The Beauharnois Bridge, Montreal

4th Construction Specialty Conference 4e Conférence spécialisée sur la construction Montréal, Québec May 29 to June 1, 2013 / 29 mai au 1 juin 2013 The Beauharnois Bridge, Montreal Alejandro Acerete Eng. - Nouvelle Autoroute 30 CJV - Engineering Manager Abstract: The bridge over the Beauharnois Canal is the main structure of the Nouvelle Autoroute 30 project, with a total length of 2551 m. It crosses the Beauharnois canal and the St-Laurent Seaway (SLS) with a twin deck, one for each highway direction. This article describes the development of the design and construction process of the Beauharnois bridge carried out by Acciona, Dragados, Aecom and Verrault for Nouvelle Autoroute 30 in Montreal. It describes the construction procedures in order to achieve the works in a schedule severely limited by the technical clauses of the contract and the winter conditions. The article emphasises on the different prefabrication systems used on the bridge construction and the incremental launching technique used to erect the steel box girders of the west part over the Beauharnois Canal and the Saint Laurent Seaway (SLS). Above all, the main constrain for the bridge was to cross over the SLS. This critical navigational infrastructure for both, Canada and USA, governed the design and the decision to erect the east approach (over water) with the launching procedure. The west approach (over land), was designed with different configuration due the better access. This division of the bridge superstructure in two different decks had as a result the following structural solutions: The east part, formed by closed composite structure with a steel box girder spanning over 82m, and the west part, that was designed with a concrete deck composed of five concrete prefabricated beams spanning a typical length of 45m. The East Part with a length of 1456m, had been designed with no expansion joints. This side of the deck includes the main span of 150m and was entirely assembled at the east abutment and put in place with the incremental launching technique. 1 Description of the structure The bridge, with a total length of 2551,4 m crosses from East to West the Beauharnois Canal (BHC) and the SLS twin deck. The SLS is an international navigational infrastructure connecting the Great Lakes of North America with the Atlantic Ocean through the Québec region in Canada. This strategic point governs the conditions under which the Beauharnois Bridge has been designed and built. CON-xxx-1 The bridge length is divided in three parts:    West approach, over land, from west abutment (axis 1) to the transition pier 26 Area of the main span over the SLS between pier 26 and 31 East approach, over the Beauharnois Canal waters, between pier 31 and the East abutment (axis 44). An overview of the Bridge is presented in Figure 1. EAST PART WEST APPROACH MAIN SPAN Figure 1 -Beauharnois Bridge general view- The design of the bridge carried out by Arup is divided in two different parts: The East Part which includes the area of the 150m main span, and the East approach with typical spans of 82m. The 14m width twin composite decks are formed by a steel box girder. The West Approach has 1094 m divided in 25 typical spans of approximately 44.5m. It is composed of continuous decks with 5 precast concrete beams. 2 2.1 Foundations Design The pier’s foundation design on the water side presented an important challenge. The rock was located 8 meters under water level and it’s composed of Quartzitic sandstone with siliceous cement of high compressive strength (average values of 150-200 MPa up to 350 MPa) it’s also very abrasive. The piers 26 to 44 are supported on 2.0m diameter piles with a minimum rock socket of 4m and a maximum of 7.5m. The design takes advantage of the high strength rock, transmitting important forces through the rock socket, and to confirm these assumptions two load tests with the Osterberg cell were realized. (See figure 2 Figure 2 - Test Pile with Osterberg cell- On these foundations, the design was based on piles and pile-caps over the water for different reasons: - Since piles were drilled from a barge, they were easier to construct over the water, rather than other underwater solutions with coffer drams. - The environment permit required minimum surface distortion at the canal bottom. - The hydroelectric power plant located downstream required minimum head losses due to the bridge construction, so a minimum surface affection on the canal waters was seek. Each pile cap, with 3 meters depth, consists of 2 groups of 3 piles placed in an equilateral triangle and interconnection by a tie beam. Foundations of the west approach were designed with 96 steel driven piles and with a “double T “ shape pile cap with similar configuration as described above East approach: 6 bored piles 2000 mm Ø West approach: 96 Driven Piles320 mm Ø Figure 3 -East and west deck pile caps plan view- 3 2.2 Construction Piles were casted using 2 mm steel casing embedded on the rock. The piles were drilled from barges. Their position was controlled and located at all times using GPS system. The construction rate achieved was one pile per week and machine. The pile caps were casted in 2 phases in order to minimize the formwork structure. Soffit was supported on piles with sand jacks to ease formwork removal. The first pour of the pile caps had 1 meter high. Once the strength is reached, the soffit formwork was removed using the sand jacks. The formwork was moved to the next pier while the second phase of concreting can now take place Besides reducing the concrete weigh supported by the formwork, this system also allows to work in two piers in parallel. Also the reduced concrete volume has as a consequence less hydration heat produced in the concrete mass. Figure 4 -Execution of foundations on the water- The foundations of the west approach were built on land with driven piles equipment. Figure 5 -West approach foundations- 4 3 3.1 Piers Typical piers Typical piers of the bridge are void circular with 3.6m diameter and 400m thickness and their height varies along the longitudinal profile of the bridge between 4m to 38m. The design of the piers was originally based on in-situ construction. A precast alternative was developed in 2009 as a means to speed up construction and improve quality in view of the site conditions. Two options were analyzed: a full precast solution and a partial precast solution with an in-situ design at the bottom. The important issue of ensuring ductility at the base of the column and the fact that foundation construction was starting at this stage, lead the project to the partial precast option. The in-situ part was divided in two lifts: the first lift of the pier is where plastic hinges could be formed during a seismic event , the second lift is where the anchors of the post-tension system were located. The design was developed with post-tension tendons to attach together all precast elements of the top part. The pre-cast part of the pier was fabricated with match cast segmental method, where each segment is cast using the previous as a bottom formwork. With this technique the control of the pier alignment and verticality had to be done at the shop as well as on site. Once the in-situ part was built, the assembling of all the precast elements reached a production rate of one pier per day and crane crew. Figure 6 3.2 -Erection of precast columns and pier heads- Transition pier 26 The transition between the two different decks is done at pier 26 (see Figure 7) with a singular pier head than combines the support for both decks at two different levels. This pier has been designed as a mobile point on the West approach and as a fix point on the East Part. This double function is achieved with a seismic retainer that links together the box girder with the pier head. (see Figure 7) The volume of this pier head made not possible to prefabricate it, but the rest of the column shaft was prefabricated. 5 Figure 7 -Pier 26- 3.3 Main Piers The Main Piers are the fixed points of the East Part;;since there are no expansion joints they attract most of the seismic forces of the 1457m deck. The combination of ductility and resistance required for these piers has been balanced providing strong connection with the deck and the foundation and slender double columns. To achieve this at the base of the pier, a 5m tall and 1.2m wide concrete wall connects both piers with the foundation. The connection between the main piers and the deck combines different elements: (1) a massive pier head 20m long and 6m wide, (2) a concrete block cast inside the box girder to connect the pier head with the deck, and (3) steel haunches* bolted to the bottom of the box girder to provide enough section for the deck to resist the hogging moments of the main span. These connections are shown in Figure 9. *(a reinforcement of the steel section with a nose shape) Figure 8 -Main Piers- 6 The main piers have been constructed in two phases: during construction as a mobile point and during permanent condition as fixed piers. Along these two phases the pier head had a different structural function and configuration: with the deck on simple supported sliding bearings during the launching and with the deck stitched to the pier in permanent condition. Several construction stages were needed to cast it and the haunches pieces were erected and attached to the pier head once the incremental launching of the steel box girder was finish. (See Figure 9). Figure 9 3.4 -Additional connections outside and inside the box girder- West Deck It is designed with precast concrete beams with standard NEBT a(New England Bulb Tee) section with a height of 2000mm. The total length of 1094m has 25 typical spans of 44.5m divided in 3 segments by the expansions joints located at axis 1, 8, 17 and 26. The deck is 14.2m wide and is continuous over the piers. Figure 10 3.5 -West deck - East Deck The east deck is 13.7m wide and has a composite section with a steel box girder 8.1 m wide and 3.7 m height. The entire length of 1450m is divided in 17 spans with typical spans of 82m and the main span of 150m. It has been designed as a continuous element with no intermediate expansion joints. The box girder was assembled at the east abutment and erected with the incremental launching technique described further bellow. The concrete deck is designed with full width precast slabs. In order to be able to support the construction loads during concreting phase, the slab has to be pre-stressed. All the elements required for the drainage support are also included in the precast slab. 7 The purpose of the design was to maximize the deck elements included in the precast section of the deck. The objective was to improve productivity on-site. With this system, once the slabs were installed, the rest of the deck was cast with rates of 82m per 3 days and concreting crew. Figure 11 4 -East deck construction- Construction singularities Beauharnois Bridge has been a challenge from different points of view. Different constrains form: (1) the contract technical requirements, (2) the tight construction program and (3) the severe winter; have lead us to introduce the following singularities in the design and construction procedures:     4.1 It has partial precast piers with the anchorage zone in the in-situ base of the column instead of in the foundation. The East Part, 1500m long, has been constructed with no expansion joints using the launching procedure. The assembly and launch operations are singular due to the variable curvature of the longitudinal elevation profile. The 150m main span over the Saint Lawrence Seaway channel has been erected without affecting ship traffic, using the launching technique. Hybrid steel mechanical joints were used, combining welding and bolting. Partial Precast piers The typical piers have been prefabricated with two peculiarities: 1. The post-tension system that joins all the precast elements has the anchor block at the base of the pier. Usually the anchors are located at the foundation, where there is enough concrete volume to be able to spread the concentrated loads of the post-tension system. In our case the piers had to be designed like this to be able to introduce the precast solution even when the foundations were designed for in-situ piers. 2. The transition between the in-situ and the precast part was solved with a “humid joint”. This joint was cast in place with high strength and non-shrinkage grout. The levelling operations of the first precast segments were critical to achieve the proper alignment of the pier. 8 3. The pier heads were designed also prefabricated with the requirement to be able to lift them with standard cranes. As a result, the weight of these pier heads were reduced to 100 Tn on water, and to 150 Tn and 250 Tn (expansion joint pier heads) on land. At the base of the pier, the firs in-situ lift is designed to be able to develop plastic hinges in the case of an earthquake. The result of these constrains is a design with congested reinforcement at the blister section and the pier heads. At these areas, 3D modelling tools were used to assist the design to be able to optimize the reinforcement detailing and to define new reinforcements compatible with the assembling process. Also real scale trials were needed solve all problems prior to starting the production work. The partial precast piers solution has been proof to be efficient to advance works in winter conditions. It allows to work in parallel with the in-situ part and the fabrication at the shop. Figure 12 - Partial precast piers critical areas4.2 Incremental Launching Initially the constrains of the contract only left a 4 month winter construction season per year over the navigational channel. During the bid design, the incremental launching was selected as the most adequate construction method to erect the steel structure over the main span.(See Figure 13 to Figure 15). Figure 13 - Launch over the SLS- 9 Figure 14 -Launching over the main span- Each deck of the East Part, with a weight of 7,500 Tn and 1457m, was erected using the incremental launching method. Non-standard launching design and equipment were required to achieve what has been a launching record in Canada and North America, including temporary stayed tower and special temporary supports combined with jacks. The operations have been carried in 20 different launching stages (10 for each deck). At each stage 160m were put in place with an average speed of 7m/h. This method has allowed erecting a total of 2900m of steel box girder in less than a year with production peaks of 270m per month. In order to cross the main span (150m) with the launching, it was decided to install 4 temporary bearings with 850Tn jacks in the main piers. Each pair of bearing at each side of the pier, were separated 20m apart and connected hydraulically. The result was a “swing bearing system” that reduced the effective span to launch to 130m. Figure 15 -Launching over the Beauharnois Canal- The steel box girder cross section was selected for the typical span. This section was designed to resist the launching forces of the typical span of 82m, but it was not enough to launch the box girder over the main span. For this reason a temporary stayed tower had to be used to provide enough resistance to the box girder. This tower also allowed to control the deflection at the tip of the launching nose. In the Figure 15 it is shown how the deflection of the deck is recovered with the stayed tower. 10 4.3 Assembling with Hybrid mechanical joints The longitudinal profile of the box girder is divided in 40 segments with lengths that go from 18m to 40m. Each pair of segments is joint using a new hybrid mechanical joint that combines welding and bolted techniques in the same joint: top flanges and webs were bolted and the bottom flange was partially welded and bolted (See Figure 16) Figure 16 -Hybrid mechanical joints- The reason for this system is be able to slide the box girder supported on the bottom flange. This 300mm wide sliding area was required to be continuous all along the entire length of the bridge with no bolts. For this reason the sliding area had to be welded at each mechanical joint. On the other hand, to optimize assembling works, especially during winter, all other parts of the section had to be bolted This double constrain introduced in the project a new concept of hybrid joint with two technological obstacles: 1. If welding is done first with no constrains from the bolting, the geometry of the joint could not have been controlled since the weld deformation introduced important variations in the joint angle. 2. If bolting was fix at the beginning of the operations, the welding introduced important tensions in the nearer bolts that were not considered in the joint design. The final method statement was sought through a process to fix the geometry with some bolts and pins introduced at the holes surrounding the weld. These pins were removed when the first passes of the weld were done. 11