously during the construction process in order to access the site and assemble the box steel frames. It was a main support system during the completion of the arch itself. The phasing sequence transitioning one bridge to the other is unique and the overlap presented unique engineering challenges and solutions.

Arch bridge A_preexisting cable bridge 2

Pre-existing cable bridge

Arch bridge A_preexisting cable bridge underlay
3d Rendering Engine: Twin Motion

Much of the rendering for this project was carried out through Epic Games' Twin Motion. The process was intuitive, and effective in conveying a realistic tone for the bridge.

Screenshot 2023-04-11 at 11.38.24 AM (2)

Twin Motion rendering interface

Bridge facts

As of 2000, the bridge carried 12,800 vehicles per day.

Oregon City, Clackamas County, Oregon, United States.

Structure Type:
Metal Solid Ribbed Through Arch, Fixed and Approach Spans: Metal Stringer (Multi-Beam), Fixed.

Construction Date and Builder / Engineer:
1922 By Builder/Contractor: A. Guthrie and Company of St. Paul, Minnesota and Engineer/Design: Conde Balcom McCullough (Oregon State Highway Commission).

Rehabilitation Date:

Main Span Length:
360.0 Feet (109.7 Meters)

Structure Length:
745.0 Feet (227.1 Meters)

Roadway Width:
18.4 Feet (5.61 Meters)

1 Main Span(s) and 10 Approach Span(s)


Ben Gramann is a Designer at Fat Pencil Studio