A bridge with stuff to strut
Rob Carson; The News Tribune
Building the tower struts for the new Tacoma Narrows bridge would be a challenge, even if workers had the luxury of putting them together on the ground.
The six struts - cross braces that connect the tower legs - are precisely engineered concrete boxes big enough to house locomotives and as heavily reinforced as bomb shelters.
Their strength and scale is daunting. But what makes them especially difficult is that they are being built in midair, hundreds of feet above the Narrows.
The concrete and steel in each strut will weigh as much as 3.5 million pounds. Hanging the struts between the sheer vertical faces of the tower legs promises to be one of the more demanding aspects of building the bridge.
"It's a challenging little piece of work," engineer Dave Stegeman says, with characteristic understatement.
Stegeman is the superintendent at Tacoma Narrows Constructors and has been assigned the responsibility of building the struts. He's only 34, but already has earned the respect of his crew of 10 ironworkers and carpenters, all seasoned veterans with decades of experience, and, like top athletes, all handpicked for the strut team.
"When we've got a problem, he's got the answer," one crew member said recently. "He always comes up with something. He's amazing. The guy must go home and think about this in his sleep."
STARTING AT THE DRAWING BOARD
Deceptively simple, the struts are, in fact, the products of thousands of calculations, including wind tunnel tests and seismic simulations.
As drivers on the 1950 bridge across the Narrows have noticed - hundreds have called the bridge office to mention it - the tower legs are canted slightly inward. They lean like the sides of capital letter A's, which makes the struts' job pretty obvious.
"They tie the tower together and give the legs more stiffness," Stegeman said.
The struts turn the 510-foot tower legs into rigid frames. That way, forces conspiring to tip over the towers are distributed across the entire surface of the towers instead of individual legs.
In calculating the necessary strength of the struts and tower legs, bridge designers were most concerned with movement perpendicular to the bridge.
In the bridge project office in Gig Harbor, state Transportation Department engineer Tim Moore uses a wooden ruler to demonstrate why. Moore is the state bridge designer who checks all design aspects of the new bridge.
He stands the ruler on end, and, with the palm of his hand, presses down on its top. The ruler bends, as one would expect, along its flat face.
"It's never going to try to buckle the other way," Moore said.
The tower legs have a similar shape and react similarly, too. The sides of the legs facing the roadway are flat faces, beginning at 29 feet wide at the bases and gradually tapering to 19 feet at their tops. The narrower sides - the edges of the ruler - are 14 feet wide all the way up.
When the 50 million pounds of cable and bridge deck are suspended from the tower tops, the weight will exert tremendous downward pressure on the tower legs. The struts keep the legs from buckling.
NO MORE 'GALLOPING'
The towers will have to stand up to forces other than the dead weight of the bridge, Moore noted.
Wind also was a design consideration. It can blow through the Narrows at 100 mph and felled the first bridge there, "Galloping Gertie."
At this point, the tower legs are about only three-quarters of the way to their eventual height. But tower workers already can feel them swaying, even though there have been no winds to speak of this winter.
Designers knew wind would be a problem during construction. To counteract the movement, they ordered temporary struts - odd-looking assemblages of steel pipes and plywood - installed at the 270-foot level to keep the movement down.
Engineers say the completed structure should be able to withstand sustained winds of 109 mph and gusts of 127 mph, both of which theoretically occur just once every 10,000 years.
The wind will constantly test the new bridge, but it will be a minor threat compared to another natural force.
"Earthquake is the controlling load criteria on the entire tower design," Moore said.
The Narrows is within striking distance of two major faults, the Tacoma Fault and the Seattle Fault, both of which are capable of sending powerful waves of motion through the bridge.
In addition, the subduction zone off the Washington coast, where the Juan de Fuca plate dives under the continental shelf, also poses a threat.
The motion from any earthquakes will begin at the bottom of the tower foundations, 200 feet below the water surface. It will be transferred upward through the towers.
"The towers are just the tail of the dog," Moore said.
In 1949, when construction crews were building the current Narrows bridge, they had just finished the towers when an earthquake measuring 7.1 on the Richter scale hit April 13.
The Tacoma tower swung so violently that it shook off a 21-ton saddle, the steel cradle that holds the main suspension cable. The saddle fell 500 feet, crashed through a work barge and sank to the bottom of the Narrows.
This time, bridge designers used a 9.1-magnitude earthquake as a theoretical maximum for engineering purposes. That's the size of a quake thought likely to occur once in 2,500 years.
The real maximum force is unknowable, Moore said. But at some point, engineers must choose a reasonable limit.
"If the bridge were designed for no risk, we couldn't build the towers big enough or put enough steel in them," he said.
Moore, by way of comparing the forces on the struts and towers likely to be exerted by winds and earthquakes, noted that calculations indicate the tops of the new bridge towers will move 5 or 6 inches in strong winds.
In the strongest earthquakes, he said, the tops could move as much as 4 feet.
In an earthquake that big, some repairable cracking and flaking of concrete is expected in the tower legs. The legs can't be designed to be too rigid, or they would snap, a point driven home in the Northridge earthquake in California in 1994. That's when brittle freeway columns snapped under the force.
But because the struts are critical to the structural integrity of the tower, any damage to them, even in a 2,500-year earthquake, is unacceptable. They will need to ride out even the strongest quakes undamaged.
HIGH-WIRE CONSTRUCTION
Calculating the required strength for the struts was one thing. Putting the beams into place is something else entirely.
The problem of how to hang the big concrete and steel boxes on the sheer tower walls, hundreds of feet in the air, had Tacoma Narrows Constructors designers and engineers still considering their options last fall - well after all the rest of the design work on the bridge was finished.
At TNC construction headquarters in Gig Harbor, engineers batted around ideas about how best to construct the struts.
The criteria were practicality, cost, safety and speed.
"We brainstormed for quite a while as to how to support the load and still build them as quickly as we could," Stegeman said.
The lowest struts, while the biggest, turned out to be the least difficult from a practical point of view. Because they are perched only 156.5 feet from the base of the towers, their weight could be supported from below.
On both towers, the strut crew erected a temporary steel framework between the tower legs. The base of the framework rests on the tops of the caissons, the concrete structures as big as a 20-story apartment buildings, that extend beneath the towers and are planted in the floor of the Narrows.
Construction on the bottom struts began in October, and crews are removing concrete forms this week.
The middle and top struts will be more difficult. The middle strut will hang 351 feet above the tower base, the top, 495.5 feet. In both cases, there was no practical way to extend supports up that far from the bottom.
"The scheme we came up with was pretty innovative," Stegeman said. It is a two-part system that begins with bolting support brackets to the faces of the legs.
The brackets will support the weight of the strut floors. Then workers will use a technique called "post-tensioning" to increase the strength of the concrete in the floors enough to support the weight of the rest of the structures.
Beginning this week, workers will hang in a swing suspended from above and, like rock climbers setting anchoring spikes on the face of a cliff, attach four brackets on each tower wall.
The brackets will support the base of a massive steel framework that will stretch across the 75 feet of empty space between tower legs.
JOB OF A LIFETIME
The plan sounds straightforward enough until you consider the scale. Each bracket is as big as a washing machine. The bolts that run through them are the size of flagpoles.
Last week, at the TNC field office at the west end of the bridge, Stegeman carried one of the hexagonal nuts that will be threaded onto the ends of the bolts into a conference room and set it on a table. It was so big it was difficult to pick up with one hand.
Each bracket will have a screw jack on top so a steel girder can be placed atop and precisely leveled along the tower faces.
Using the tower crane, workers will then bolt on steel beams that span the distance between the towers.
Again, the scale is mind-bending. Each cross beam is nearly 4 feet high and weighs 431 pounds per linear foot. Each of the 10 beams will weigh 32,000 pounds.
"In the world of construction, that's a pretty big beam," Stegeman said.
In fact, they are the biggest beams available in North America. Anything larger would have to be specially ordered from steel mills overseas.
Big as they are, the temporary steel support structures will be inadequate to support the full weight of the completed struts. They will be able to support only the weight of the 4-foot-thick strut floor.
To support the rest of the weight, workers will beef up the floor by tensioning it with millions of pounds of pressure.
"The idea was to get the floor to work a little harder for us," Stegeman said.
To stress the concrete, workers will stretch steel tendons through tubes in the concrete floor and through the tower legs on each side. They'll anchor one end and, using a hydraulic ram, pull on each tendon like a rubber band until it registers more than 1 million pounds of force.
It's a potentially dangerous process, because if the tendons break, they can shoot out the ends of the tubes like spears. TNC hired a California subcontractor to do the post-tensioning and keeps other workers well out of the way while he works.
When stressed, the floor will form a reliable base for the strut walls and tops, which will be poured by hauling concrete up, bucket by bucket, with the tower crane.
The uppermost strut, which connects the tops of the towers, is the smallest of the three.
In some respects that will make it easier to build. But, Stegeman notes, working at that height - about only 100 feet lower than the top of the Space Needle - makes every move more psychologically challenging.
And there will be other complications up high as well. The top struts, in addition to their own jobs, will have to support all the heavy equipment of the crews that spin the 19,115 miles of cable across the tower tops.
To attach the spinning equipment, hundreds of predesigned couplings and embedded fasteners will be built into the top struts, most with tolerances measured in fractions of inches.
Far from being daunted by the task ahead of them, Stegeman and the crew are thrilled by it.
"You like to think that this won't be the pinnacle of your career," Stegeman said, "that some other time in your life you will get on a job that's more exciting. But in this case of this bridge, you really have to wonder.
"I'm looking at this as what will probably be a once-in-a-lifetime opportunity," he said, "and I know most of the other guys out here are looking at it the same way."
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Rob Carson: 253-597-8693
rob.carson@thenewstribune.com
