There are times when the two dimensions of a piece of paper—whether physical or electronic—are not enough to convey what something looks like or how it can be taken apart into manageable pieces. The Sundial Bridge in Redding, Calif., is such a case, and it provided plenty of 3-D challenges for the engineers, fabricators and contractors who worked on constructing it.
The biggest challenge—and the most striking and artistic structure—of the bridge was the pylon. The single pylon is a sort of shark’s fin-like steel blade that pierces 217 ft into the sky at a 41° angle pointing north.
The bridge was conceived by renowned Spanish architect and engineer Santiago Calatrava as a giant sundial. During winter, the pylon/sundial casts its shadow too far away into the McConnell Arboretum to mark the time. Even at the summer solstice, the pylon only functions as a sundial from 11 a.m. until 3 p.m., but its artistic function is accomplished year-round.
They were after a signature bridge, a very elegant gateway to both sides of their exploration park,” Bob Elliott, project sponsor for Kiewit Pacific Co., a Vancouver, Wash., subsidiary of Kiewit Corp., told Roads & Bridges.
Sundial Bridge connects the two sides of the Turtle Bay Exploration Park, which owns the bridge, across the Sacramento River.
Sundial Bridge won the Eugene C. Figg Jr. Medal granted by the Engineers’ Society of Western Pennsylvania. The Figg Medal is given for a single, recent, outstanding achievement in bridge engineering that, through vision and innovation, provides an icon to the community for which it was designed. The bridge is the first steel inclined-pylon, cable-stayed bridge in the U.S.
Kiewit acted as the construction manager, representing Turtle Bay’s interests, and as general contractor in constructing the bridge. Kiewit was awarded and primarily self-performed both of the two contracts for constructing the bridge: one for the pylon and the tubular, triangular truss that stretches across the river and one for the glass-and-granite deck that is embedded in the truss.
“We were attracted to the job because it was a very complex but very functional sculpture. It had kinda the best of both worlds,” said Elliott. “On top of all that, the erection method as conceived by Calatrava was really complex.” Calatrava originally envisioned that the bridge would be almost completely constructed parallel to the river and then swung on an axis at the pylon out over the river and fixed in its permanent position.
“I thought, Hey, that’s challenging,” said Elliott. “There’s an opportunity for us to go in and help the owner and the designer obtain their thoughts and concepts and bring it to reality.”
The fin is key
In the end, the bridge was erected in a more conventional way. First the pylon was assembled. Then a truss section was cantilevered onto the pylon over the river, and a stay cable was attached. Then another truss section was lifted into place with a crane and attached, and another stay cable was strung, and so on.
Both Calatrava’s and Kiewit’s erection method avoided placing any bridge structure or construction equipment in the river for environmental reasons. The river is a salmon-spawning area, so it was important not to disturb the aquatic environment.
The key to the entire job was the pylon, and the pylon was an incredibly complicated geometric shape.
Tensor Engineering, Indian Harbour Beach, Fla., was subcontracted to detail the steel pieces of the pylon. One of the first things Walter Gatti, president of Tensor, did when he got the architectural drawings for the bridge—after several days of staring at the sketches and struggling to comprehend—was to build a model out of cardboard. Having a three-dimensional representation gave him a better grasp of the complex geometry of the elegantly curved pylon.
Tensor also translated the cardboard figure into a computer model using software developed by Tensor.
“We have a very sophisticated computer program that we developed back in the late ’60s, early ’70s to do three-dimensional objects,” Gatti told Roads & Bridges. “We do nothing but bridges. And we do a lot of complex bridges . . . so we developed a pretty sophisticated geometric program that can calculate just about anything in space. We used that program, along with several new routines.” The result of Tensor’s work was a set of detailed drawings of each piece of the structure so the fabricator could take steel plates and bend them into the shapes needed. There were about 1,200 different pieces in the pylon alone, and each one was an irregular polygon shape that had to be cut at different angles in order for it to weld to another plate.
One of the puzzles the detailer, fabricator, engineer and constructor collaborated on solving was how to get the whole structure welded together.
“The pylon was a double-wall structure, and the internal wall was not in the same plane as the outside wall,” said Gatti. “It was another wall that was converging from the bottom up to the top.”
The real problem developed at the top, where the inside and outside walls of the pylon were close together.
“The welding was so complex and the angles were so extremely sharp that physically you couldn’t get in there and weld some of this material,” said Gatti. “A lot of it had to be welded from one side only with backing bars, and those backing bars required a lot of (calculating) just to figure out how to get the backing bar shape.” Once Tensor had worked out the details of the pieces, consulting with Kiewit, erection engineer Buckland & Taylor and the fabricator on what was well-engineered and constructible, they produced fabrication drawings and delivered the 3-D geometric model to Buckland & Taylor.
Buckland & Taylor used the 3-D coordinates of all of the key points on the pylon and deck to calculate the forces and deflections of the structure.
“Our primary task was to make sure we didn’t overstress the pylon or do anything to it during erection of the bridge,” Don Bergman, vice president of Buckland & Taylor, North Vancouver, B.C., told Roads & Bridges. “So essentially we have to model the partially erected bridge through all the stages of erection and check that complex form to make sure that we’re not overstressing it or doing anything to it during the erection process. And that’s a very challenging process as well.”
Buckland & Taylor ended up creating its own very complex 3-D, finite-element model of the pylon to check the stresses on the plates in the complex structure. The model was complicated by the fact that the single plane of stay cables is attached off center to the center of gravity of the deck. The arrangement produces torsional displacements as well as longitudinal and transverse displacements in the deck and pylon.
One issue they found through the analysis was the lack of stiffening in the pylon. The plates in the pylon needed stiffeners to hold their shape. Working out all the complexities of the Sundial Bridge added to the cost to construct it.
Up and away
In fact, the cost of the bridge ballooned from an estimate in the neighborhood of $13 million to a finished cost of $23 million. The majority of the funding was provided by the McConnell Foundation, a private, independent foundation in Redding. The remainder of the funding came from the Redding Redevelopment Agency, the Federal Highway Administration and Turtle Bay Exploration Park. Value engineering by Buckland & Taylor actually saved the owner money. The engineering firm redesigned the bridge’s foundations, redesigned the pile caps to better suit Kiewit’s construction methods and re-engineered some of the retaining walls and other structures in the plaza area under the pylon. Buckland & Taylor’s foundations, which were built, used large-diameter drilled shafts.
“A good deal of the price increases came with increased scope,” said Elliott. Finishing the design to the point of being able to detail the steel pieces and model the stresses on the structure took many man-hours and much back-and-forth communication between all of the parties involved. The engineering analysis needed to step through the erection process added to the cost. Resulting changes to the design also added to the cost.
Have a slice of pylon
Transporting the Sundial Bridge from the fabrication yard in Vancouver, Wash., to the construction site in Redding was what Elliott called a “logistical nightmare.” The fabricator, Universal Structural Inc., preassembled nearly all of the pylon in Vancouver, Wash., to make sure the pieces would fit together before they were transported to the construction site.
The triangular, tubular truss was transported in sections by truck down I-5, the main highway on the west coast. Since the sections were about 25 ft wide, and I-5 is only two lanes wide for much of its length, the entire southbound interstate was blocked by the convoy of trucks hauling bridge sections plus auxiliary trucks. The whole convoy consisted of 12 trucks with arrow trucks, warning trucks, trucks going ahead to make sure the road was clear and other trucks.
The convoy could only move through Oregon in the early hours of the morning on weekends. When those hours had passed the trucks were required to pull off the road at an agreed location and wait for the next night.
“In the end, we took nearly all of the truss sections down I-5 and several of the pylon pieces,” said Elliott. “But in the end we found it just too cumbersome to deal with all of the logistical nightmares associated with that, and we basically took the major components of the pylon and loaded them on a barge and shipped them down to Sacramento.” From Sacramento they were trucked to the bridge site along a corridor where I-5 is wider and it was less difficult to deal with government requirements.
Transporting the largest possible sections reduced assembly work in the field and ultimately saved money.
The bridge sections were lifted into place with a 240-ton 410 Series II Manitowoc crawler crane in a ringer configuration to increase its reach and capacity: first the pylon, then the truss sections starting at the pylon. When the truss reached the middle of the river, Kiewit set up the crane on the other side of the river and continued connecting sections until they reached the river bank. The deck, made up of glass and granite panels, was installed as a separate contract, also performed by Kiewit Pacific, after the bridge structure was erected. The installation was completed with the help of overhead gantries designed for the job by Kiewit to carry the panels. Kiewit placed about 2,245 glass panels approximately 12 in. wide by 4.5 ft long and weighing 200 lb each.
To cap off the precision engineering on the Sundial Bridge, Buckland & Taylor used its finite-element model to calculate the center of gravity of each section of the pylon. Knowing the center of gravity, they could figure out how to lift the sections in the right orientation.
“Because of the form of the pylon, even that exercise is a bit of a challenge,” said Bergman, “to get the piece hanging right so that when you set it down it mates to the previous section and you can bolt it up temporarily and then make the welds.” After all the number crunching, Kiewit finished the Sundial Bridge last year without a single lost-time accident. Turtle Bay Exploration Park and the city of Redding received an elegant work of functional sculpture for visitors to admire.