By Maqsood Ahmed
At the specialty structures division of Halcrow Yolles, every request comes with unique challenges. Airport Architects Canada, — the architects for the Pearson International Airport, Toronto, Canada — had the challenge of designing the fast track, re-locatable walkways that are elegant and fit within the budget that was given by the airport authority.
The $1.5 million covered walkway canopies serve the east holdroom satellite terminal expansion of the airport.
Ongoing expansion at the airport and reconfiguration of the aircraft stands required walkway canopies to provide a sheltered environment for passengers walking from the satellite terminal to their aircraft. The canopy project consists of two separate walkways located at the opposite ends of the satellite terminal.
Along with meeting the budget constraints of the airport, the design and construction team had the additional challenge of having only four months to design, award, fabricate and erect the north canopy, with an additional two months for the south canopy. Airlines would occupy the facility on a preset date and the contractor had no room for errors.
The goal of the walkways was to provide a cost-effective system that could be erected quickly (while the satellite terminal was in operation), which would provide shelter from wind, rain and snow while providing maximum use of natural light and maintaining the view to the aircraft. A lightweight modular system of curved steel frames with translucent tensioned membranes was selected to achieve these goals and also to provide a unique visual impression to passengers.
The curving frames were designed to open up facing the airside to provide good views for the passenger and staff as well as to enhance the wayfinding aspects of the walkway. It was critical in the airport environment that we select a Class-A fire rated material. A fiberglass membrane with Teflon™ coating meets this requirement as well as provides maximum durability and ease of maintenance. The fabric, which is 70% reflective, diffuses the sunlight during the day and provides maximum indirect reflection of the electric light at night. A concrete foundation pad in the original design was eliminated to save additional cost. This decision created a new challenge of using the existing un-reinforced slab for anchoring the frames. A typical design check of the anchoring bolts for uplift resulted in insufficient capacity of the existing slab. Consequently, a more refined finite element model was run to obtain satisfactory results and determine the capacity of the slab. New footings had to be poured at a few locations.
Specifications for the fabric structure were stringent — wrinkle-free fabric with no visible seams was specified in addition to all the other requirements for a fabric structure. Sheerfil-V fabric was specified to get better stretch and tolerance in both warp and fill directions and less likelihood of wrinkles. The radius of the frames was revised to facilitate fabric installation and avoid stress concentration.
In coordination with the design intent, curved moment frames were selected to create the three dimensional shape required for biaxial tensioning of the membrane and provide slopes to avoid ponding of water. Fabric roll width and fabric seam locations were carefully considered when determining the spacing of frames and intermediate horizontal girt placement. No fabric seams are seen in the completed canopy — from front or back views. Moment frames in two sections were assembled on site without an extended end plate connection. Instead, a flush plate bolted moment connection was used to connect the two sections of the moment frames on site along with the side frames.
Approval was given to proceed with rolling of steel prior to shop drawing approval process. Details were reviewed and fine tuned during the fabrication process to achieve the desired looks.
Pre-fabricated fabric panels and steel frames were erected during the harsh winter of January 2007 and allowed the opening of the North terminal canopy at the end of January for the airline. Sectionalized fabric panels and field bolted connections were used to ease the possibility of dismantling at a later date and to speed construction to meet the schedule.
In addition to dealing with record cold temperatures, the on-site contractor had other surprises to deal with: A concrete slab below the finished layer was found to be missing in a few locations; new footings were designed, poured and cured in the extreme cold, yet the project still met the deadline. As the canopy was getting built, new opportunities were discovered to make the canopy meet the existing building terminal. Additions were designed, fabricated and erected just in time for the airlines to occupy.
The Terminal is expected to further expand and be reconfigured in the next seven years. Relocation of canopies will leave no trace of current existence except for anchor rods that can easily be cut off.