Cables and Bolts of the Tilikum Crossing

Evan Kristof shares his love of design and the Tilikum’s role in expanding Portland.

Portland’s newest bridge, The Tilikum Crossing, looks especially elegant from the rooftop deck of the old Washington High school in southeast. The river is obscured looking towards downtown over the spread of leafy green and dusty roofs, but the crossing’s clean white peaks mark the river’s trail, along with the faded green Hawthorne Bridge and the bulky concrete span of the Ross Island and the Marquam bridges. Something about the crossing’s simplicity had me envisioning the construction workers building it with impossible coinciding ease, like the way you see a bed made in a Disney movie or a cotton commercial. Lifting the sheet in the air, letting it fall slowly in gentle billows. The bridge looks frozen in this motion mid-air, its deck rolling across the river in a soft long wave. It also has the simple look of an Ikea bridge, if Ikea designed bridges. Like you could fold it up and bring it with you to reassemble in a new city.

To hone my thoughts on the bridge from fantasy to reality, I spoke with Evan Kristof, former lawyer and professor of engineering at Portland State University, to learn more about the bridge, which seemed to rise rather quietly and off to the side of Portland’s busier, more central hubs. He informed me of the people and organizations involved and indeed, how unquiet and complicated the steps of a project like this can be. He worked on the Sellwood Bridge repair project and spent two years at T.Y. Lin International, an engineering firm with offices and projects worldwide and the Engineer of Record for the Tilikum.

Tilikum's Cables
Tilikum’s White Cables

“Most people would lump the Tilikum under the cable-stayed type,” says Kristof, but the bridge, which is owned by Trimet, is a new hybrid cable-stayed plus extradosed bridge. “A cable-stayed is different than a suspension bridge like the St. John’s,” Kristof explains, saying that the St. John’s bridge has one big cable that curves and goes from one tower to another, with vertical cables running from the deck to the tower. In the case of the cable-stayed type, “all the cables are straight and they don’t attach from one tower to the other, they go from the tower down to the deck.”

An extradosed bridge is basically a short cable-stayed bridge. It’s a special bridge type where the towers are shorter and the cables are not parallel to each other, instead creating a fanned arrangement of cables. Kristof tells me they mostly exist in Europe. The intermediate length of the Tilikum at just over 1,700 feet lent itself to incorporating aspects of this model. I’ve since read that this type is typically expensive and can be inefficient material-wise, but its elegance draws architects and designers anyway. “There are more efficiencies with the cable-stayed bridge,” says Kristof, so the hybrid model along with concepts introduced by TYLI and the project’s architects, San Francisco-based MacDonald Architects, helped to reduce costs which were estimated at $134.6 million, compared to say, the Sellwood project, which is costing over $300 million to repair.

There are four towers on the Tilikum and ten singular cables run through each, starting at the deck and threading through the tower to stretch down to the deck on the other side of each tower. The result is the clean, white triangular peaks and intentionally on the part of the bridge’s architects, MacDonald Architects, they are like tracings of Mt. Hood. Riding by in a car on the Ross Island Bridge after speaking to Kristof, my eye easily followed the lines of the cable arrangement below, taking me up and down the slopes. 

One of the efficiencies of the cable-stayed type is that the bridge can support itself during the construction phase, so that temporary supports are not necessary. “You can balance out the construction, build a new section, connect a cable to it, and it can support itself. It balances out the other side, kind of like a teeter totter,” Kristof says. For the Sellwood Bridge, a steel deck arch bridge, this is not the case. “We have to put up temporary supports to hold arch segments up before it all gets bolted together.” Temporary pilings have to go in the water, Kristoff told me and then he supposed aloud that this may not have to be the case with an arch bridge, recalling the Fremont Bridge. It is the second longest tied-arch bridge span in the world and was lifted in place in one piece in 1973, weighing 6,000 tons, making it the heaviest lift ever at that time.

I watched the time-lapse videos capturing the four-year building of the Tilikum. You get to see what Kristof means by the construction balancing out. Two decks protrude from construction sites on both the east and west banks of the river. Under all the clear, dense, and stormy skies of the last four years flickering in fast motion above the water, where barges and boats scurry in and out of the scene like game board pieces, the two sides of the deck eventually meet in the middle like two arms connecting in midair.

I recommend listening to Portico Quartet’s “City of Glass” as you watch it. Here is a stream.

The engineers have to consider many types of load when designing a bridge, like “pedestrians, wind, vehicles, snow, vessels hitting the bridge,” Kristof lists off. “Each of those might end up controlling different parts of the design. One might end up controlling how big the bolts are, or how much concrete you need in one spot, it’s really complex. They run analysis models that take into account all those possible permutations.” Kristof smiles and recalls going out on the Sellwood Bridge on a tour with his students. “To look at the plans on paper and then go out there and realize how big some of this stuff is — they have four-inch diameter bolts out there — it’s just incredible, the scale of it.”

The Tilikum is open to public transit, pedestrians, and bikes, but since cars are banned, that alters design options slightly. The greatest control here is determined by the Tri-Met’s new Orange Line, the heaviest load. “The streetcar and max are both pretty heavy but the good thing about it is you have a pretty good idea of what they weigh.” Considering bridges built for vehicles, Kristof adds, “whereas overloaded semitrucks and things like that, there’s a lot more uncertainty about people not following weight restrictions.”

The conversation around the Tilikum in news stories has emphasized how the vehicle-banning bridge reflects Portland’s values of alternative transportation. Kristof echoes this. It’ll make his commute easier, as he occasionally rides his bike to work over the Ross Island which “is awful, with just a little tiny sidewalk on one side. It’ll improve safety for sure.”

The structure is also an indication of Portland’s growth. Kristof has mixed feelings about how the bridge may change things in the Central Industrial Eastside, specifically the potential for it to become like many other parts of the city, light commerce and no industry. “During the economic downturn, industrial commercial output stayed pretty strong, so having a commercial industrial core over there has been good for the city.” He mentions that although “different people are trying different things down there,” plans to protect the industry have always been a goal. “Long term, I’m not sure, because it is really valuable property.”

“The need for the new bridge was there and now we are linking it,” says Kristof. There is interest in extending the urban core to the south waterfront which has seen immense development over the last decade. Having grown up off Scholl’s Ferry Road in Southwest Portland, he brings up how the history of bridge building in Portland mirrors the city’s progress. There was an active ferry service where the Morrison and the Sellwood bridges were built. “People driving in Portland, on Taylor’s Ferry Road, Boones Ferry Road or Scholl’s Ferry, those were roads that led to a river, that led to ferry lines to get to the other side.” There was a lot of commerce on the other side, and just like then, the Tilikum is closing a gap.

By Kathleen Dolan

I studied writing and English at Purchase College in NY State and graduated from PSU with an English degree. I contribute content, edit, and brainstorm at THRU.

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