Friday, November 4, 2011 - 15:23

BRIDGE CONSTRUCTION: Must-slide rule

Designers faced with tough task of moving existing span

The Old Capilano River Bridge was a two-span steel through truss structure, originally built in 1929.
Carrying all westbound traffic between North Vancouver and West Vancouver on the busy Marine Drive corridor, the bridge also was a key link to the Lions Gate Bridge, connecting the city of Vancouver with the Sea to Sky Highway. With daily traffic in excess of 25,000 vehicles per day, the two narrow lanes of the old bridge were a frequent bottleneck to traffic, including the many transit buses crossing daily.
With rehabilitation of the functionally obsolete, aging steel bridge not practical, and recognizing that a new three-lane bridge was needed, the bridge owners, the BC Ministry of Transportation and Infrastructure (BCMoT), had long-term plans to replace the bridge. The potential to advance the timetable for this occurred in early 2009, when the Canadian federal government offered matching stimulus funds for the project. The catch: The project had to be substantially complete by March 31, 2011, in order to get the money.


Lateral move
The deadline imposed by the stimulus funding left less than two years to hire consultants, design, tender and build the new bridge, including provision of a detour, demolition of the old bridge and roadway improvements. Within this short period there were two other significant schedule constraints. One was the river itself—a major salmon stream, with a hatchery upstream of the bridge, it was subject to environmental protection requirements that limited construction work in the river to a two-month period from mid-July to mid-September. This meant there would be just two “fish windows”: one just three months after the start of planning and one a year later. The other schedule impact came from the 2010 Winter Olympic and Paralympic Games, held jointly in Vancouver and Whistler, two communities connected by the Sea to Sky Highway, including the Capilano Bridge. Word came from high in the provincial government that there would be absolutely no construction activity on any Olympic transportation corridors for the months of February and March 2010.
With the busy road being one of only two links over the Capilano River, no interruption of the traffic was acceptable, and there was no possible new alignment for the bridge. A construction detour was therefore a necessity, and planning for a detour bridge began immediately. The modular bridge concept developed would require three temporary piers in the river, require purchase of the temporary bridge and would trap the old bridge between the eastbound and detour bridges, complicating its demolition and removal. As these constraints were realized, the BCMoT decided to investigate laterally sliding the existing bridge to become the construction detour. This would leave the original alignment open for construction, with only the old substructures left to be removed.
Two months into planning for the project, the BCMoT engaged Buckland & Taylor Ltd. (B&T) to prepare an engineering assessment for sliding the old bridge. The two organizations have a long history of collaboration on complex projects, including a recent project that replaced the entire suspended spans of the nearby Lions Gate Bridge while maintaining daily traffic. B&T had two weeks to review the bridge, determine the effects of the move on the aging structure, identify potential risks and acceptable mitigation strategies for each and develop the preliminary concepts for the temporary structures and equipment that would be needed. The results of this work confirmed the feasibility of the sliding concept in June 2009, whereupon the BCMoT quickly decided to move ahead with it.


Fast acting
With the limited time available, it was imperative that the BCMoT “hit the ground running” on the project as soon as funding was confirmed. To do this, they took advantage of two existing general engineering services contracts with road- and bridge-design consultants. This allowed the immediate commissioning of the sliding study, as well as the design of the detour roadway and the temporary structures needed for sliding the bridge. The construction of these temporary works, which had to begin immediately to meet the 2009 fish window, was managed directly by the BCMoT, purchasing materials directly and using previously approved subcontractors. The breathing space provided by these initial procurements gave the BCMoT time to follow its normal consultant-selection procedures and open tendering process for the main bridge design and construction contracts.
B&T was next retained to provide detailed design for the temporary river pier for the detour, which used two lines of steel pipe piles projecting from the riverbed to the bridge elevation. The piles were topped by a heavily reinforced concrete pier cap, tied into the old bridge pier, which would also be the sliding runway for the four pier bearings. Documents were prepared and tenders let in under a month in order to get the contractor into the river during the fish window. Once the temporary pier was constructed by Surespan Construction Ltd., it was left until the bridge was slid in June 2010. Meanwhile, the detour was designed by Urban Systems Ltd. and McElhanney Consulting Services. The approaches and abutments were built prior to the Olympic break, using geogrid-reinforced granular fills and modular concrete blocks and then also left until the bridge sliding.


Adding some width
The BCMoT selected B&T as the designer for the new bridge, including final design of the sliding operation and design of the demolition of the old bridge. B&T was supported by EBA Engineering Consultants Ltd. for geotechnical and roadway design and by PBA Engineering Ltd. for electrical design. Just over three months was provided to complete the work, about half the time normally expected. This intensive effort was successful due to close collaboration and effective communication between the owner, the multiple consultants and environmental and other agencies.
The design of the new bridge replaces the 450-ft total length of the two old steel-truss spans with a 380-ft two-span steel plate girder bridge with a composite concrete deck. The new width of 57 ft provides three traffic lanes, shoulders and a wide shared pedestrian/cyclist path, replacing the original two narrow lanes, without shoulders, and a narrow sidewalk. The two-span design needed just one pier in the river, which is aligned with a pier of the existing eastbound bridge downstream. Abutments are integral with the superstructure and there are no deck joints in the new bridge. The five weathering steel girders were designed with a level soffit, but the top flange follows a symmetric vertical curve provided for roadway drainage, which also provides increased depth to the girder over the pier.
Foundations for the bridge utilize steel pipe piles, 60-in. diameter at the pier and 30-in. diameter at abutments. Piles are drilled through boulder layers and founded in granular materials at depths of 50 to 60 ft, cleaned out, and filled with reinforced concrete. At the pier, the steel piles terminate at the riverbed, and the reinforced concrete core of each pile is continued to pier cap level as columns, with a curtain wall infill between columns. The bridge is designed for the potential high seismicity in the area, and is classified as an emergency route.
The bridge deck is a departure from the usual BCMoT practice in the region, which normally tops concrete decks with a waterproofing membrane and a 4-in. asphalt overlay. The restrictive construction schedule at the Capilano River Bridge would have put the membrane and paving activity in the worst season for this work, January to March, and so a design compromise was reached that utilizes the concrete deck as the running surface, but employs all stainless steel reinforcing in the deck to provide durability.


Making the move
The design of the actual sliding scheme was included in B&T’s mandate, rather than leaving it solely up to the contractor, for several reasons, including: the need to pre-construct temporary elements, thereby locking in certain parameters; the very limited time available between contract award and the date the bridge had to be slid; and the crucial nature of the bridge to the BCMoT. The sliding design incorporated into the tender documents included the overall configuration and geometry of the slide, the sliding runways, sliding tracks, guiding system, vertical jacking details, sliding shoes, existing bridge reinforcement, and the overall procedure. Left up to the contractor to design, to prescribed parameters, were the actual jacking and pulling systems for the slide, detailed procedures and contingency plans. The design for the sliding runways at the abutments, which tied into the temporary abutments, was completed early and provided to the BCMoT so this work could be constructed along with the detour pre-build.
Once the successful bidder, Neelco Construction Inc., was on-site following the Olympics break, preparations began for sliding the bridge. After reinforcing some areas for jacking and installing sliding tracks, sliding shoes, guides and pulling jacks, the bridge was jacked vertically during short night closures to remove old roller bearing nests and install sliding shoes, then left on these sliding shoes while traffic resumed. The pre-built detour approaches were set to match the elevation of the bridge deck once it was set on sliders, so that once the bridge was in its new position, no vertical jacking was required; it was left on the sliding shoes for the life of the detour.
A maximum of 36 hours from closing the bridge at 6 p.m. Saturday until reopening it to traffic at 6 a.m. Monday was allowed in order to complete the sliding operation. At 6 p.m. on June 19, 2010, one lane of westbound traffic was diverted to the three-lane eastbound bridge, so that Marine Drive traffic was uninterrupted, and the contractor began sliding the bridge. Hydraulic jacks, working in pairs and cycling up to 6 in. at a time, pulled high-strength thread bars to move the bridge on PTFE pads sliding along greased, polished mild steel tracks. The bridge was rotated in plan as it was moved, to accommodate the detour alignment, moving 36 ft at the west end and 56 ft at the east end.
This sliding path was guided only along a curved guide track at the pier, with the abutment sliding shoes running unguided. Just before midnight, less than six hours later, the bridge arrived in its new position, then work continued through the night to install restraints at the bearings and weld deck joint covers in place. In the morning, traffic barriers were moved and new lines painted, then the bridge was re-opened to traffic by 10:30 a.m. on Sunday, just over 16 hours after starting the slide, and with some 20 hours remaining in the permitted closure.


Building up, taking down
With the old bridge slid out of the way, and the 2010 fish window looming, the contractor proceeded to demolish the old abutments and begin piling work for the new abutments. Once allowed into the river, the contractor demolished the old pier, installed the new pier piles and completed the concrete pier construction to above high-water level before the fish window ended. The remainder of the substructure work was then completed outside of the river and the new bridge approaches constructed.
The contractor chose to erect the bridge girders using incremental launching from the east bank, cantilevering the first two sections of assembled girders over the river, launching them partway, then adding the remaining girders and completing the launch. After jacking down the girders, deck formwork was completed and the deck poured in a careful, symmetric sequence to limit cracking and lock in the integral abutments after girder end rotations had occurred. Final approach backfill was completed after the abutments, then approach slabs, parapets, railings and paving finished the work.
The new bridge experienced some construction delays, mostly related to structural steel delivery, and was opened to traffic on May 20, 2011. In the end, the Canadian government had extended the completion deadline, so the matching funding was still secure despite the delayed completion.
After opening the new bridge, the old bridge was demolished, along with subsequent grading, paving and landscaping completion. Concrete deck slabs were sawn into sections and removed by tracked excavator. Environmental and health precautions included collecting all of the saw-cutting runoff water and measures to deal with lead paint during structural steel cutting. Once bare, the 250-ft east span truss was lifted off by a large crane and set on blocking to be cut up, while the 180-ft west span was temporarily supported at mid-span and cut in half, with the river half being lifted off in one piece and the land half being cut up in place.  
The Capilano River Bridge Replacement Project included some very challenging schedule, environmental, site and traffic constraints, many of which were addressed by the innovative solution of sliding the old bridge laterally in an overnight operation to become the construction detour, saving an estimated $500,000 in the process. The new bridge provides a much-improved westbound link over the Capilano River, reducing traffic delays, speeding up buses and providing a much safer and more pleasant pedestrian and cyclist crossing. The integral abutment design, lack of deck joints, stainless steel deck reinforcing and weathering steel girders are all expected to contribute to a durable, low-maintenance bridge with a long life.

About The Author

Johnson is an executive engineer with Buckland & Taylor Ltd. He was the design manager for the Capilano River Bridge Replacement Project and was engineer of record for the sliding. He can be reached at mmj@b-t.com.

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