By Michael Meschino, P.Eng., Yolles Partnership
Pearson International: New Terminal 1 Building Structural EngineeringEngineering
A significant component of the redevelopment of Pearson International Airport is the construction of a new passenger terminal facility to replace the aging Terminals 1 and 2.The first stage of the new...
A significant component of the redevelopment of Pearson International Airport is the construction of a new passenger terminal facility to replace the aging Terminals 1 and 2.
The first stage of the new terminal construction consists of the Central Processor building, along with two piers for transborder and domestic flights. The facility that opens this fall comprises 261,500 square metres, part of a development that after future phases will be 424,500 square metres.
The new terminal building has large open spaces with an abundance of natural light, and promises to be one of the architectural jewels of the city. The building’s highly developed architectural qualities and complex geometry presented a number challenges to the structural design. The challenges included designing a 66-m span arched roof built using top-down construction, and a 70-m high apron control tower incorporating a 50-tonne tuned mass damper system. These and other design issues had to be resolved within a fast-track design and construction process.
Central Processor overall structure
The Central Processor building incorporates all the passenger processing functions. It is divided into two main areas, known as the “high roof area,” which houses the main Departures hall, the baggage claim hall, and greeting areas; and the “liner area” where holdrooms, security screening and Canada Customs functions are located. The high roof area is a four-level toroidal structure set out on a 2.25 degree radial grid arrangement. The liner is generally a three-level structure, supporting partial fourth levels and the apron control tower. The liner forms the transition zone from the high roof area to the piers by reversing the radial grid pattern of the high roof area and subdividing it so as to mesh with the layout of the various piers.
A main feature of the Central Processor is the long-span roof over the departures hall. For architectural reasons an arch roof system was selected to keep the structural depth to a minimum and maximize the amount of natural light entering the hall. The roof consists of 43 two-hinged arches, 1,400 mm in depth, spanning 66 metres between pin supports. The arches are oriented on a radial grid spacing that varies from approximately 8 to 11 metres. Every fourth arch is fully exposed to view within radial skylights, while elsewhere only the arch bottom flanges are exposed. All arches bear on graphite bushings over 200-mm diameter pins fabricated from grade 350 steel. Outward thrusts generated by the arches are transmitted down and equilibrated by the Level 3 floor structure below. An insulated membrane covers the 75 mm steel roof decking, while interior surfaces are finished with a combination of metal panel and drywall materials.
At the south end of the arch spans, thrusts are transmitted through elegant, architecturally exposed steel wishbone-shaped assemblies. The “wishbones” are splayed relative to the arches, and also form part of the roof’s lateral resistance system. The individual wishbone assemblies have a wide-flange cross section fabricated from plates of varying depth. By cutting back the wishbone web plates and using hidden bolts to connect the wishbone assemblies at each arch location, the designers achieved an elegant connection at these geometrically complex and structurally critical bearings.
The schedule dictated that construction begin before all the necessary program requirements could be determined for the floor structure design. As a result, it was decided to first design and construct the high roof structure, with its supporting columns and caisson foundations, followed by the floor structure. This top-down approach was complicated by the fact that the arch roof system required the Level 3 floor structure below to resist the outward thrusts generated by the arch action.
To overcome this challenge, one central interior bay of columns and caisson foundations was designed and constructed to enable one line of the interior columns to be extended up to the roof level. These components temporarily support the arches until the floor steel could be designed and installed under a follow-on contract. Since the temporary support system was highly dependent on the trade contractor’s method of erection, the high roof steel was tendered at a stage when design was only 60% complete in order to obtain early input from the trade contractor on this critical aspect of the design.
High-lift column grouting
At the north end of the arch roof system, thrusts at each arch are resolved down to the Level 3 floor system using a built-up plate diagonal tension member between two architecturally exposed circular columns. The tension member generates a 5,600 kN (1,260,000 lb.) vertical compression force in the northern 700-mm diameter columns. The structural design of the arch roof system required that the 700 mm diameter tube columns be filled with concrete and reinforced with twelve 30-m bars.
Due to the top-down construction approach, these columns had to be erected full height (approximately 31 metres). Their upper 24 metres also remain architecturally exposed. In order to eliminate the need for grout ports in the architecturally exposed portions, the columns were grouted full height in one continuous operation using a single port at the base of the each column.
After a single test column proved that the grouting techniques, concrete quality and consistency were sound, production grouting began. Excellent speed was attained, with the entire operation taking only 15 minutes per column.
Transparent curtain wall
Elegance and transparency were the primary architectural design objectives for the Departure Hall’s curtain wall system. Yolles engineers worked with the architects, Airport Architects Canada, to create aesthetically pleasing details and structural form. Thin structural elements were designed to coincide with various horizontal and vertical mullion locations to minimize the obstruction of light. Member slenderness was maintained with a minimum of structural components by ensuring that all components functioned together in providing overall support to the curtain wall system. The result is a very light structural back-up system that compliments the curtain wall and achieves the desired transparency.
The north wall system is 13 metres high, and over 300 metres in length, with four vertical expansion joints. The east and west wall systems follow the shape of the arched roof, varying in height from 13 metres at the north end to 20 metres at the peak, and down to 8 metres at the south end. Since the arch roof drifts horizontally as it deflects vertically under snow and wind loading, an expansion joint isolates the east and west wall systems from the arch movements, while providing the necessary lateral support against wind forces.
In both wall systems, the curtain wall panels are connected to a series of horizontal channels spaced vertically at 4-m centres. These coincide precisely with the elevation of horizontal mullions to enhance the transparency of the system. The channels and curtain wall weight are supported by a system of double solid rod hangers, 25 to 30 mm in diameter, located to coincide with vertical curtain wall mullions and curtain wall-to-channel connections. Lateral wind loads are resisted by elegant, castellated brackets that support the channels at the curtain wall connection points. The brackets transfer the wind forces to the 700-mm diameter concrete filled columns at the north wall, and to parallel chord trusses of varying height at the east and west walls. By prestressing the hanger rods, the brackets, channels, and trusses function together to maintain stability and thereby eliminate the need for additional bracing elements.
Apron control tower
The apron control tower rises 43 metres above the liner roof, and sits 70 metres above the apron level. Structurally, the tower is supported by a rectangular braced core of six wide flange columns within its 10-m diameter shaft. The tower core columns are transferred at the liner roof lev
el (Level 4) to the main liner column grid by a full-storey transfer system. The transfer system consists of five 3,100-mm deep plate girders, each weighing approximately 30 tonnes, supporting the tower core columns.
Fast-tracked within a construction managed process, the final design commenced in December 1998. The Central Processor high roof steel was issued for tender the following February and by December 1999 approximately 18,000 tonnes of structural steel had been designed and issued for the construction of the Central Processor high roof area.
By October 2001, virtually all the 32,000 tonnes of structural steel and all concrete work for Stage 1 had been designed and issued for construction. This period included a one-year major redesign of the liner area to suit updated program requirements. In the first three years, over 300 milestone structural drawing issues were made; the equivalent of two major issues per week, every week, for three consecutive years!
Owner: Greater Toronto Airports Authority
Structural Engineer: Yolles Partnership (Hugo Blasutta, P.Eng., Michael Meschino, P.Eng.)
Architect: Airport Architects Canada (SOM/Adamson/Moshe Safdie)
Construction Manager: PCL/Aecon Joint Venture
Collaboration on Central Processor Arch Roof Concept: Ove Arup & Partners