Reaching an excavated depth of 81’ below Market Street and through approximately 70’ of solid bedrock, made this project unique right from the beginning. The depth was not the only major constraint on the project; the site has 2 view planes from Citadel Hill that pass over portions of the building at varying heights, as well as a maximum allowable height due to sight line restrictions from Citadel hill. To make it even more challenging we put a road through the center of the building. The site is bounded by 4 streets, Argyle and Market which slope gradually to the North and Prince and Sackville which slope steeply to the East. These extreme grades added an additional layer of difficulty to an already challenging project.
From the beginning, we knew that this project would require an entirely new design process. We could not simply draw and design such a complex building using traditional 2D drawing methods so we turned to Autodesk Revit for the solution – to create a three dimensional model of the entire building. This allowed us to both manage the building and drawings in a much more efficient way, and also to visualize intricate parts of the structure to allow us a much more efficient design but also allow us to take the structural design to new heights. The architectural and mechanical contractors also used Revit for this project to help in the collaboration and clash detection for the project.
During excavation, the entire site had to have permanent rock bolts installed around the entire building to hold back the rock supporting the adjacent streets and buildings. The installation of the bolts needed to follow the excavation into the ground to prevent rock falls as well as provide a permanent tie back to the surrounding rock. The formwork for the foundation walls had to be specially designed to allow for the narrowest of cavities between the wall and the rock face to take advantage of as much square footage as possible on the constrained site. A special “one off” form system was designed using large steel plates on the exterior of the form that could be lifted up without needing to have a person work on the outside of the wall. Since access to the exterior of the wall was limited, the concrete contained special additives that would create a waterproof concrete so that no exterior waterproofing was required. A concrete ramp from Grafton street wraps around inside the building down to the lowest point in the structure where there are two full levels of parking.
After several design changes and public consultations, it was decided that the below grade convention center ballroom would be moved above grade. This required the entire design team, to find a way to support a large ballroom over an active street, and all above an equally large exhibition hall. This was accomplished by spanning Grafton street using five, 160’ long steel tub girders, traditionally reserved for bridge construction. Cherubini metal works and Harbourside engineering proposed a design for the girders that maximized the ceiling height below Grafton Street. They were so massive that they had to be shipped to the site in 3 pieces each, assembled on a temporary platform and then rolled into place. The girders have to support the street traffic, and also the retail space on level 4 and the ballroom on level 5, so we were concerned with vibrations. Tuned mass dampers were added to each girder to control structure borne vibrations along with the physical mass of the girders and the street landscaping.
A prominent feature of the building is commonly referred to as the “Blue Box” and was in of itself an extremely difficult design challenge. It required us to develop another “one off” design to allow the structure to cantilever up to 24’ form the nearest columns. Since this piece of the building houses the main ballroom we knew that is needed special attention. We landed on a solution of using composite columns, which is a steel column inside a concrete column. This allowed us to build the concrete office towers above the blue box and at the same time, to cantilever steel beams off of the steel columns deep inside the concrete columns. Another formwork challenge was how to create an architecturally exposed round column with a piece of steel beam sticking out the side. The solution was to provide a structural steel collar where the cantilevered steel beams meet the concrete column, creating a clean uniform finish. The blue box also features a series of slanted tapered columns along both Sackville and Argyle streets. These columns not only tie the floor and roof together for support of the roof and deflection control, but also act as a truss and bracing system all in one. This allowed the architect to achieve the clean and unobstructed look they were going for. The ballroom roof received nothing ordinary either, 160’ clear span steel trusses that were restricted to a maximum depth of 6’ at one end due to one of the view planes and the drive to maximize floor to floor headroom rounded out the design.
The convention center required a truck loading bay, which had to be designed to allow for 4 transport trucks and 2 large compactors to fit inside the building on the fourth floor. This required us to use sloping and stepped concrete structure as well as an extremely large capacity freight elevator to quickly move the freight to the required floors.
Even the office tower and the hotel tower forced us to rethink normal design and go outside the box. With concrete slab spans of up to thirty feet and some cantilevers up to twenty one feet, this was not an ordinary building. We were able to design columns that, while larger than normal due to the long spans, we were able to help the design team create an extremely open floor plan with minimal number of columns.