Construction Engineering Consultant
City of Toronto
1.15 km, 66 spans
The Gardiner Expressway Rehabilitation Project involved the full deck replacement of 1.15 km of elevated roadway from Jarvis Street to Cherry Street in Toronto, Ontario, including the York/Bay/Yonge Westbound off-ramp, Sherbourne Westbound off-ramp, and the Jarvis Eastbound on-ramp. The project replaced all the bearings, steel girders, and concrete deck that compose the Gardiner Expressway’s bridge superstructure.
This rehabilitation was commissioned by the City of Toronto to ensure the Gardiner Expressway can continue to safely serve as a vital artery for the city of Toronto, serving an average of 50,000 vehicles each day. The City and its Consultant developed the plan for the rehabilitation as a design-bid-build project while AECON collaborated with Entuitive to define the means and methods.
Entuitive provided Construction Engineering Support for the fabrication and installation of the approximately 400 unique prefabricated superstructure components, including the production of shop drawings for the precast deck panels, the design of formwork and access platforms, and the specification of removal and installation procedures.
We used a high-level of slag in the concrete. This meant we had a high percentage of recycled content in the mix, greatly reducing embodied carbon emissions for cement manufacturing.
The project optimizes human comfort with radiant heating and cooling (with hot water coming from the District Energy building two blocks away) while minimizing the need to condition outdoor air thanks to a dedicated outdoor air system (DAS). The system supplies outdoor air to the space at a low level and low velocity, known as displacement ventilation, which efficiently removes unwanted heat and contaminants as the supplied air rises through the space.
The nodes of the 55-metre truss consist of five piles of 100-mm-thick plate. The largest node weighs the same as some battle tanks at 12 metric tonnes. The two diagonal web members, which support the largest compression forces of 33 and 38.5 meganewtons, comprise four piles of 100-mm solid plate. These were welded using partial joint penetration welding along the edges in the shop, requiring high pre-heating and careful post-heating to minimize potential weld-induced stress concentrations and indicators. The site welding required for connecting the truss web members to the largest node took approximately 150 man hours to complete over multiple days, with induction heaters running 24 hours a day.
The library’s entryway is designed to resemble a Chinook arch cloud formation. The three-storey, 18-metre-tall trusses feature architecturally exposed web members, with the largest members comprising four built-up piles of 4” thick plate. The largest truss spans 55 metres, is curved, and supports another truss spanning 30 metres.
The library sits over Calgary’s busiest light rail transit (LRT) line. This presented many unique structural challenges. The curved shape of the LRT line helped achieve the library’s distinctive shape by mirroring the curve along its longitudinal axis. Cost-effective structural solutions involving long-span floor framing and long-span, mega-trusses provided large, column-free spaces at the main entrance and oculus.
Each truss was too long, too tall, and too heavy to ship as one piece. As such, each piece was individually shipped and erected 10 metres above grade and temporarily braced back to the concrete cores that were advanced first. A temporary bracing system was used to stabilize each truss as additional floors were cast. Welded in shop along the long edges in partial joint penetration, the truss pieces required careful pre- and post-heating to minimize potential weld-induced stress and steel cracking. Two-end dialog b members support the largest compression forces of 33 and 38.5 meganewtons.
Meet the Team
The primary challenge on this project was the accelerated timeline. Work started in 2018 and was essentially complete by July 1, 2021. Like all successful projects of this type, this work involved a design that had to be constructed rapidly, a tremendous amount of planning prior to the start of construction, and flexible implementation to address unanticipated situations.
To facilitate the accelerated timeline, our team developed an automated analysis design and drawing production process to provide rapid, accurate response to stakeholders throughout the project.
As multiple undersized beams had to be replaced, the structural team worked closely with the contractor to ensure that the means and methods were the most economical and feasible, while ensuring that the replacements would not adversely impact the new building envelope components that would be installed overtop.
The extensive decay to the timber framing required much of it to be replaced in-situ, including corrections to poorly executed carpentry work found once the walls were fully revealed. Some of the decay was so severe that tools were not required to remove the affected components. (The team could remove them by hand.)
The new superstructure is primarily composed of precast elements consisting of a pair of steel girders and a precast concrete deck. The precast elements were cast in an off-site, temporary facility and then brought up onto the deck for placement following the removal of the existing deck and girders. The new girder pairs were set onto new bearing pedestals constructed between the existing girder supports, with the joint between the precast elements cast with ultra-high-performance fibre-reinforced concrete.
BSc(Hons) CEng MIStructE
Gardiner Expressway Rehabilitation
The casting of approximately 400 precast concrete deck panels on girder pairs was a major industrial operation, set up in the first stage of the project and then fully removed. The steel girders were fabricated by Canam at sites in Quebec and Ontario, and then shipped to the precasting facility, constructed by AECON at a yard near Cherry Street and Lakeshore in Toronto. The girder pairs were then taken into fabrication tents and the formwork set.
Each girder pair had a specific camber and screeds. The initial analysis developed the cambers to control haunches to a narrow range and to provide screed elevations to match road profile once installed and keep a tight tolerance at the joint between precast units.
The precast units were unevenly loaded by the deck, complicating the screed design, and introducing a lateral movement during casting that also had to be accounted for. The setting out information had to account for this vertical and horizonal movement. Once the girders were on site, they were re-measured and the screed and offset tables were adjusted to suit the as-constructed dimensions.
The deck was replaced in a multi-step process that started with the creation of new bearing pedestals under the existing bridge. This was followed by the removal of structure bracing, saw cutting to create individual panels, lifting those old panels out, and then placing the new panels in.
Entuitive provided extensive quality verification work on this project. Each panel was inspected prior to casting concrete. Each bearing pedestal was inspected for dowel placement, then for reinforcing placement, and finally for bearing placement. Link slab and barrier wall reinforcing was also confirmed.
Jacking designs were prepared for both panel lifting and the major jacking of large steel bents, the latter done under full live load for the replacement of part of the existing columns. Our team ensured we were on call and available to our client at all times during the project.
Vice President, Structures
Achieving the proper final elevations for each precast deck element required developing screed elevations based on the prescribed highway geometry and the expected long-term deflections. Our team used the automated approach to calculating these screed elevations, allowing the specified casting geometry to be checked for compatibility with the actual girder dimensions as the steel assemblies arrived at the precast yard. This also allowed for adjustments to be made as required without delaying the fabrication schedule.
Entuitive was engaged directly by the construction manager to provide a third-party review of all temporary structural work used in the project, including hoisting, scaffolding, shoring and formwork, perimeter netting and pedestrian protection. In addition, work included support for the two large tower cranes needed to erect the 40-ton precast concrete segments. These Favco M1280D cranes are the largest tower cranes in New York City and have been used only a handful of times in the history of the city, including for the erection of the World Trade Center Transportation Hub.
Due to the large lateral loads imposed on the existing structure by the cranes, Entuitive designed temporary steel bracing to the building for the construction phase, before the new concrete cores could be relied upon for full lateral resistance.
Originally conceived as cast-in-place, the client challenged the design team to develop an innovative solution with precast, post-tensioned concrete elements, similar to the methodology used in segmental bridge construction and also used on the Manhattan West platform, a previous Entuitive project.
Over 200 factory-built, precast C-shaped wall units were stacked vertically without any physical connections at the horizontal joints except through the vertical high-strength bars prestressed to clamp the units together. Horizontal monostrands were used for connections at vertical joints. The segments are 30 feet long, matching the width of the core, and vary from 9’-8” to 12’-10” in height; the heaviest individual segment weighs approximately 45 tons.
In the cellar, a conventional cast-in-place concrete “starter core” is supported on new and existing caissons, while a new pressure slab also acts as the elevator pit slabs. Steel beams link the precast segments at each floor level and support the precast elevator lobby slabs. The precast walls feature an architectural board-form finish that will be exposed to view in the completed building.
The precast concrete cores benefitted the project by reducing the structural construction schedule by two months, thereby reducing the cost of the general conditions for the project and facilitating an earlier occupancy by the building’s tenant. Other benefits included reduced onsite welding, improved worker safety, maximized use of repeatable forming systems, and the opportunity for the walls to be exposed as part of the interior architecture.
The landings are finished in terrazzo, set on a stiffened steel landing structure with concrete infill and the treads are also capped with a terrazzo finish. The plate material varies in thickness from 5/8” for the landings and risers/treads, to 3” in thickness for the heavy inside ballustrade, which supports the stair between landings. The 3” thick inner balustrade, which is approximately 72” in height between landings, reduces to 9” in depth under the landings, and ultimately to 5 ½” in depth where it extends beyond the landing to 1 ½” dia rods that support the landings from the existing structure.
In addition to meeting all necessary strength and deflection requirements, the stair was designed to meet stringent vertical and lateral acceleration requirements, without the need for any supplemental damping systems.
Tent-Enclosed Precasting Facility
Typical Panel-Reinforcing Steel Prior to Concrete Placement
Yellow Lifting Frame Panels up to 85t
Providing clear information to the site was critical. The crew in the yard needed information that worked with the formwork and site procedures and was easy to read. To this end, we developed drawings for each girder pair, rotated from the final longitudinal profile to a flat profile in order to provide clear tabular and scaled drawings for the fabrication work. The development of the screed and geometry files required for casting the deck sections was automated using scripts developed by our team.
Our additional work in the yard included the design of a lifting frame that was adjustable for the variable centre of gravity of the girder pairs and designed for low headroom. We also detailed the stacking procedure and support pads. The adjustable lifting frame was required to ensure the panel could be lifted flat, facilitating lifting with minimal twist and ease of placing in the field.
At peak delivery four panels were removed and replaced every 24 hours.
The panels were delivered to the location via specialized heavy load trailers and were then lifted by cranes onto the deck. When the panels were initially placed, they were supported on high-capacity sand jacks our team had designed, capable of full traffic loads. Once all the panels were in place in a stage, the permanent bearings were positioned and grouted, followed by the release of the sand jacks.
AECON carefully sequenced the work, always maintaining a drive aisle and crane access. To achieve this, the deck removal operation followed an alternating pattern like that of the black and white keys of a piano on one half of each stage.
The deck was removed in every second bay (the “white keys”) with the cranes sitting on the bays in between (the “black keys”). Once the “white keys” were replaced, AECON went back and completed the operation by removing the “black keys”. The other half of the stage was the drive aisle. When one half was done the pattern was switched with the new deck as the drive aisle.
The first segment was installed in November 2019 and the last in November 2020.
Old Deck Panel Removal and Placement onto Transporter
A critical part of the rehabilitation plan was the use of mobile cranes to complete all removal and erection work. The cranes offered great flexibility and avoided the cost and fabrication time of custom equipment.
It is rare for mobile cranes to be set up and operated on a bridge structure, as the weight of a crane plus the load it is lifting will far exceed the design live loads for bridges. To add to the challenge, the accelerated schedule required the cranes to be moved on the structure while fully fitted with counterweights and for crane outriggers to be positioned close to edges and on overhangs.
Our work required analysis of both the existing and new structure under more than 3,000 crane load cases. This work was made possible through the development of a parametric model of the structure that facilitated both the analysis and the drawing production. Our analysis of the cranes on the bridge deck required a full grillage review of the structure, with the cranes moving with counterweights in place, and complex interaction with the precast element transporter. The analysis was automated to the extent possible using internally developed scripts to define the models and pull out the pertinent results, coupled with automated drawing production for the deck removal and replacement work, including the crane location and positioning.
This automation allowed for completion of the large volume of analysis required, and, critically, allowed for refinement of the procedures and rapid evaluation of any changes and alternatives. Sixty-six removal and erection plans were detailed, using cranes with capacity between 145 and 200 tonnes.
The automation required substantial design efforts before the first drawing could be produced but paid for itself many times over during the course of the project. The ability to rapidly assess and refine alternates, in hours instead of days, was critical to respond to field situations and maintenance of schedule.
Jacking Tower for the 6 Columns at the Steel Bents supporting 1750t
Final Panel Installation
Solution: Parametric Analysis
Lifting Frame used for Stacking Panels in the Tent in Accordance with the Stacking Procedure
Yellow Lifting Frame Panels up to 85t
New Structure Beside Existing Structure
Old Deck Panel Removal and Placement onto Transporter