Pillars of strength
Rob Carson; The News Tribune
The new Tacoma Narrows bridge is 40 percent finished, but most of the work has been underwater and out of sight.
That will change dramatically in the next few weeks, as the bridge's two towers - each nearly as tall as the Space Needle - begin to spring skyward from their foundations.
The sight of the massive towers rising next to the existing bridge promises to be such a spectacle that the state Department of Transportation has issued a warning to drivers: Keep your eyes on the road to avoid collisions.
When the 510-foot towers are finished in April, they'll be the tallest such structures built in the United States in 30 years.
Their height and strength - necessary not only to bear the weight of the milelong bridge but also to withstand catastrophic earthquakes - make them engineering and design marvels.
Sophisticated new materials and advanced computerized modeling will place them among the most sophisticated such structures ever built, according to engineers.
"Some of the best experts in the world have worked on this," said Tim Moore, the Transportation Department's bridge expert. "They've not necessarily cut new ground in construction generally, but with regard to suspension bridge construction, they certainly have."
Moore has worked on the Narrows bridge project since the earliest phases of design in 1996. He is responsible for checking all the work of Tacoma Narrows Constructors design partner Parsons Corp., which used about 60 designers in Manhattan, San Francisco and Bellevue to draw up plans for the suspension bridge, including the towers.
Designing a suspension bridge tower begins with calculating the forces to be placed on it, Moore explained. In the Narrows, those forces present a range of challenges for the towers to stay upright and keep from crumbling, twisting or breaking away from their bases.
The towers must resist "axial loading," which means the weight of the bridge itself, and "lateral loading," which means the infinite combinations of forces that tend to move the towers from side to side - including wind, temperature changes and the movement of cars and trucks across the roadway.
The towers also must withstand the complex forces likely to be placed on them by earthquakes, which produce an unpredictable combination of rolling, heaving and lurching.
"It's a whole lot of number crunching," Moore said. Computer programs model the maximum stresses likely to be placed on every inch of the
towers, and designers use the results to set parameters for construction.
A critical starting point in designing a suspension bridge is the towers' height, which determines how much stress will be placed on the cables and the anchors at each end. The higher the towers, the less stress on the cable system.
In his Gig Harbor office last week, Moore demonstrated the point by extending his right hand, fingers pinching an imaginary rope.
"Pretend I'm holding one end of a rope," he said. "You've got the other end, and we've got a rock hanging from the middle. The less the rope sags between us, the harder we have to pull. That's just one of the properties of a cable."
For the new bridge, external factors also figured into the height equation.
"The closeness of the Narrows airport made it pretty much mandatory that the towers not be any higher than the current ones," Moore said.
Because the existing bridge is listed on the National Register of Historic Places, designers had to pay attention to historic preservation guidelines. They specify that new structures built alongside historic ones be of similar size and scale.
Designers decided on a tower height of 510 feet above sea level, virtually the same as the existing bridge.
RESISTANCE TO FORCE
The shape designers came up with for the new towers - a rectangular frame - was chosen in part because of its ability to withstand lateral forces. The pressure of the 21-inch main cables that will be placed at the top of each tower will increase its resistance to movement.
Lateral forces on the towers transfer most of their energy to the tower bases, so the connections there have to be especially strong to keep them from toppling like tall trees in the wind.
To accomplish that, the towers are fastened to the caissons with heavy steel reinforcing bars, each about as big around as the grip of a tennis racket, embedded 12.5 feet into the concrete "distribution cap" on top of the caissons.
The bars, 208 of them spaced 10 inches apart around the perimeters of the tower legs, end in what resemble the heads of bolts to keep them from pulling out.
As the towers are extended upward, ironworkers will thread extensions onto the ends of the vertical bars so that each effectively becomes a single continuous piece of steel running from bottom to top.
If you imagine the tower trying to tip over, Moore said, the side on which the force is acting tends to want to stretch, while the opposite side tries to compress. The steel/concrete combination being used in the new towers is an ideal partnership for resisting these forces, he said.
The steel is good at resisting stretching, unlike concrete. But concrete is remarkably effective at resisting compression.
EFFICIENCY IS KEY
Viewed from their fronts, the tower legs widen, like someone standing with feet slightly spread. That stance is stable, Moore explains, and it means the roadway can be hung straight down from the main cables and away from the tower legs. That's safer than the current bridge, where the legs are inches from traffic.
In addition to leaning toward each other, the tower legs taper as they rise, going from 29 feet at their base to 19 feet on top.
That's efficient in two ways, Moore said. It reduces the material needed to build each leg, and, because the legs need to be stiffer at their bottoms, putting more material at their tops would be counterproductive. Greater mass at the top would put more stress on the bottom.
The tower legs are connected by three horizontal struts, which transform them into rigid frames, like walls in a house constructed of two-by-four studs.
The steel inside the horizontal struts is tied to the steel skeleton of the legs, and it is further reinforced by what's known as post-tensioning.
Once the struts are in place, 44 steel tendons, each made of 31 strands of 0.6-inch steel cable, will be threaded through the struts and tower legs. Then they'll be pulled taut with 1.4 million pounds of force, locking legs and struts together.
PERFECTION ISN'T EVERYTHING
Technology unavailable to previous bridge builders will help build the towers.
For example, a sophisticated GPS locating system will allow builders to keep the towers in position as they rise.
The GPS measurements are so precise that sunlight on one side of the tower or another will throw off the results, Moore said. For that reason, they'll take readings before the sun comes up and after it sets.
When the towers are finished, Moore said, their tops will be within 1 inch of perfection.
Interestingly, perfection won't mean straight up and down. Computer programs calculated that, once the main cables are attached and the weight of the bridge added, the tops of the towers will bend slightly toward each other.
For that reason, the east tower will be pulled back 2.4 feet toward Tacoma and the west tower will be pulled 2.3 feet toward Gig Harbor.
That way they'll be straight when the weight of the bridge is added.
The computer indicated another correction: The weight of the 8,000 cubic yards of concrete in each tower - combined with the weight of the cables and the suspended roadway - will cause the towers to slump slightly.
To compensate, they'll be built 2 inches taller.
"The towers will shorten approximately 2 inches due to the superstructure dead load and its own weight," Moore said.
"They are therefore constructed slightly taller to compensate."
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Rob Carson: 253-597-8693
rob.carson@thenewstribune.com
