First the Kingdome, now Nalley Valley viaduct

When John Christiansen heard the Nalley Valley viaduct was being torn down, he reacted almost as if there had been a death in his family.

There was a long pause on the phone.

“It stuns me to hear,” Christiansen said. “I just cannot fathom the Washington Department of Transportation. My opinion is that a concrete structure like this should last several hundred years or more.”

The viaduct, the distinctive Tacoma bridge that for 40 years carried state Route 16 across Nalley Valley on its leaning, space-age legs, was Christiansen’s baby.

He designed the viaduct and its distinctive “tetrapod” support columns, now being battered to pieces by demolition crews as part of a $200 million project to streamline the interchange between Route 16 and Interstate 5.

Christiansen is now 84, retired and living on Bainbridge Island. The quarter-mile-long viaduct is not the first of his monumental structures to go.

He and his Seattle-based firm, Skilling, Helle, Christiansen and Robertson, did the structural engineering for the World Trade Center in New York, destroyed by terrorists on 9/11. Christiansen personally did the structural engineering for Seattle’s Kingdome, imploded with explosive charges in 2000.

“It’s getting so I’ve seen a lot of my structures destroyed,” Christiansen said. “I’ve lived too long, I guess.”

The viaduct’s most distinctive feature is the shape of the concrete tetrapods, which extend up and out from narrow bases like frames of inverted pyramids.

The legs taper from about 5-feet square at their bases to 3 feet at the top and range to a maximum length of 67 feet. They lean 30 degrees off vertical, and when viewed from below give the impression of a forest after a heavy windstorm.

Geoff Swett, head of the state’s team of bridge engineers that designed the viaduct’s replacement, said as far as he knows Tacoma’s tetrapods are unique in highway bridge construction.

“I’m not aware of any other places where they were used,” he said. “Certainly not in Washington.”

The tetrapods are a classic example of modernism in civil engineering, nearly as emblematic of the 1960s as Bob’s Java Jive giant coffee pot restaurant, another Nalley Valley landmark, is of the 1920s.

Christiansen insists he wasn’t trying to make an architectural statement with the viaduct.

“The design was a result of the circumstances of the structure,” he said.

The fact that bridge needed to cross over two busy city streets, as well as railroad tracks, meant the distances between support columns needed to be relatively long, he said.

“It was an interesting problem to get across that valley,” he remembered. “It required that the piers be rather far apart, which required very long spans.”

The idea with the tetrapods, Christiansen said, was to reduce the length of the spans – and therefore the overall cost of the structure.

By canting the legs 30 degrees outward, Christiansen was able to stretch them farther from their bases and therefore cut the length of the spans to about 70 percent of what they otherwise would have been.

“It was a design element to reduce the size of the structure and the cost of the structure, which is what we do,” he said.

The original Nalley Valley viaduct cost $3.67 million in 1970 dollars.

Christiansen remembers that in 1969, when he unveiled his tetrapod plan to transportation department officials, it got a cool reception.

“They weren’t crazy about the design, but they accepted it,” he said.

Swett said that, regardless of what people thought of the unusual design, the old viaduct remained sound throughout its 40 years.

The only problem with it was that traffic volume rose beyond its capacity, he said. “It’s essentially impossible to widen it.”

About 40,000 vehicles used the viaduct per day when it opened in October 1971, according to transportation department counts.

Since then, the traffic count has risen to 132,000 vehicles a day.

TAKES TIME

The World Trade Center towers collapsed in under two hours. The Kingdome was reduced to dust and rubble in just 16.8 seconds.

The viaduct will take much longer to demolish.

Demolition crews began with a short section on Dec. 1. This week, they’ve been working on the section between South Tacoma Way and Center Street, first pecking off 60,500 square feet of pavement from above and now hammering away at the understructure.

The chief difficulty, according to Neal Uhlmeyer, the state’s project engineer, is that the demolition needs to be accomplished in two phases in order to accommodate eastbound traffic.

Crews with excavators equipped with giant hoe rams, claws and nippers currently are tearing down what was formerly Route 16’s westbound lanes, while the eastbound lanes, just a few feet away, remain in use.

Once the old westbound lanes are out of the way – around the end of January according to the construction schedule – crews will begin building a new, larger viaduct in its place.

When eastbound traffic is shifted onto the new viaduct, the last of the old viaduct and tetrapods will be torn down. That’s not scheduled to take place until about this time next year, according to Claudia Cornish, a transportation department spokeswoman.

When the demolition crews move to the sections spanning South Tacoma Way and Center Street, they’ll block off traffic and lay down foot-thick blankets of sand on the roads for cushioning, then top them off with steel plates to protect the pavement.

That precaution came from recent lessons learned during the demolition of Seattle’s Alaskan Way Viaduct, Swett said, where pavement was damaged by falling concrete.

The demolition of the viaduct, unique though it may be, has not caused any perceptible protest from Tacoma residents.

Which is not to say the tetrapods are without admirers.

“I think they are beautiful, fantastic and marvelous,” said Amy McBride, arts administrator for the City of Tacoma. “I’ve always loved them. They’re sculptural. I don’t think I’ve ever seen anything like them. I’ll be sad to see them go.

“The new one (viaduct) is just kind of a straight, standard deal. Pretty unexceptional.”

The design of the new eastbound viaduct will echo the westbound portion, which opened earlier this year. It will be a precast, prestressed concrete-girder style bridge, supported by square vertical columns, a style Swett called the Transportation Department’s “bread and butter bridge.”

“They’re the most efficient to build and pretty economical,” he said.

Girders on the new bridge will span as much as 195 feet and will be slightly over 8 feet deep. That’s 3 feet thicker than Christiansen’s girders, which, thanks to the tetrapods, needed to span just 120 feet.

Christiansen has not seen the plans, but, generally speaking, he is not thrilled with the state’s design choices.

“I don’t think the Washington Department of Transportation ever wants to make an architectural statement,” he said. “In a design sense, they are extraordinarily conservative. They don’t do anything unusual. They do standard stuff all the time.”

Swett took issue with that.

“I’d debate that with him if we were face to face,” he said. “Maybe back when he was working with the department that may have been true. Today, I think we would be considered one of the more innovative departments in the country.”

But, Swett conceded, the new Nalley Valley viaduct will not be evidence of that.

Rob Carson: 253-597-8693 rob.carson@thenewstribune.com