The western extension of Highway 407 in Ontario, Canada, from Mississauga to Hamilton required a major crossing of the environmentally sensitive Bronte Creek Valley. Commitments made to minimize the impacts of construction on the valley and creek led to the conclusion that a bridge structure, launched from the top of the valley, was the optimal solution. Some key components of interest are described: during the design and construction of the resulting incrementally launched, precast girder design.
In 1993, the planned extension of Highway 403, in Ontario, from Mississauga to Hamilton required a major crossing of the environmentally sensitive Bronte Creek Valley. At the crossing location, the Bronte Creek Valley is characterized by very steep slopes. Access into the valley is not feasible without extensive environmental impacts. Commitments made to minimize the impacts on the valley and creek led to the conclusion that a bridge structure, launched from the top of the valley, was the optimal solution.
Preliminary studies were completed for both steel and concrete bridges, ultimately leading to the completion of contract drawings for both a launched steel box girder bridge and a launched, precast concrete girder bridge. However, at this point, the Province of Ontario started investigating the concept of a toll highway across the top of Toronto that would eventually pass through the Highway 403 site, and the contract drawings were shelved.
In 1999, the Province of Ontario decided to extend the Highway 407 Toll Road westerly, and the planned extension of Highway 403 became the westerly extension of Highway 407. A design-build contract was awarded to SLF Joint Venture. The Bronte Creek crossing was recognized as a key component of the work, since it was required to provide access for the construction of the remainder of the highway. A complete redesign was required because of geometric changes in the highway.
Preliminary studies were undertaken for both precast concrete and structural steel alternatives. Preliminary quantities were developed for costing by the design-build contractor, with the result that the precast concrete girder alternative was the determined to be the more economical structure.
DESCRIPTION OF THE BRIDGE
The bridge crossing consists of two separate structures, supported on common abutments, but independent piers. The span arrangements of the two structures is controlled by the alignment of the Bronte Creek, which flows diagonally to the highway, resulting in non-symmetrical span arrangements for the eastbound and westbound structures. To simplify construction, the same 35m - 60m - 55m span arrangement was chosen for both structures, and simply reversed for the eastbound and westbound bridges, respectively. The heights of the bridges rise about 23m above the valley floor. Refer to Figure 1. for a general view of the twin structures.
The cross-sections of the twin bridges each consist of a 225mm thick concrete deck slab supported on seven, precast concrete girders. The center to center girder spacing for both structures is 3.1 m. Each bridge deck carries four lanes of traffic (eastbound and westbound Highway 407, respectively) and is approximately 21 m in width (see Figure 2).
The design of the twin bridges was undertaken in accordance with the Ontario Highway Bridge Design Code (3rd Edition). A discussion of some of the key design issues follows.
GIRDER FABRICATION/POST-TENSIONING SEQUENCE
The girders were fabricated in the Pre-Con Company precast plant in Woodstock, Ontario. The 60m and 55m span girders were too heavy to be transported on municipal/provincial roads. Therefore each girder was fabricated and shipped to the site in three segments (see Figure 3). The 35 m girder segment was just within the permissible weight restrictions and was fabricated and shipped as a complete unit.
The centre segments for the 55m and 60m girders were pretensioned for self-weight alone while the shorter, exterior units were reinforced with normal reinforcing steel. The girders were fabricated with five, draped post-tensioning ducts, two of which were used to splice the individual segments of the girders into complete girders (Stage I Post-Tensioning) and three of which were used to splice the individual spans into one continuous structure (Stage II Post-Tensioning). The 35m span girders were pretensioned both for self-weight and the slab dead load. Accordingly, these girders were fabricated with only the three draped post-tensioning ducts required for Stage II Post-Tensioning. Straight pretensioning strands were located in the bottom flanges of the girders to keep the girder webs clear for the draped post-tensioning ducts.
Individual segments of the 55m and 60 m spans were placed on concrete pedestals at the site and aligned. The post-tensioning ducts of the adjacent segments were then coupled and the girders were spliced with cast-in-place in-fill closure pieces. Each assembled girder was subsequently post-tensioned (Stage I Stressing) for the assembled girder dead load and for the slab dead load, and the ducts were grouted. Girder assembly was carried out in an area located approximately 200m west of the bridge site.
Once assembled, the individual girders were transported along a girder delivery system to the structure and along a launching truss running between the twin bridges to their respective span locations. The girders were transported laterally along another delivery system from the launching truss to their final locations on the piers and abutments. Once in position, each girder was transferred onto its permanent bearings. Steel angle cross frames at third points along the girder spans were installed for stability.
The Ministry of Transportation of Ontario requires all bridges be constructed to permit future deck slab replacement. Consequently, the continuity (Stage II) post-tensioning was carried out prior to the placement of the deck slab. The ducts for the through cables running the full length of the structure were then coupled at the piers once the girders were in-place. Support diaphragms were cast at the piers enclosing the ends of the girders in adjacent spans. The fully continuous girders were then post-tensioned (Stage 2 Stressing) with cables running the full length of the structure. The ducts were grouted and the deck slab was subsequently cast.
The Bronte Creek bridges were designed utilizing a traditional, cast-in-place deck slab design. Given the difficult access conditions and schedule constraints, however, the Contractor decided to use normally reinforced precast concrete deck panels, with a reinforced cast-in-place top surface. The total thickness of the panels and cast-in-place topping matched the original design's 225mm cast-in-place slab (90mm thick deck slab + 135mm thick cast-in-place topping). The deck panels were designed in accordance with the requirements of the, Canadian Highway Bridge Design Code.
PRECAST CONCRETE GIRDER
The original 1993 design was based on a modified version of the largest commercially available precast girder of its day. To stretch the capacity of the 2.35m deep girder to reach the 60 m maximum span in this structure, a very complex, multi-stage post-tensioning sequence was concieved.
The new design, completed in 1999, employed the new CPCI 2450A girder (Figure 4). The benefits realized by replacing the original girder with the new girder (reduced post-tensioning requirements and a greatly simplified post-tensioning sequence) resulted in significant cost savings. This project is only the second application for these girders in Ontario (the other application was for the Perley Bridge replacement over the Ottawa River, between Hawkesbury, Ontario and Grenville, Quebec, where the girders were combined with haunched, precast sections at the piers to achieve a maximum span of 68.5 m). The Bronte Creek girders resulted in a structure with the longest precast girder span for a constant depth section built in Ontario. The center-to-center girder spacing was 3.1 m. The girders were constructed with 50 MPa concrete. The required concrete strength at-transfer was 30 MPa.
The following table summarizes the prestressing and post-tensioning requirements for the individual girders:
1.Prestressing strand is low-relaxation seven wire strand, size designation 13 (0.5"), grade 1860MPa.
No. of strand and total strand area given.
2.Post-tensioning strand is low-relaxation seven wire strand, size designation 15 (0.6"), grade 1860MPa.
No. of cables, cable designation, and total cable area given.
GIRDER SPLICE DETAILS
The structure employed two different types of girder splices: intermediate girder splices connecting together the girder segments within the 55m and 60m spans, and splices over the piers which provide continuity between spans.
Intermediate Splices - The splices were located close to the inflection points within each span to minimize the flexural stress on the spliced section. Rather than using a match cast approach, splicing was achieved by casting a 150 mm wide, keyed, concrete infill section between the girder segments (Figure 5). This approach had the advantage of ensuring a perfect fit between girder segments regardless of the deformation of the individual segments under dead load and the initial prestress. The splice concrete was a superplasticized High Performance (50 MPa) concrete mix using 10mm aggregate. The splices were reinforced with 15M hairpin bars embedded in the webs and bottom flanges of the girders. The bars were offset 15mm from the centerline of the girder to facilitate lapping of the bars when the girder segments were placed end to end. After Stage I Post-Tensioning, four 30M bars were spliced (welded to an overlapping angle) within a blockout in the top flange.
Pier splices -The splices between the ends of the girders at the piers were carried out after the girders were erected in-place. Concrete diaphragms along the support lines provided the infill concrete between the ends of the girders. Positive moment reinforcement was provided in the bottom flanges primarily to control cracking in the bottom flange during stressing of the continuous (Stage II) continuity post-tensioning. A sectional analysis for a cracked section was carried out to determine the stresses in the concrete and the required positive moment reinforcement. The resulting crack widths were within code limitations, and any cracks that formed were subsequently closed up by the addition of the slab dead load.
SPECIALIZED CONSTRUCTION ENGINEERING
The incremental launching of the girders had a dramatic effect on the erection process. The equipment required to carry out the work was almost exclusively purpose-built equipment. The logistics of coordinating the girder assembly, the girder delivery systems to the bridge, the launching equipment across the bridge and the girder placement required a high degree of planning. Some of the key components that required specialized construction engineering were as follows:
· The girder assembly yard
· Three separate carriage delivery systems to transport the girders from the assembly yard to their final resting positions on the piers and abutments.
o Side-shifter carriages in the assembly yard to transport the girders laterally from the position in the yard where they were assembled to the longitudinal delivery system leading to the bridge.
o Transporter carriages to carry the girders along the longitudinal delivery system from the assembly yard to the bridge and across the launching truss.
o A second, different, side-shifter carriage system to transport the girders laterally from the launching truss (located in the median between the eastbound and westbound structures), across the tops of the piers and abutments, to their final resting positions on the piers and abutments
· Five individual launching trusses (three unique span lengths, 35m, 20m and 40m) to span between the piers and abutments
· Various stabilizing measures required to prevent the girders from tipping over under construction wind loads, during assembly, during transportation on three different carriage systems and, once erected, prior the deck slab being cast.
GIRDER ASSEMBLY YARD
The 55m and 60m girders were fabricated in multiple segments and spliced together on site required a large assembly area to the west of the bridge site. A large enough working area to assemble two spans of girders simultaneously was created (i.e. 2 bays of 7 girders) to maintain a reasonable girder assembly speed. This allowed the assembly and pouring of splices to take place on the girders for one span while previously spliced girders were having their splices cured and Stage 1 post-tensioning.
The assembly area for each bay of girders consisted of four concrete sleeper slabs to support the ends of the three girder segments in each spans (i.e. for the 55m and 60m spans). A running surface was provided for the side-shifter carriages that carried the girders transversely from their assembly position in the yard to the longitudinal delivery system that transported the girders to the structure. The longitudinal grade on the delivery system was about 3%.
INCREMENTAL LAUNCHING OF GIRDERS
The erection/launching sequence was as follows (Figure 6). The side-shifter carriages and longitudinal transporters are shown in Figures 7 to 9):
1. Deliver the girder segments to the yard and unload by conventional crane.
2. Assemble individual girder segments into full-length girders, cast and cure cast-in-place intermediate field splices; and, stress stage 1 post-tensioning.
3. Shift the assembled girders laterally across the yard to the delivery rail on side-shifter carriages.
4. Transfer the girders from the side-shifting carriages to the longitudinal transporters that run on the delivery rails.
5. Push the girders down the delivery rails to the bridge using a small bulldozer.
6. Connect a winch from the far side of the bridge to the longitudinal transporters to winch the girders along the suspended launching truss to the girder's respective span location.
7. Transfer the load of the girder from the longitudinal transporters to side-shifter carriages running across the tops of the piers and abutments.
8. Pull girders on the pier/abutment side-shifters laterally across the piers and abutments to its final position.
9. Lower girder onto its bearings and install temporary bracing.
Photograph 1 shows an aerial view of the site. Only the remaining three girders in the center span of the eastbound lane structure remain to be erected (east is up). Half of the assembly area is visible at the bottom of the picture. The delivery rail system runs longitudinally from the assembly area to the structure, between the westbound and eastbound structures. Two girders remain in the assembly bay. The remaining girder is sitting on the launching truss in the middle of the bridge, waiting to be transported laterally to its final resting position on the eastbound bridge.
The launching truss (see Figure 6) consisted of a series of custom-made steel trusses, erected in the median between the twin bridges. The trusses were largely constructed from hollow structural steel members and were supported on brackets cantilevered off the sides of the piers.
The staggered pier arrangement greatly reduced the spans of the individual trusses between supports in comparison with the girder spans. The launching truss ran the full length of the two bridges offering maximum flexibility in sequencing the placing of the girders in any span. The longest truss span between the piers was 40m. For this span, it was necessary to post-tension the bottom chords of the truss (see Photograph 2).
CARRIAGE DELIVERY SYSTEMS
The delivery of the girders from where they were assembled in the assembly yard, to their final resting position on the structure required three different carriage delivery systems.
These were as follows:
Assembly Yard Lateral Side-shifter System (Fig 7) - The side-shifter system in the assembly yard consisted of a pair of support carriages in each bay, which transported the assembled girders laterally to the longitudinal delivery system, which would transport the girders to the bridge. Each carriage was supported by four Hillman Roller bearings. The rollers ran on two steel bars embedded in each of the two exterior concrete sleeper slabs. The carriages were powered by long stroke hydraulic jacks pulling prestressing strand that were attached to the carriages.
The steel grillage carriage housed a pair of jacks that were used to jack the girder off its temporary support. Once the girder was raised off its supports, the jacks were blocked, and the girder was tied to the carriage using hi-strength thread bars. Longitudinal stability was provided to the girder (i.e. along the axis of the girder) by a second pair of thread bars, anchoring a spreader bar across the top flange of the girder to a stabilizer bar below the bottom flange of the girder. The stabilizer bar was rigidly connected back to the carriage. Once the girder was fully secured to the carriages, it was side-shifted across the assembly yard.
Longitudinal Transporter System (Fig.8) - The longitudinal transporter system consisted of a second pair of carriages, one at each end of the girder. Again, each carriage was supported on four Hillman Roller bearings. Each pair of rollers ran on a steel bar running surface that was welded to the top flange of a steel rail beam. The steel rail beams were attached to timber ties sitting on a granular pad. Once the grade was achieved on the running surface, gravel was added to raise the pad level with the top flange of the rail beams. The steel rail beams were fastened directly to the launching trusses at the structure.
The longitudinal carriages were a simpler design than the side-shifter carriages in two ways. First, it was not necessary to house jacks within them, because transferring of the girder from one delivery system to another was always achieved using jacks in the side-shifter carriages. Second, a wider wheelbase could be used under the carriage, which provided the carriage with superior stability and only a single pair of high-strength thread bars was required to tie the girder to the carriage.
Once the girder was secured to the transporter carriages, a bulldozer was used to push the girder along the rails, down the 3% grade to the bridge. A winch was used to pull the girder across the launching trusses to the span where the girder was to be placed.
Structure Lateral Side-shifter System (Fig. 9) - The side-shifter system on the structure, again consisted of a pair of support carriages (one at each end of the girder), which would transport the girders laterally from the launching truss to their final resting position on the bridge. Each carriage was supported on only two Hillman Roller bearings (as opposed to four bearings for the other carriage systems). As a result, the side-shifters could not be made wide enough to be stable on their own and it was necessary to stabilize the side-shifters to the girders themselves with tie rods connected to spreader bar across the top flanges of the girders. The tie rods were attached, in turn, to a stabilizing arm below the bottom flange. Additional lateral stability was provided to the stabilizing arm by a castor, running on a rail supported off the face of the pier.
The width of the pier caps were wide enough to accommodate the side-shifter rails as well as the permanent bearings. It was necessary to increase the height of the pedestals to provide vertical clearance for the side-shift equipment as the girder was being transferred across the pier. At the abutments, the side-shifter rails were supported on steel brackets attached to the face of the abutments.
The side-shifters ran on a set of Hillman Rollers across the top of the pier in four specially installed channel tracks. As with the side-shifters in the assembly yard, the mode of power for the carriages was by long stroke hydraulic jacks pulling on prestressing strands that were attached to the shifters.
Girder Stability during Erection - While on the various transportation devices, it was necessary to stabilize the girders against falling over sideways, particularly under the effects of wind load. This was achieved through threaded tie rods attached to the base of the transport carriage elements and tied either through the top flange of the girder, or to a spreader beam across the top flange of the girder. The tie rods provided sufficient stability to the girder for a 40 km/hr wind speed, with gusts up to 60 km/hr. The margin against instability was adequate but not excessive. Provisions were provided for tying off the girders, in the event the girders had to be stopped for an extended period on the transport carriage. The girder tie-offs were designed for a 1 in 5 year wind (81 km/hr with gusts up to 115 km/hr).
Once erected on their bearings, additional measures were installed to ensure the stability of the girders (see Photograph 3). The first girder erected in each of the longer spans (55m and 60m spans) was braced by a steel stabilizing truss attached to the top flange - the truss provided lateral support to the top flange. Subsequent girders were connected to the first girder and received additional stability provided by the stabilizing truss. In addition, the first three girders erected in each span were torsionally restrained at the piers and the abutments by post-tensioning strand connected to the top flange and anchored to the bearing seat. Once all the girders in a span were installed, the girders could be cross-braced in a way that allowed removal of the stabilizing truss before forming and pouring of the deck. The design wind for the erected girders was a 1 in 10 year wind (88 km/hr with gusts up to 125 km/hr).
Construction of the Bronte Creek bridges was started in mid-November 1999. The substructure was completed in the spring of 2000. Precast girder fabrication started in February 2000, with delivery to the site starting in May. Fabrication of the temporary launching truss was started in March 2000. Field assembly was completed in June, with completion of installation of the launching assembly in July. The girders were launched commencing in early July and the launching operation was completed at the end of August. Placement of the precast concrete deck panels took place from early October to mid-November. One of the cast-in-place decks was placed in December. The remainder of the work was completed by April 2001.
Owner: Highway 407 ETR
Engineer: McCormick Rankin Corporation
General Contractor: BFC Construction Group/Diamond Stonebridge Contracting
Precast Girders: Pre-Con Inc.
Additional Participants: Aluma Systems Canada, Inc., Lafarge Canada Inc., LIUNA Local 506, Otter Brown Engineering Limited, Totten Sims Hubicki Limited