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Spanning the Saint Lawrence

By Marwan Nader, Ph.D., P.E., Eng., M.ASCE, Alex Sanjines, P.E., Eng., Carol Choi, P.E., Eng., James Duxbury, P.E., George Baker, P.E., Eng., Hardik Patel, P.E., S.E., Sam Shi, P.E., Tim Ingham, Ph.D., P.E., S.E., Eng., and Hayat Tazir, P.E., Eng.

The recently opened Samuel De Champlain Bridge is an iconic gateway to Montreal. Its signature structure, an asymmetrical cable-stayed bridge, features a 240 m long main span over the Saint Lawrence River. With an overall width of 60 m, the new bridge is the world's widest to have two planes of cables.

Spanning the Saint Lawrence River between Île des Soeurs—or Nuns' Island, a suburb of Montreal—and Brossard on the South Shore in Quebec, the new 3.4 km long Samuel De Champlain Bridge is part of a larger Samuel De Champlain Bridge Corridor Project. In addition to the new bridge, the larger C$4.2-billion (U.S.$2.95-billion) project includes highway reconstruction, widening of the federal portion of Autoroute 15, a new 470 m long Île des Soeurs bridge, and improvements to ramps on the Brossard side. The project features four highway lanes in each direction, a central transit corridor for the forthcoming light-rail, and a multiuse path for cyclists and pedestrians.

The new Samuel De Champlain Bridge comprises three structures. From the west, a 2,044 m long west approach connects to Nuns' Island and climbs at a constant grade over the Saint Lawrence River. This structure consists of 26 spans that are typically 80.4 m in length, divided into four expansion units. Next, the cable-stayed bridge extends over the Seaway Navigational Channel. With a clearance of 38.5 m required for the vessels, the asymmetrical, single-pylon, cable-stayed bridge has a main span of 240 m over the channel and a back span of 124 m. (See the illustration on page 51, top.) Further east, the bridge descends at a steeper grade to the east abutment in Brossard over a length of roughly 780 m. The east approach is divided into two expansion joint units and typically has 84 m long spans as well as a 109 m span over an existing six-lane arterial road, Route 132. To preserve the aesthetic intent of the owner, architectural requirements were defined in a set of definition drawings that prescribed the essential external forms and dimensions of the structures. Among the key parameters established were the minimum standard approach-span lengths, pier caps each shaped in the form of a W, the allowable number of expansion joints, the cable-stayed bridge main-span and back-span lengths, the tower shape, and the harp-style stay-cable arrangement. Essential geometric parameters were defined, including the alignment, seaway channel geometry, clearance envelopes, and no-construction zones.

For the entire bridge, the superstructure consists of three independent decks, built as box girders with constant structural depth throughout. (See the illustration on page 52.)

The technical challenges included the region's severe winter climate and the navigational requirements established by the Saint Lawrence Seaway Management Corp., which manages the seaway. The site's natural geographic, geologic, and climatic conditions presented a challenge to meeting the owner's desire to establish a 125-year design life for the facility. Severe conditions, including ice loading, wind, seismic activity, liquefaction, light-rail transit loading, and vessel collision loading had to be addressed. One of the most notable challenges was the fast-track schedule that the new bridge was put on, which was necessary because of the deteriorated condition of the existing bridge. The design-build team was tasked to complete the design and construction so the bridge could be opened to traffic within a mere 48 months.

T.Y. Lin International (TYLI), which has its headquarters in San Francisco, was the managing partner of the design joint venture and the engineer of record for the cable-stayed bridge. TYLI's involvement began in 2014 with prebid design. Upon notice to proceed in April 2015, TYLI performed detailed design and construction support.

One of the most prominent elements of the Samuel De Champlain Bridge is the signature cable-stayed segment. The structure includes a 170 m tall single-pylon main-span tower comprising twin masts configured in the shape of a tuning fork. Inclined lower tower legs echo the inclined approach-pier legs.

The main-span tower comprises two shafts built of precast and cast-in-place (CIP) concrete segments on a CIP footing supported by piles. The choice of precast segments considered the season in which the concrete would be cast and erected. The hollow tower shafts provide passageways for elevators, ladders, and utilities and are connected by a lower cross beam (LCB) and an upper cross beam. The LCB is monolithically framed into the bridge superstructure, while the upper cross beam, nicknamed the "bow tie" for its resemblance to the sartorial item, is above the clearance envelope of the transit corridor.

The lower portions of the shafts up to the bow tie are sloped at 1:7 (H:V), while the upper portions are vertical and freestanding. This upper vertical portion, standing on the rigid A-frame of the lower shafts and cross beams, supports the stay-cable anchorages. The architectural requirements of the shape of the shaft and the eccentric placement of the stays in the shaft section result in an eccentric downward component of the stay force onto the shaft. This arrangement produces a permanent moment in the shaft about the bridge's longitudinal axis, requiring an initial transverse camber, bowing the shafts inward, to offset the permanent outward dead-load deflections.

In addition to forming the backbone of the lower A-frame of the main-span tower, the LCB supports about 60 m of the back-span and main-span superstructures and resists any twisting due to differential loads in the back and main spans. Structurally, it is one of the most rigid components of the entire bridge.

Functionally, the LCB serves as a major cross-passage between the three longitudinal girders, a center for the coordination and distribution of utility lines in the superstructure and main-span tower, the chief elevator service landing, and a base station for the underbridge maintenance gantry.

The LCB at the main-span tower consists of two composite U girders with a top and a bottom concrete slab connecting each girder. The LCB is 48 m long and 8.8 m wide, with a height of nearly 3.5 m at the typical pylon section.

The composite section of the LCB was designed to provide the stiffness and strength of a large concrete section with the steel framing needed for compatibility with the superstructure box girders. The concrete is prestressed transversely, longitudinally, and vertically to address the interaction with the main-span tower and the girders.

The demands used for the service-limit-state and ultimate-limit-state design came from a global analysis model created with RM Bridge software produced by Bentley Systems, of Exton, Pennsylvania. A model of the staged construction of the LCB was created in CSi Bridge software, from CSI Inc., of Walnut Creek, California, to evaluate the stresses in the concrete and steel elements during construction. This analysis included the effects of placing the bow tie on the LCB during its erection.

Because of the large volumes required, the concrete for the LCB needed to be placed before the beginning of winter to facilitate curing. To accelerate the back-span erection schedule, the north and south boxes of the LCB were installed early and used to support the first back-span girders ahead of prestressing. The LCB placement was a critical path activity as it also enabled construction to proceed on the upper tower shafts.

The bow tie joins the two tall shafts of the main-span tower and controls the effects of the asymmetrical span loadings. The owner defined the shape of the bow tie, and its design and construction were based on this geometry.

For large demands, prestressed concrete was found to provide a feasible solution, given the need to provide durable, compatible connections to the tower shafts. Because the bow tie's complex shape made forming difficult, the forming was performed on the ground. Casting on the ground enabled the use of heating for casting during the winter.

Erection involved placing the bow tie on a holding frame over the LCB, erecting the tower segments to the level corresponding to the planned top of the bow tie, and then raising the bow tie into position for connection.

The superstructure of the cable-stayed bridge comprises three longitudinal girders that support the northbound and southbound roadways and the center transit corridor. The steel box girders are composite members with precast deck panels that support all the design demands, including axial compression for the stays and earthquakes, wind, and extreme events. Unbalanced spans and stays, important to the overall architecture of the bridge, required the use of concrete counterweights in the shorter back span to achieve overall balance at the tower. The concrete counterweights consist of a structural lift, which is reinforced and made composite with the girders, and a ballast lift, which is noncomposite. Counterweights are also used in the main span to balance the southbound roadway transversely with the wider northbound roadway.

Scheduling constraints dictated the choice of composite steel box girders as the primary superstructure system. Although concrete segments may have been structurally feasible in the back span, they would have taken longer to erect during the winter, potentially extending the overall schedule by months.

A cross beam at each pair of stay cables connects the three steel box girders, forming a two-dimensional grid system in plan. The cross beams transfer the weight of the girders to the stay cables, distributing the stay forces to mitigate twisting of the upper tower shafts. The cross beams and the three girder segments form the basic assembly unit for the erection of the main span.

The cable system consists of stay cables, anchorages, and link beams, with the link beams being the key to the modular construction of the tower. The link beams are composite with the tower's upper shafts and support the independent anchorages of the back-span and main-span stays. The lower stay anchorages are located inside the cross beams clear of the transit corridor deck.

Although they vary in size, the stay cables generally consist of 127 seven-wire strands conforming to ASTM A416 Grade 1860, Standard Specification for Low-Relaxation, Seven-Wire Steel Strand for Prestressed Concrete. The hot-dipped galvanized wire strands have a guaranteed minimum breaking strength of 279 kN per individual strand. Each strand is waxed and sheathed in high-density polyethylene (HDPE). The strands of each stay are placed into an HDPE stay pipe. Stays were installed with the dead ends in the tower head and the jacking end in the cross beams.

For the piers supporting the cable-stayed bridge, similar to those throughout the approaches, the 11.4 m pier caps consist of steel box sections having an interior matrix of diaphragms and stiffeners. The pier caps form two triangles that rest on the more prismatic pier legs and join in the center. (See the illustration below.) The top members of the triangles are tension members.

Although it would have been possible to construct the pier caps in concrete, the W shape would have required complex formwork and long curing times in large staging areas. At the same time, the top tension members would have required a high concentration of posttensioning, the placement of which necessitates several additional construction steps. Finally, lifting such a heavy concrete element would have been challenging.

With these factors in mind, the design-build team chose to make the pier caps of steel, which can be fabricated off-site year-round. Steel required fewer assembly stages during erection and could be positioned by smaller cranes because of its lighter weight. All these factors reduced construction time and effort.

To save significant time during construction of the pier legs in both the cable-stayed bridge and the approaches, the design relied heavily on precast construction. The pier leg segments were precast, enabling work to continue during cold winter periods when placing in situ concrete would have been difficult. The segmental pier leg substructures consist of hollow concrete box sections stacked atop one another and joined together by means of posttensioning. Precasting also enabled casting and erection to be carried out in parallel rather than in series, as is done in conventional construction.

The foundations of the Samuel De Champlain Bridge consist of spread footings and pile foundations, depending on soil conditions at each location. Of the four piers supporting the cable-stayed bridge, those called W01, W02, and E01 have foundations that consist of cast-in-drilled-hole piles with CIP pile caps, whereas the E02 foundation, located on land, is a CIP shallow spread footing. (See the illustration above.)

The bridge's tower is founded on 21 1.2 m diameter cast-in-drilled-hole piles connected by a CIP pile cap. The tower foundation's structural design is based on strength and stability-resistance to sliding, overturning, and uplift-under the governing load combinations. In addition, the piles are designed to resist liquefaction of the surrounding soil.

Among the 37 piers and foundations for the west and east approaches, concrete spread footings were precast on land in a controlled environment, enabling work to continue through cold winter periods when placing concrete would otherwise be difficult. The precasting yard was outfitted with rolling tarp shelters to enclose the production in its entirety and enable the interiors to be heated. Precasting the footings also reduced the amount of marine work, enabling most of the foundations to be placed in an accelerated fashion.

The major challenge in the erection of the cable-stayed bridge was crossing the Saint Lawrence Seaway, the main waterway of eastern Canada and the Great Lakes region. No temporary structures were permitted in the channel, and overchannel clearances had to be maintained with limited effects on shipping.

The severity of winter in the region makes placing concrete difficult and reduces overall productivity. Therefore, erection of the main span proceeded through the springs and summers of 2017 and 2018. Each segment was lifted to a gantry, which transported it under the main-span soffit to the end of the previously installed segment over the seaway.

There, another gantry lifted the segment into position for connection to the previously erected girders. The transit over the seaway took several hours. Once lifted into place, the segment no longer obstructed shipping clearance. During the segment erection cycle, restrictions to shipping were limited to several meters of vertical clearance during the course of a few hours per month.

The traditional method for constructing cable-stayed bridges is cantilever construction: first erect the steel box girders, then erect the stay cables, and finally place the concrete deck slabs. To accelerate construction for this project, this procedure was modified, and the steel segment was erected with most of the concrete deck panels, alignment devices, and counterweights already in place. Although this approach increased the construction cantilever moments at the tip of the girders, the cycle time per segment was reduced. When the segment was lifted and erected, the concrete slabs were not yet connected, which saved lift weight and also reduced cracking in the deck slab caused by transverse bending. As the erection of each segment was completed, the deck panels were joined by concrete stitch placements before the stays on that segment were tensioned.

A total of 15 segments, each weighing approximately 900 metric tons, were erected using a heavy lift. The main-span construction involved multiple steps of stay stressing, which were essential to control the negative bending in the previously installed girders while at the same time providing the construction team an advantage with respect to the schedule.

The final key to success for the new Samuel De Champlain Bridge was an important innovation in the erection scheme. In late 2017 the contractor made a strategic decision to relocate the bridge deck closure joint of the cable-stayed bridge to enable concurrent erection from the east and west banks of the navigational channel. This erection scheme, however, resulted in a 50 m cantilever from the first east pier that could not be supported by means of belowdeck temporary shoring, because of the no-construction zone requirements in the navigational channel.

After numerous engineering analyses and constructability evaluations, the design-build team arrived at a bold solution: to support the cantilever by means of a king-post tower with temporary stays anchored at the second east pier (E02) and at main-span segments MS12 and MS13. Effectively, the east cantilever was supported by a temporary cable-stayed bridge.

The inception and execution of the alternate erection scheme using the temporary king-post tower exemplified teamwork among the designer, steel fabricator, and contractor. Through effective communication and coordination, the team succeeded in finalizing and implementing this erection scheme within a very aggressive schedule.

The analysis of the cable-stayed bridge was performed using RM Bridge Advanced V8i. The analysis considered the sequence of bridge erection to achieve the dead-load state of the structure and account for the creep and shrinkage of concrete. Loads applied to the model included dead load, service loads (live, wind, temperature, etc.), and seismic effects.

The design adopts a holistic approach to fulfill the durability requirements to ensure that all structures and components maintain their levels of serviceability during their design lives. Comprehensive durability plans were developed jointly by the design and construction teams to address such issues as material selections, design detailing, corrosion-protection systems, operation, construction quality, inspection, maintenance, repair, and replacement intervals.

The bridge was fully opened to vehicular traffic on July 1, 2019. At press time, a central transit corridor was under design and construction to accommodate the forthcoming Réseau express métropolitain (REM) light-rail network. When completed, the light-rail system will have 67 km of tracks and 26 stations across Greater Montreal.

The design of the new Samuel De Champlain Bridge gave utmost consideration to the accelerated construction schedule. Working alongside the contractor, the design team made innovative use of precasting, modular construction, and erection sequencing to meet the project's fast-track schedule while overcoming the site-specific hazards and severe winters in Montreal.

Additional Insight: Defining the Design

This article is based on two papers—"Design of the Cable-Stayed Bridge Signature Span of the New Samuel De Champlain Bridge" and "The New Samuel De Champlain Bridge-Performance and Design Criteria"—that were originally presented at the 36th Annual International Bridge Conference in National Harbor, Maryland, June 10-12, 2019

PROJECT CREDITS Owner Government of Canada Developer and operator Signature on the Saint Lawrence Group, consisting of SNC-Lavalin, headquartered in Montreal; ACS, headquartered in Madrid; and HOCHTIEF, headquartered in Essen, Germany Contractor Signature on the Saint Lawrence Construction, consisting of SNC-Lavalin Major Project Inc., of Montreal; Dragados Canada Inc., of Toronto; and Flatiron Constructors Canada Limited, headquartered in Richmond, British Columbia, Canada Designer TYLI-IBT-SLI Joint Venture, consisting of T.Y. Lin International, of San Francisco; International Bridge Technologies Canada Inc., of Laval, Quebec, Canada; and SNC-Lavalin

Marwan Nader, Ph.D., P.E., Eng., M.ASCE, is a senior vice president, Alex Sanjines, P.E., Eng., is an associate vice president, Carol Choi, P.E., Eng., is a senior associate, James Duxbury, P.E., is an associate vice president, George Baker, P.E., Eng., is an associate vice president, Hardik Patel, P.E., S.E., is a senior associate, Sam Shi, P.E., is a senior associate, Tim Ingham, Ph.D., P.E., S.E., Eng., is a vice president, and Hayat Tazir, P.E., Eng., is an associate vice president for T.Y. Lin International, which has its headquarters in San Francisco.

Civil Engineering, April 2020, © American Society Of Civil Engineers. All Rights Reserved


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