The Burnside Blog

Posted By Randy Gragg on 12/07/2008

It’s a big week ahead in Portland bridges . Thursday, the Willamette River Bridge Advisory Committee—overseeing the first new bridge in downtown Portland in more than three decades and the first transit/pedestrian/bicycle-only bridge in the country – is likely to vote on which scheme or schemes it will recommend be unveiled to the public. Friday, bridge designer Bradley Touchstone shows his first schemes for the Columbia River Crossing to that project’s urban design advisory group.

Both are worth watching closely. After the CRC’s committee blasted an early scheme’s aesthetics, the Washington State Department of Transportation brought Touchstone in to add what project consultant Ron Anderson calls “an aesthetic element” (or possibly what Touchstone’s website calls one of his No cost and low cost aesthetic solutions TM). Anderson assures it will likely have little to do with the bridge’s actual functioning engineering. Stay tuned on that (or go to the meeting yourself, 7:30 a.m., Dec. 11, at WSDOT’s SW Regional Office, 11018 NE 51st Circle, Vancouver.)

The Willamette Crossing is at a far more critical juncture. The designers – Miguel Rosales working with Schlaich Bergermann and Partners – have explored numerous schemes. But TriMet and the advisory committee have narrowed the choice is down to two designs: It’s a cable stay vs. a “wave” bridge design. The choice will have huge consequences. The cable-stay, thus far, a pretty standard version of the genre with a pair 250-foot high towers rising higher than the Marquam Bridge. The wave would be both lower and the first of its kind: Typically the undulating, parallel, steel waves providing the structural support are solid; Rosales’ version would be an open web. Preliminary estimates suggest the custom design could drive up the cost by around $10 million.

The advisory committee, thus far, favors the wave bridge. Behind the scenes, TriMet engineers appear wary. I caught up with Rosales by phone from his Boston office to ask why.

Q: What are the pros and cons of these two bridge schemes?

The cable-stay would be completely different than anything you have in Portland. That can be positive or negative. But in this context, it will not look very good with the other bridges. It’s a completely different scale. It’s very monumental. The bridges next to it are kind of low key. The Ross Island Bridge is an arch bridge that is fairly transparent from a distance; it blends with the landscape. And, of course, the Marquam Bridge is not very attractive, but, to me, it’s just a background bridge. And if you see the rest of the bridges – the Hawthorne and others – they all have a low profile with a truss system with a combination of diagonals and verticals. They’re all kind of transparent.

The cable-stay will have 250-foot-tall towers. I think it will clash. It sticks way up into the horizon. It’s a much more dominant structure. The wave bridge, by contrast, always stays below the horizon of the hills and the mountains. When you see the cable-stay bridge, it will always be much higher than the horizon. To me, a cable bridge creates the sense of a gateway into someplace. But this bridge is more a connection between the two sides of the river. It’s not really a gateway into anything. It’s about flow from one side to the other.

Also, to make the cable-stay bridge a little bit more contextual or more special, it will require additional funding. If the cable-stay is selected, I’d really need to change it. The advisory committee doesn’t like it. I don’t like it either. This cable bridge is, basically, the cheapest alternative within that span.

Q: Looking at the cable-stay completely as a layperson, it’s surprising that it is cheaper.

I actually think they are in the same range. But that’s not what the contractor says. That’s why we’re having a workshop in San Francisco: to arrive at a better cost estimate. If you were going to ask anybody which is more expensive, any engineer with no bias would say the longer span is the more expensive. The wave bridge is 680 feet. The cable-stay is about 900 feet. The length of the span always has an exponential increase in cost. That’s well known.

What might make the wave bridge more expensive is that it is more unique. There are not many bridges like it. That has a premium. It uses steel. The steel market is up and down. Right now, it’s going down. But there’s a factor of risk. The cable bridge also has steel in the cables and reinforcement. But the wave bridge has more steel. And it’s a more specialized steel—well, actually, this is a point of strong contention. We think you could make it work with a more common steel. We are looking into that. The contractor says the steel we first recommended is harder to find. So that price would be higher.

Q: Why does the wave bridge require higher-grade steel?

With a lower-grade, the plate has to be thicker or the cross-section has to be different. The loads it is carrying is always the same. It’s just about how you take the loads with different materials.

HNTV who hired this contractor has done many cable-stay bridges. This particular bridge they’ve done almost identically. That’s what they want to do. That’s what they know how to do. But there have been a lot of studies done on the wave bridge because TriMet’s engineers keep asking, What about this detail or that? They’re trying to figure it out. We’re way into the design now. It’s more than preliminary design, very detailed. We could go farther, but everybody is keeping their distance. I don’t know what’s behind that the decision.

Q: Is there advantage to the cable-stay’s longer span?

The navigation will be better, in theory. But the Ross Island only has a 500-foot span, and the Marquam has about a 450-foot span. So even if the cable-stay gives more space, the 680 feet we are proposing with the wave bridge is more than enough. Everybody dealing with navigation on the river has acknowledged that. And when you make the span bigger, it puts the piers closer to the environmentally sensitive areas – in water that’s less than 20 feet deep. So while you might get some advantage for navigation, it’s not as good for the environment. That’s a big advantage for the wave bridge. It jumps over the sensitive areas. The piers are in deep water.

Q: So it sounds like the cable-stay would end up being a more off-the-shelf design?

If the details are not done well, it can be very unattractive. Have you seen the details on the Sauvie Island Bridge for the arch? They are terrible. All the connections of the cables are very awkward. Basically, there is no finesse in any of the details. In that arch, the connection between the approaches and arch are also very awkward from underneath.

When you do a bridge that is more unique, a lot of the details have to be figured out more carefully. You cannot take them off the shelf. They cannot be copied from another bridge. If the preliminary design is done well and the details are resolved correctly – which would be our hope given how many hours we’ve already spent working on the wave bridge already – it’s harder to change. Engineers won’t change something that hasn’t been done before without further calculations because that’s a risk.

Q: So by designing something unique, it keeps the designer at forefront of the process.

Yes. More questions get asked than if you just say, OK, we’re going to copy another cable-stay bridge from somewhere else and just put it in Portland. A lot of the new cable-stay bridges built through the design/build process they are terrible looking. This bridge will be design/build. The intention is to do about 25-percent of the design then release it into design/build. Once it’s there, there’s always a lot of pressure to cut costs.

I am supposed to stay through the process. I will do some guidelines for what they can change or not. But I don’t know if I will prevail. There is enormous pressure to cut costs. Unless the city is very involved, I don’t know what the oversight is in your city. Unless someone really understands the technical side, they can’t really help.

Q: Have you ever achieved acceptable results with design/build?

Miguel: Yes, once: over the Panama Canal. But the contractor and engineers were both German. They have a much higher standard of design. They respected the design and it looks very good. I generally don’t get involved in design/build. The contractors so often don’t care about the visual results. It will be up to TriMet to enforce it. I don’t know if they have the experience. They’ve never done anything like this.

Q:Experientially, the two bridges would be much different, particularly for pedestrians and cyclists. The cable-stay’s outer guard rail is solid. The wave bridge’s is more transparent which seems way nicer for pedestrians and cyclists. Why the difference?

The cables are anchored there. You have to have a concrete edge. The wave bridge is cantilevered so it can be more open. It’s like a wing. The views are clean and clear, and there is a separation between the pedestrians and buses and light rail because the wave goes in between. That’s a very big positive. You will feel like you’re in your own space. With the cable-stay, you will all be together. There is only a small barrier between light rail, buses and pedestrian.

Q: There’s no way to do the cable-stay with a cantilever?

That will add cost. Plus, you also have to change the tower. So far in our explorations, it’s much uglier. There might be a solution, but we haven’t found it yet. The problem is the angle of the cables. To keep them out of the bus lanes, the angle has to be very severe.

Q: If the cable stay is selected, you said you would want to improve it. What would be the major things you would change?

The towers. Those triangle towers are very alien. I would try to put the cables between the pedestrians and trains and buses. The issue is getting TriMet to commit to a range of cost. If they say, it’s $82 million and that’s it, then we can’t change it.

Q: Unless you can bring the wave bridge’s cost down, it seems doomed.

That estimate we have is not correct. There were too many mistakes. The quantities were all wrong. But you know, it’s not always terrible to be the first to do a design. At least three of the bridges I’ve designed were the first. They were not the cheapest of all the choices, but we kept them within the final bid amount. And they have brought a lot of attention to the places they were built. Does anybody regret them? Not at all. But did the agency take a risk? Yes. When you do something that hasn’t been done before, you have to pay more attention, there are more questions, the engineering has to be better. But would those bridges as successful if they were copies? No. Would people be as proud of them? No. How do you put a price to that? The question is: Is there the demand in Portland for something unique?