The Siuslaw Bridge

The first time I crossed the Siuslaw Bridge in 2020, I was driving approximately 60mph on the 101 to hike in the Oregon Dunes. I didn’t know the bridge’s name and was more focused on the view of the dunes than on the bridge itself. It wasn’t until I joined Fat Pencil Studio in March of this year that I began to look more closely at the bridge and its history.

Tasked with upholding the enduring FPS tradition of 3D modeling and rendering an Oregon bridge, I perused our archives to find an older bridge that hadn’t yet been modeled by my colleagues. It had to have character, a great view, and, ideally, one or two architectural plans to help me, as a non-architect and non-engineer, build a bridge based on photos, Google Maps, and hand-measurements. Forgetting I had driven over the Siuslaw Bridge before, I chose it for its majesty and intricacy. Decked out with Art Deco details, Gothic-style piers, and decorative spires, it’s the kind of bridge that isn’t built anymore; the kind of bridge that wouldn’t have been constructed at all if not for dramatic tectonic shifts, the violent displacement of the Siuslaw people, and the advent of the Public Works Administration, nestled between the first and second world wars.

Siuslaw Bridge, August 16, 1936. Image from Historic Bridges.

To begin the modeling process, I worked on figuring out how to best utilize the axes in SketchUp to build a 3D bridge based on a 2D architectural plan of the side facade of the bridge. How closely should I trace the measurements of the architectural schema once I’d brought it into SketchUp? And how could I best reconcile those measurements with the measurements from Nearmap (which provides aerial photos and geospatial data, including high-resolution vertical, oblique, and 3D imagery)?

Ultimately, a SketchUp YouTube tutorial demonstrating how to model the Sydney Harbour Bridge provided the answers to these questions. This video also marked a turning point in my modeling process, helping me release my attachment to perfectly precise and accurate measurements. I learned to create components on an x-axis, using the original architectural plan as a reference while simultaneously building out the vertical bridge nearby on a y-axis.

Original bridge drawing from ODOT. Image from Historic Bridges.
Original bridge drawing from ODOT showing details of the steel eyebars for the bottom chord (tie) and how the concrete rainbow arch is reinforced by solid steel pieces (not just rebar). Image from Historic Bridges.
Original bridge drawing from ODOT showing the bascule trunnion as well as the operating rack and pinion. Image from Historic Bridges.
Birdseye view of modeling process on an original bridge plan in SketchUp.
Perspective view of modeling process on an original bridge plan in SketchUp.

A Drive to the Bridge

Still figuring out the structural grid and axes in SketchUp, I drove to the Siuslaw Bridge to take accurate measurements and photos in person. Thanks to a friend’s drone footage services, a long reel of measuring tape, and my drawings of bridge details accompanied by measurement notes, we captured key details of the bridge in under four hours of wind-buffeted investigation.

Drone footage of the Siuslaw Bridge, facing south.
Drone footage of the Siuslaw Bridge from above.
Drone footage of the Siuslaw Bridge from the east side.

Sunburst obelisk on the Siuslaw Bridge, taken during our bridge visit in May.
View from the bridge, facing west.
View through the small railing arches, facing west.

What wasn’t clarified by our visit, however, was the history of the bridge. The Siuslaw Pioneer Museum was closed when we passed through Florence and I had many questions remaining regarding the surrounding geological and Indigenous history. How did the Siuslaw river come to be? What happened to the Siuslaw Tribe when white settlers arrived? Why was the bridge built, and how was it funded?

Tectonic Shifts

The Siuslaw River region holds a layered history that long predates the graceful concrete arches of the bridge. About 25 million years ago, the uplift of the Oregon Coast Range began, and rivers cut through the raised land formation to form river valleys. Then, about 2 million to 11,000 years ago, melting glaciers caused sea level to rise, forming bays and inlets, and sediment deposits along valley floors made them habitable. Siuslaw River valley—a complex mosaic of braided channels, tidal swamps, and freshwater wetlands–was made possible by such a process.

For thousands of years the Siuslaw people inhabited the Siuslaw River valley. They managed its forests, estuaries, and salmon runs, and called the river itself iktat’uu. When Great Britain ceded the Oregon Territory to the United States in 1846, the Oregon Organic Act recognized Indigenous land rights—at least on paper. In 1855, the Tribes signed a treaty that promised food, education, health care, and the right to remain on their ancestral lands. Unsurprisingly, the treaty was never ratified by the U.S. Senate, and those guarantees were ignored.

Siletz ancestral tribes and homeland, with an overview of the languages spoken in those areas. Image from Confederated Tribes of Siletz Indians.

What followed was a period of displacement and suffering, often overlooked in histories of the Siuslaw Bridge. During the Rogue River War of 1856, federal forces rounded up the Tribes and confined them at Fort Umpqua. By 1860, they were marched 60 miles to a reservation near Yachats, where harsh conditions, disease, and starvation claimed roughly half their population. For 17 years they were forced to farm land ill-suited to agriculture, cut off from their traditional ways of life. When the Yachats area was opened to pioneer settlement in 1876, survivors were released only to find their homelands irreversibly changed and their former villages gone. Many became itinerant workers, taking farm or seasonal jobs while maintaining their cultural identity through monthly gatherings and ceremonies.

Despite these hardships, the Tribes endured. In 1916 they formed an elected tribal government that continues today, and by 1941 the Bureau of Indian Affairs placed a small parcel of land in Coos Bay into trust as a reservation, complete with a tribal hall that still stands as a symbol of resilience. This deeper context frames the development of Florence, Oregon and the Siuslaw Bridge itself. Completed in 1936, the bridge is often celebrated as an Art Deco landmark of Oregon’s coastal highway system—but it also spans a river whose history includes centuries of Indigenous stewardship, a painful era of dispossession, and a remarkable story of survival and self-determination that continues to shape the Siuslaw region today.

The Building of the Bridge

In the wake of WWI, anxious about defending an inaccessible coastline, the United States military establishment supported a bond measure put forth by the Oregon legislature, authorizing the issue of $2.5 million to complete the Roosevelt Coast Military Highway, now known as the Oregon Coast Highway, which was being constructed piecemeal to connect the isolated Oregon Coast with the rest of the state.

Construction of Oregon Coast Highway (101) at Neahkahnie Mountain, February 1940. Courtesy Oregon Department of Transportation. Image from Oregon Encyclopedia.
Construction of Neahkahnie Mountain Chasm Bridge, 1941. Courtesy ODOT, Bridge Engineering Section. Image from Oregon Encyclopedia.
Construction of Oregon Coast Highway (101) at Neahkahnie Mountain, April 1940. Courtesy Oregon Department of Transportation. Image from Oregon Encyclopedia.

Construction of Oregon Coast Highway (101) at Neahkahnie Mountain, April 1940. Courtesy Oregon Department of Transportation. Image from Oregon Encyclopedia.

By the Great Depression, however, the roads and small bridges making up the highway had yet to be fully connected, and five channels in the southern half of the state–Siuslaw River, Coos Bay, Umpqua River, Alsea Bay and Yaquina Bay–were still crossed by ferries. While they were a scenic tourist attraction, the ferries proved inadequate for increased traffic resulting from the highway system and hindered travel. Financing the construction of those bridges given the economic climate proved challenging and initial federal loan programs stalled as presidential administrations changed. Eventually, however, the state secured Public Works Administration funding under President Franklin D. Roosevelt, jump-starting the design and construction of the Siuslaw Bridge.

When the federal funding came through, Conde B. McCullough, Oregon’s legendary bridge engineer, rapidly expanded his design team, working day and night to deliver plans for and complete the five major coastal bridges on the Oregon Coast Highway—including the one at the Siuslaw—within an astonishing six months. To meet PWA funding requirements, construction was labor intensive, involving handsaws and wheelbarrows. Together, the five bridges employed more than 1,000 laborers, creating jobs and bolstering the local economy.

The reinforced-concrete tied arch span, a defining feature of the Siuslaw Bridge, was a relatively new bridge type in the United States at the time. Unlike true arches, which rely on solid rock abutments at both ends to keep the arch in compression, reinforced-concrete tied arches are self-sustaining, meaning they don’t require natural geologic strongholds. The bridge contains its own thrust, which proves helpful when building on sandy river banks.

McCullough built with reinforced concrete because it resists salt air far better than exposed steel and allows for plasticity of design. Wooden falseworks, temporary structures made primarily of timber, were molded to accommodate the forms designed by McCullough and his team, then filled with concrete and reinforcing steel.

Siuslaw Bridge, March 1, 1935. Image from Historic Bridges.
Siuslaw Bridge, April 19, 1935. Image from Historic Bridges.
Siuslaw Bridge, April 23, 1935. Image from Historic Bridges.
Siuslaw Bridge, June 19, 1935. Image from Historic Bridges.
Siuslaw Bridge, June 29, 1936. Image from Historic Bridges.
Siuslaw Bridge, June 29, 1936. Image from Historic Bridges.
Siuslaw Bridge, June 29, 1936. Image from Historic Bridges.

Bridge Restoration

All things considered, the Siuslaw Bridge has held up remarkably well, but the bridge has undergone significant preservation and repair work over the years, including cathodic protection to prevent corrosion, repairs to concrete support columns and the bridge surface, replacement of bridge railings, seismic upgrades, pedestrian access improvements, and replacement of bridge joints. Most interesting of all these repairs is the cathodic protection process, whereby a zinc coating is applied through a three-step process involving scaffolding, concrete repair, and a final zinc application.

Modeling and Rendering

Once I had overlaid the architectural plan with a structural grid based on confirmed measurements in Nearmap, I began building components. I started with the 140 foot steel double-leaf bascule span, which opens upward to allow maritime traffic to pass, and then drew the massive concrete piers, where the counterweights for the bascule span are housed.

The center of the bridge, from the northern arches to the southern arches, was largely modeled in quarters because of its symmetry. The curved spans on either end of the bridge, however, had to be modeled as whole parts due to their asymmetry. For the bridge railing, I created a unique profile in Profile Builder, a SketchUp plugin, which allowed for one-click extrusion along the curved end spans as well as the straight roadway in the middle of the bridge. This was challenging due to the difficulty of spacing the railing dividers and small arches along curved lines that differ on either side of the bridge roadway.

Railing dividers arranged on roadway in SketchUp using Profile Builder.
Railing dividers and small arches arranged on roadway in SketchUp using Profile Builder
Railing assembly extruded along a curved roadway in SketchUp.

There was another difficult and time consuming aspect of modeling: Art Deco sunburst obelisks that adorn the pointed pillars crowning the main support towers. Initially, I used Flowify, a SketchUp extension that bends groups or components along a quad target surface. This did not work as well as expected given the inset sunburst details on the obelisks, so I reverted to draping the sunburst details on the four faces of the obelisks, manually hiding and cleaning up the geometry on the faces of the obelisk.

Tracing of sunburst pattern draped on obelisk in SketchUp.
Finalized sunburst obelisk in SketchUp.

When it came time for rendering, I geolocated the bridge in SketchUp and then directly linked the SketchUp file to D5 to experiment with elements such as environment (sun position, sky, weather), materials (metallic, base color), lighting (brightness, intensity), and assets (sailboat, cars).

Somewhere between figuring out the structural grid in SketchUp and rendering the final bridge scenes, I became more comfortable relying on photos and Google Maps to gauge the proportions and relationships within the bridge model. This entire project was a practice in maintaining attention to detail while staying focused on the big picture.

Along the way I also picked up a good deal about the landmark’s history and discovered several useful SketchUp’s extensions. Profile Builder, Flowify, Thomthom’s Selection Tools, Solid Inspector, FredoTools, JoinPushPull Classic, and CleanUp₃, to name a few.

Now, when I drive over bridges, I slow down to note how the bridge is supported, what it’s made of, and what time period it was built in. I pay close attention to the details and wonder what history surrounds it. What was the bridge built on? How was it stabilized? Who was displaced? And what repairs have been needed? After this experience, it’s safe to say I’ll never look at a bridge quite the same way again.