December 4 2020
A Closer Look at Toronto’s Gardiner Expressway Strategic Rehabilitation Project
Currently underway, the Gardiner Expressway Rehabilitation project involves the full deck replacement for 1.1 km of elevated roadway, from Jarvis Street to Cherry Street, as well as several off and on ramps.
One of the most common questions we’ve heard throughout this project is, “Why are you replacing the deck when the substructure is falling apart?” We spoke to Stephen Brown, Entuitive Principal and Bridge Engineering Leader, to dig deeper into the details of this exciting project.
The answer is that there is more than meets the eye here. A layman’s view, formulated by driving or walking under the bridge, is radically different from the reality of the structural issues with the bridge itself. What appears to be in disrepair is in fact mostly cosmetic. The structure is strong. The bridge deck itself is what is truly in need of repair and is what is driving this strategic rehabilitation.
This is accelerated bridge construction on a large scale, rehabilitating a critical piece of infrastructure. Like all successful projects of this type, this work comprised a design that was to be constructed rapidly, a tremendous amount of planning prior to the start of construction, and flexible implementation to address the inevitable unanticipated situations.
Work started in July 2018 and will be complete in 2021. This is an extremely tight timeframe for work of this magnitude and is possible only because of the design and construction efforts of the City of Toronto, WSP (the owner’s consultant), and AECON, with their subcontracting team and engineering support, including Entuitive.
Below, we look at how each project partner is contributing to the novel approach that has enabled project speed and efficiency.
The City of Toronto: A Creative Approach & Strategic Planning
The new superstructure is primarily composed of precast elements consisting of a pair of steel girders and a precast concrete deck. The precast elements are cast in an off-site, temporary facility and then brought up onto the deck for placement, with the joint between the precast elements cast with ultra-high-performance concrete.
The interesting strategy here involves holding the entire bridge up on the diaphragm between the girders, rather than on bearings at the ends of the girders.
This solution has enabled the team to build pedestals to support the new bridge in advance of taking the old bridge out – the result being a substantial compression of closure time, and therefore a continuous construction schedule.
The pre-cast elements are landed on temporary supports that have been set to the required final location and elevation of the deck, and the bearings brought up to the underside of the diaphragm and grouted, ensuring a good fit and adjustability.
This innovative idea – brought to the project by the City’s design team – means that construction doesn’t require using the traditional process of lifting a section, fixing the pedestals, then bringing in a new piece, which on average, would take considerably more time.
The City’s approach focuses on the fundamental requirement of replacing the deck structure, leaving the pier bents for later work. The pier bents appear to be in poor condition, but evaluation of their capacity showed satisfactory capacity due to multiple layers of reinforcing within the body of the piers.
As such, they can be safely left for a later rehabilitation project. Furthermore, the new design is well-suited to accommodate their reconstruction due to multiple full-capacity shoring locations available at the ends of the girder pairs.
Aecon: Construction Solutions that Maximize Efficiency
The approach to panel removal and replacement was critical to making the system work and is part of the art of construction brought to the project by Aecon.
Aecon developed a precast operation, deck removal, and deck placement process to meet project requirements and schedule. After the precast elements are cast and stacked in the yard, they are brought to the deck on a conventional heavy girder transporter and lifted into place with cranes.
A two-panel-wide path has been maintained at all times to enable access to the multiple working areas. This approach depended on the ability of the existing deck to support the heavy loads from the cranes and transporter.
The deck removal operation followed an alternating pattern like that of the black and white keys of a piano. The deck was removed in every second bay (the “white keys”), with the cranes 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”.
This has proved to be a cost-effective and flexible approach that is adaptable to any number of special conditions.
Entuitive: Combining the Science of Bridge Engineering with the Practicality of Construction Engineering
Aecon retained Entuitive to provide engineering support because of our experience with cranes on bridge decks, which was critical to their approach, and because of our ability to develop the geometry control for the precast girder pairs.
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 drawing production for the deck removal and replacement work. This automation allowed for completion of the large volume of analysis required, and, critically, allowed for rapid evaluation of any changes.
The development of the screed* and geometry files required for casting the deck sections was similarly automated using the prescribed highway geometry to generate girder camber and deck edge plan and elevations for casting each of the 400 precast elements. 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. An automated approach to calculating these screed elevations was used, allowing the specified casting geometry to be checked for compatibility with the actual girder dimensions as the steel assemblies arrived at the precast yard, and adjustments could be made as required without delaying the fabrication schedule.
This project involved adopting an unusual approach to bridge deck replacement, implemented on a large scale. The individual design elements are largely conventional, but combined, they have produced a structure extremely well-suited to rapid replacement, something that Aecon has been able to implement very effectively through careful planning and skilled engineering support.
For more information about our Bridge Engineering Team, contact Stephen Brown.
*Screed is the elevation of the top of concrete withou the weight of the concrete, therefore when the concrete is added (i.e. deflects due to dead load) it is at the correct highway road elevation.