Tilikum Crossing under construction (photo by Brian Libby)
BY BRIAN LIBBY
A couple weeks ago, I wrote an article for CityLab on Portland's new bridge, Tilikum Crossing, which is already gaining attention nationally and internationally for being the first multi-modal bridge in the United States that will be off-limits to private automobiles. As part of that article, I interviewed TriMet executive director of capital projects Dan Blocher about the process; I've decided to share the interview here in its entirety.
Do you think there’s a precedent here doing a multi-modal bridge without cars?
Yes I do. But it came about because of the environment, and not because TriMet or the city of Portland or the project partners had an aspiration to build a bridge of many modes but not private automobiles.
What makes this one so unique is basically because it’s a product of its environment. We started out by looking at where from a land use standpoint we wanted to have the bridge location. With the major presence of South Waterfront development and the tram being built nearby, it made a lot of sense to take the bridge south so we would have connectivity with all that. And then to cross over and come into the OMSI area, which is where it had always been planned. That was the starting point.
At that point we looked at our surroundings, and what we found was major bike and pedestrian infrastructure right at that bridge. So it certainly made sense to put bikes and pedestrians on it. What was difficult, however, was there was really no major network of streets on either side to support a highway network, certainly not in any kind of major capacity. So that really didn’t work very well as a design parameter. But it occurred to us that in the same space that the rail could run, so could buses. That’s not a whole lot of traffic. It’s not like having to build a major road network on either side to support it. But as with all of our projects the transit project helps the road network by providing alternatives for those who can use them. It’s not for everybody. A lot of people like to drive their cars because of their scheduling needs or childcare needs or so forth. But for everyone who does ride the transit system, that’s a car off the road. It has to be looked at as a total transportation system.
The bridge seems noteworthy, then, less for its physical design than as an act of urban planning through transit, and the collaboration that made that happen.
Definitely. Its environment isn’t just physical but it’s also cultural in the city it serves. There would probably be many other cities that would look to put a rail line there and then would say there really isn’t a need to put pedestrians or bicycles on it, either because other infrastructure does that or there’s no supporting infrastructure. But Portland, being so advanced in bicycle use and bicycle commuting, we already have that infrastructure on either side. So it becomes a very reasonable thing to do to support it with the crossing. Portland’s well ahead of the game in that regard. On day one, we expect to have a lot of pedestrian and bike use on the bridge.
Despite these urban planning goals that enabled Tilikum, how seriously did TriMet look at putting MAX on the Hawthorne Bridge? Certainly it would have been far less expensive at a time when TriMet’s budget is quite constrained.
Very generally I would say it was a combination of the difficulty of putting it on that bridge and finding room for it on that bridge. The Hawthorne Bridge is pretty far north from the other uses we described on the west side. That helped to move the alignment further south.
Tilikum Crossing under construction (photo by Brian Libby)
Could you discuss the process of choosing the bridge type and designer?
We assembled a community stakeholder group to investigate what kind of bridge would be appropriate for this setting. The alignment, the physical location, was set. The Willamette River Bridge Advisory Committee was chaired by [former mayor] Vera Katz and had a number of architecture, landowner and river representatives. We did that as a very public process. Every one of their meetings was a public meeting. TriMet also hired two consultants to give it technical advice: HNTB and [Boston-based bridge designer Miguel] Rosales. We began the discussion with the committee to look at the universe of bridge types and which ones were suitable for the location, the architectural appearance, the span width, river height, constructability, and cost. We got to the point where they selected this one type [a cable-stay bridge] to move forward. Along the way, a whole lot of bridge types were considered but went off the list because of reasons like cost or being too much bridge for the need.
The finalists came down to a new design called the wave frame, and the cable stay bridge. Then there was some additional research done by TriMet and our consultants to look at those two types and see which one was the most suitable. Ultimately we came away with the cable-stay bridge as being the committee’s first choice. The hybrid type was considered later. But we did some slight adjustments to the cable-stay design they looked at. One example is the committee was concerned about the height of the towers. There was the belief that if they got too tall or too massive it would not be the right architectural appearance for that setting. So we worked with some consultants, HNTB in particular, to design the type of cable-stay bridge we’re using out there, which is somewhat unusual.
How so?
Most times those cables are connected to the column at the center and are independent but opposite each other as they go down to the deck. As you can imagine, having a bridge like that, when you load it, that puts forces that tend to want to turn that tower back and forth. We designed a type known as a saddle, where the cable goes from the deck, up through the tower and back down to the other side as one continuous cable. That’s very different, because as it goes through the tower it goes through the saddles, and so there’s not as much force that tends to want to move the tower back and forth. The forces tend to be much more vertical. What that allowed us to do was to make the tower narrower and shorter, which fit perfectly with the committee’s desire for the appearance of that bridge.
How much push-pull was there over the height of the span from boat traffic versus pedestrians?
There definitely was a balancing act regarding the height over the river. We were constrained at how high we could make the bridge, because we were meeting landings on both sides that had to function within their surroundings. On the west side of the bridge, we actually did raise the ground level 14 feet. That actually worked pretty well insofar as because of the leftover contaminated materials we don’t want to dig down. So we built up 14 feet. There was a limit on the east side of how high we could go because we have to get underneath the 99E structure. Then as far as the rise of the bridge to the center, we were limited by ADA limitations. Unless we put a lot of landings on it and made it stair-step, we were limited to a five percent grade. We went a little bit under that, because that gives us a little bit of tolerance. And the height over the river is the third one. Coast guard approval is based on river traffic. We did a complete analysis and really found one owner, which was the Portland Spirit, which did have some issues with the height and where it is within the center span. We made some minor modifications to one of the vessels as well as extended their dock. That was just a matter of reaching an agreement that satisfied their needs.
Tilikum Crossing under construction (photo by Brian Libby)
Bridges are often civic symbols. How do you make it more than the sum of its parts while satisfying different constituencies?
We have a lot of constituencies to satisfy on a bridge like this. There’s waterway uses, land uses on each end, the requirements of the bridge itself. It required 34 different permits to build that bridge. That’s a lot, even in our type of work. And then as far as beauty, most people can sort of viscerally recognize an inherent beauty when the bridge is properly designed for its need. It’s possible to make a bridge much bigger assuming you have the funds to do so, but it might look sort of ponderous in its setting. It’s also possible to make one that’s very slender and minimalist, but again, that may not exactly fit the purpose. I think you know when you’ve got it right when the completed product just seems to fit, just like it belongs there. And I believe we feel very good about the feedback we’re getting on this bridge now that you can see what it’s going to look like. People seem to be really responding very positively.
In bridge architecture, that’s part of the picture, that appropriateness of size and scale. One aspect of the bridge’s design is that it’s very slender. It’s very thin. One way that you can often build a structure like that is to build a box cross-section. Yet that makes it deeper. We chose not to do that. We have a two girder with four-beam system, which allows it to be flatter. It gave us a little more height over the river but also gave our designer more of a challenge to make it work.
What did TriMet see in Donald McDonald that made him the right choice?
He was TriMet’s architect during the period when we were developing the proposal on which the contractors would bid. So while he didn’t do the detailed design of the finished product, he guided the architectural appearance of the final product. And that appearance was specified for the contractors to bid on. He had a major role in working with TriMet to specify the appearance and has worked with us all the way through as our architectural advisor.
The design seems to have a kind of angularity and kinetic energy. Where did that idea come from?
Really the inspiration for all those features, but especially the cables, comes from this basic shape, which is Mt. Hood. The columns themselves could be square, but they’re not. One side of them has that same triangular shape. The belvederes [the middle of the walkway that juts out] highlight that shape. There are reasons for the belvederes: they separate the walkways from the towers, and that has some help in relieving some wind loads that are at that location. Overall, it’s very graceful. There’s also something about the white cables and what that does visually that really makes the bridge shine through.
What might be some takeaways for you, or things you are particularly proud of?
There’s a lot of emotion that swirls around. I started my career as a bridge engineer in the mid-Atlantic for a bridge construction firm. I think what really moves me about the whole bridge is just the notion that Portland is Bridgetown. We have a famous bridge poster that shows the whole inventory of bridges over time. And now we’re adding another one. It seems to me that the process that the community drove the design of this bridge and was key to the selection of it, and yet it is different from every other one of those bridges. So it is not only appropriate for its use and its location but its time. And all those things come together in a way that seems almost magical. It wasn’t one party or the project team or TriMet saying, ‘This is the bridge we want.’ It wasn’t just engineers saying, ‘This is the right type of bridge.’ It was really that collaboration, which is so Portland. It becomes, I think, a perfect demonstration of how Portland approaches that kind of major civil infrastructure. It really is right out of the Portland story.
Downtown Portland from Tilikum Crossing (photo by Brian Libby)
Could you talk about how the two sides of the river will change?
Portland is really the poster child on the integration of land use and transportation planning. People come from all over the world to study how it’s done here. We have some advantages, like having the urban growth boundary, but it all works together. And we’ve seen that sort of land use change following major transportation investments in Portland in prior cases, and I’m sure we’ll see it here too. The South Waterfront is already taking off. And the innovation quadrant around OMSI has master-planning going on right now. We will see major changes on both sides of the river, in a positive way, in a way that’s supported by this new piece of infrastructure. So you get a lot of bang for the buck. You get more than just a transportation project; you get a full, multi-modal, city-wide development that brings more development.
A lot of this process paralleled the Columbia Crossing’s. Is that a reminder of the importance of public process and buy-in?
The public process is vitally important. The Columbia crossing did that as well, but it’s a much more complex project. It has two of everything. It has two federal agencies, two states, two cities, two planning organizations.
And I think the country as a whole is very divided on the priorities we have for spending. That was certainly a factor with Portland-Milwaukie [light rail], and that was a factor with CRC. They can be taken individually but they all have that challenge.
How different is the process for this bridge from what other cities would do?
We’re kind of ahead of the wave. Most cities wait until it’s just horrendous congestion, and by then when things are built up it’s a lot harder to do the things we’re doing here. The Portland region was very wise to get started on this and sort of get it closer to the wave of growth that the region has seen. We’re not unique, but we’re kind of a leader in this. The word gets out. And then other people do the same thing: it evolves the entire industry.
The reason other regions are quick to put vehicles on these bridges is that their lines tend to run alongside of a highway. But here it’s not an option. There’s no road network to support it. This was a transit project from conception.
Advertisements
Comments