The three-lane Lion's Gate Bridge spanning 1,517 m across the scenic Burrard Inlet in Vancouver, looks slender and elegant, even serene. But the experience of the all-star assembly of bridge engineering firms that gathered to replace its deck, seismically upgrade an approach and keep it sturdy for 70,000 daily commuters was anything but serene.
|(Photo courtesy of American Bridge)|
"We had some of the best engineering firms in the world working on this bridge, knowing it would be challenging," says Geoff Freer, project director for owner British Columbia Transportation Financing Authority. "But even with that, challenges came that nobody anticipated."
The bulk of work has been completed on the contract begun in April 1999. A joint venture of American Bridge, Pittsburgh, and Surespan, Ltd., Vancouver, won the job at a price of $58.5 million, to be completed at the end of 2000. A six-month delay resulted from a dispute over whether the contractor's original plan to use a unique movable truss ramp system would work.
|ROLLING Clinging to bridge hangers, a traveling jacking system lifted and lowered deck sections. (Photo courtesy of American Bridge)|
While engineers still disagree about that, they concur that the alternative method, a traveling jack system with continuity links, worked well in the end. Final paving will be completed by spring. And the final price tag will be about $63.4 million.
Built by the Guinness family of beer fame in 1938 with two lanes, the signature suspension bridge, supported by 109-m-tall towers, was designed with economy prioritized over durability, says Darryl Matson, project engineer for owner's bridge engineer Buckland & Taylor, Vancouver. Sold to the government in 1952, the bridge accommodated increasing traffic that eventually far exceeded its design capacity of 45,000 daily vehicles. Lane markings were altered to make three narrow 3 m wide lanes and two 1.2-m wide sidewalks. By the 1990s, the deck maintenance cost about $2 million a year.At one point the British Columbia government considered building a new design-build-operate-maintain crossing to be funded by tolls, but this did not meet with public favor, says Mike Proudfoot, director of design and construction for the owner. The decision to rehabilitate the existing landmark structure went forward, but the government at the time "basically restricted the budget and put very little in the project for contingency," says Freer.
|TIGHT QUARTERS Every morning, equipment had to be covered up to allow traffic to pass. (Photo courtesy of American Bridge)|
TOUGH START. The final decision was to replace the concrete-filled steel grid deck with an orthotropic one, widening it to accommodate three 3.6-m-wide lanes and two 2.7-m-wide sidewalks while lightening the total weight in the 473-m-long main span by 4.5%. However, the two 187-m-long side spans now are 3.1% heavier. An existing sag of 8% in the center of the profile also has been straightened, a causeway road widened and the old stiffening trusses on the bottom chord replaced by two longitudinal stiffening trusses below the deck.
Buckland & Taylor designed a replacement plan for the deck and seismic upgrades for the north approach in 1998, and American Bridge/Surespan won the contract in 1999. AB has since bought Surespan's 10% interest. Crews had 10-hour nighttime windows to work, but were expected to have the bridge fully open to traffic by 6 a.m. "The 16 months of work were really 13 months," says Mike Cegelis, AB spokesman.
|Much work was done on hangers overhead. (Photo courtesy of American Bridge)|
Restricted scheduling was just the beginning. "It's hard for someone who was not involved to realize the complexity and challenges AB was faced with," says Michael Abrahams, project engineer with Parsons Brinckerhoff, New York City, which performed independent checking. "This was an old bridge, which really didn't have a lot of capacity." Wind loads limited suspender cable capacity and live traffic during total deck replacement made the bridge so sensitive that "we had to take into account every piece of equipment during lifting...even pieces of tools and number of people standing on the jacking traveler," he says. "For instance, you could have four men on the travelernot five."
Movements also can cause deformations in the bridge profile. John Clenance, project engineer for erection engineer Steinman, New York City, now owned by Parsons Transportation Group, uses the rubber-band analogy to describe the potential for "jumping" back as sections are cut out while the bridge is under tension. Also, due to the bridge's original economics, "some of the elements have no reserve capacitywe couldn't count on the structure to take extra loads in many of its members," he adds.
Every change in the bridge geometry was calculated and matched against the reality in the field. "We ran a computer model through 60 years of history,"says Clemmons. Elements such as suspender lengths sometimes turned out differently when actually surveyed. "The cable at the north bent had slipped 66 mm...platforms had been removed, utilities added, new overlays" and a multitude of other factors changed conditions.
Parsons developed software just for the bridge. "We had to generate a complete set of calculations for each erection stage, which had 15 steps each," says Clenance. "We generated up to 600 megabytes of numbers for each stage....It almost sunk our file server." Loads varied, too. "The truss became a live load when being lowered or raised. Then, it's a dead load when in place." Wind factors and the hydraulics of the traveling jack system also had to be factored in.
PLAN A. AB/S originally had intended to use a movable truss ramp especially devised for the Lion's Gate in its bid. "This would have allowed us to work during the day with the equipment serving as a roadway bridge and as an erection gantry," says Cegelis. Using the movable truss ramp (MTR) to carry traffic as well as serving as a lifting and lowering gantry for truss sections would have minimized closures to less than 50 nights, he says.
AB/S, Parsons and Parsons Brinckerhoff collaborated on the design of the MTR, but conflict arose four months into the contract over interpretation of the contract documents regarding loading requirements. "We interpreted [load limits allowed on the bridge during construction] as being for a 20-ton truck," says Ron Crockett, AB/S contractor representative. "The client had a different opinion60-ton trucks." With the higher allowable loads, the MTR also would have to be heavier to carry those loads.
"It was a praiseworthy idea," says Peter Buckland, principal in charge for Buckland & Taylor. "But the bridge couldn't sustain the weight of the MTR and the traffic as well."
After wrangling with possible modifications for two months, "we realized if we were to continue down the MTR path, it would delay erection by one year," says Crockett. So the contractor decided to use a jacking travelera variation on Buckland & Taylor's gantry plan. "Our plan had been to lift big [20-m segments] up a ramp from the park," says Buckland. "The contractor built 10 m pieces instead, and lifted them sideways up." Buckland says the gantry had advantages in lateral lifts of bigger segments, but concedes, "The jacking traveler was light...and they didn't have to build a ramp in the park." Click here to view diagram
AB and Parsons maintain the MTR would have worked, particularly since the owner eventually backed off the 60-ton truck limit. Also, some work that was required with the MTR was done anyway with the jacking traveler, such as adjusting suspender tensions in advance of the traveler, notes Peter Whitlock, Parsons project manager.
Proudfoot maintains that "AB put a lot of effort to make the MTR work, but I think everyone recognized everything was exhausted in trying to make that scheme perform. Going back to the original strategy was right decision." AB's Cegelis concedes that "the owner could have penalized us...but they recognized a honest misinterpretation so they elected not to pursue damages. When we did change directions we moved very quickly." There were no major injuries on the job, on which the contractor was responsible for quality assurance and control.
|IN MOTION strand jacks help support deck segments carried by traveler, which has retractable legs for rolling forward. (Photo courtesy of American Bridge)|
Crockett experienced plenty of "hair-pulling" moments from the beginning. "There were timesfor example, the very first weekendwhen we got into all kinds of problems," he recalls. Removing pins from the old bridge's 166 hangers was one; the rusty pins had seized up and wouldn't come out even after being heated. The road wasn't reopened to traffic until the following afternoon. "It was never that bad again," Crockett says. Crews built frames and used a 100-ton jack to push the pins out.
Canron Construction Corp. West, Vancouver, fabricated the new bridge sections upside down and welded orthotropic troughs to the 16.8-m-wide deck plates. A total of 54 sections were fabricated and barged to the site. Work began first on the bridge's north side spans and progressing to the main spans, with old sections lowered onto either bogeys or barges.
The jacking traveler, a steel frame weighing about 55 tonnes, is suspended from hangers over the deck section to be removed and rolls forward to the next section when each deck section has been replaced. Its strand jacks lift the deck pieces up and down and support the weight of the deck section while the existing hangers are disconnected.
At one point, it temporarily filled in for a suspender that could not be used at the end of a shift because the suspender socket failed, says Clenance. "One suspender couldn't be used the normal way....We computed adjustments so that particular suspender would not be overstressed by the jacking traveler. We were able to give AB the information so they could open the bridge with the jacking traveler holding the suspender to the truss."
Once the new section is hoisted into place and aligned, temporary hanger extensions are installed and the strand jacks released. A traffic plate bridges the gap between old and new deck sections so that traffic can pass. The traveler stays on the hangers during the day, then rolls forward for the next night shift.
For wind factors and for the need to disconnect and reconnect bridge components quickly, the hydraulic continuity link, which holds the old and new bridge parts together during the daytime, was vital. The continuity link on the main span had a hydraulic system that "permitted it to have differing stiffnesses depending on...wind forces," says Clenance. "It prevented higher winds from overloading the truss." Like a giant deadbolt, the link's steel members connect to receiving ends on the new deck.
On the bridge's south side, where the terrain is rugged, the crews lifted the sections in half-lengths of 10 m, turned them sideways and lowered them into place from above. The jacking traveler on the south spans sported a turntable device that allowed the rotation of the sections.
SHAKEUP. The north approach viaduct also received what engineers claim is the world's first design-build seismic retrofit. Klohn-Crippen Consultants Ltd., Vancouver, decided on a plan to allow 24 steel bents to rock on their concrete pedestals. "We'd been joking about taking the nuts off the anchor bolts so the foundations could just lift off at the base plates," says Bruce Hamersley, Klohn-Crippen project manager. But after research was done, it became reality. "Fifty bucks to take the nuts off, $2 million to prove it worked, and we're done," he says facetiously. The approximately $3-million retrofit of the steel structure is designed for a 1-in-475-year likelihood seismic event.
The columns are rigidly connected at the bases and founded on individual spread footings. Piles were driven up to 120 ft deep at seven piers and connected to the footings with concrete pile caps, where liquefaction was a factor. Lead-core rubber bearings were installed at the north abutment and bracing added to H-bents, where pairs of frames are linked together. Individual footing pedestals were tied together to prevent differential movements. At bents where more uplift in a quake is expected, steel plates were put in at the bases to absorb energy, and internal flanges were added to columns.
|SEISMIC Bracing at bases help control movements. Photo courtesy of American Bridge)|
Hamersley compares the rocking concept to a wineglass. "If you hold both sides of the glass and then try to move the top, you will break the stem. But if you move the toe, the heel of the glass goes up and the stem doesn't break," he says. The "wineglass" foundations of the viaduct will move up in a quake, but not buckle. "Once you've got enough load in the braced frame to initiate the structure to start overturning, the quake can't put any more loads on itit just pushes it over more," he says. "We isolated the braces and limited the amount of load to protect them."
There were plenty of skeptics, but Lion's Gate engineers were convinced. "It's amazing how well it worked out," says PB's Abrahams.
HAPPY END. There were some 300 claims, says Freer, but at least 80% were resolved as the project continued, and the rest through mediation. "We had a referee in place continuously [and] it worked out reasonably well," he says. "It was tough communication, but people kept communicating." AB/S received a settlement of about $4.7 million.
The final cost is about $5 million more than the original budget. Normally, "I'd be expecting contingencies of 15% and 25% for a project like this," says Freer. "If one looks at the final numbers including claims settlements, we came in around 16%."
All's well that ends well, say all involved. "I don't mind the debates, as frustrating as they can be," concludes Freer. "They were held in a warm room, not out on a cold night on the bridge."