Steel Structures Education Foundation

2006 "crossing the divide" - a pedestrian bridge
Award of Merit

Jessica Thatcher / Jennifer Thorogood / Melissa Ouellet / Yunlu Shen
McGill University

Competition Board 1

Competition Board 1 Competition Board 2 Competition Board 3 3-D View of Bridge

Project Description

The Site
Avenue du Parc, between avenue Duluth and avenue du Mont Royal, is an area dense with both people and cars. The Mont Royal Park encompasses this westernmost edge of the Plateau, and serves as the summertime hub of the neighbourhood. The park space unrolls from the mountain, stretching eastward to avenue de l’Esplanade, where it meets the Montreal urban fabric. However, this green expanse is separated in two by Avenue du Parc, an eight lane street. How can a park be people friendly while also supporting a constant flow of vehicular traffic through its centre? The existing crosswalk, installed to accommodate pedestrians, is a make-shift solution that interrupts traffic and encourages jaywalking. The sight and sound of cars pervade and the park remains divided. We propose to situate our pedestrian bridge at this point: spanning avenue du Parc, in front of the Sir George-Etienne Cartier Monument. Here is the main point of crossing from the eastern to western side of the park. Our intervention facilitates vehicular flow, while allowing uninterrupted movement between each side of the park. In order to bring a stronger visual connection between the two park areas and to reduce the presence of motorized vehicles, the road will be lowered 2 meters in addition to constructing the bridge.

The Concept
The design was inspired by an element essential to the park: a blade of grass. As the wind blows, the grass elegantly bends and its structural capacity is revealed. The wide base tapers toward the tip, lessening the weight and thus enabling a greater height. The v-shaped cross section also adds to its ability to withstand strong winds. The form of the bridge grew out of these structural and aesthetic elements intrinsic to blades of grass. Situating the bridge adjacent to a Montreal landmark creates a complex design challenge. How can the bridge compliment and integrate with the monument, while retaining its individual identity?

The Deck Opens
The steel deck progressively widens from the eastern side of avenue du Parc, reaching towards the monument and the mountain with an open embrace, and meeting a concrete abutment that mimics the material of the monument on the western side. This material-based connection with the existing monument is emphasized by an exposed expansion joint.

The Tower Directs and Bows
The narrowing curve of the tower, from its massive base to delicate tip, points toward the angel statue at the top of the monument. When the viewer approaches the bridge from avenue du Parc, their view is pulled along this curve and ultimately directed at the angel. The lowering of the road allows the bridge to meet clearance requirements while limiting its height-- At 16 meters from the ground, the tower reaches only half the height of the monument. Together with the tower’s ability to bring one’s view to the statue, this unobtrusive height gives the impression of a homage to the monument and the mountain.

The Tower Frames
The opening at the base of the tower makes the crossing an intriguing experience. This triangular shaped opening mitigates one’s view of the statue. As the viewer ascends from the eastern side of the park toward the bridge, the statue is slowely revealed through the opening in the tower. Upon arriving at the base of the bridge, the statue is perfectly framed. Because of the complex shape and non-orthogonal configuration of the bridge, preliminary structural analysis were carried out using a finite element model in SAP2000, according to the Canadian Highway Bridge Design Code. Based on these results, the following member sections were chosen.

Components
Tower
Each leg has a hollow trapezoidal cross section, with the four sides fabricated from plates with varying thickness: 12 cm at the base and 2 cm at the tip. The two main plates are 5.5m wide and separated by 1.7m in the center. Vertical and horizontal stiffeners are used to prevent lateral buckling of the plates.

Deck
40mm thick Sandwich Plate System

Stiffening Girders and Cross Beams
To minimize deflection problems commonly experienced by cable-stayed structures, W 460 x 67 stiffening girders and W 200 x 15 cross beams are used to support the SPS deck.

Cables
22mm spiral strands, spaced at 2.0m along the arc length of the bridge deck, and 1.0 m along the arc length of the tower. The cables is pre-tensioned to eliminate sagging.

Abutment
Hollow reinforced concrete structure. Preliminary vibration analysis shows that the acceleration of the superstructure is slightly above the acceptable range stipulated in the code. However, the elastomer core of the SPS should provide sufficient damping to bring it back into that range.

Arch/Cable-stayed Combination
The bridge employs a hybrid structural system, combining the modern cable-stayed configuration with the classic arch form through the use of a unique cable configuration. In traditional cable stayed structures, the cables primarily provide vertical forces. In this case, however, the more horizontally inclined cables provide large compressive forces and introduce arch-like behavior to the steel deck, increasing the stiffness of the deck while allowing the expansion joint to minimize temperature effects on the bridge. This special combination also provides an additional measure of safety in case of vandalism. In the unlikely event of multiple cable failure, the steel deck will compress against the concrete abutment, allowing the entire bridge to behave like a classic arch bridge and remain erect. Only the expansion join will need to be replaced as a result.

Construction
The bridge is to be constructed using the push-out method. The tower can be prefabricated in one piece and transported to the site to be erected. The deck can be prefabricated in approximately 4 m-long pieces, to be connected on site with field bolts. Temporary supports would be provided at road median when the deck reaches half of the span.

Materials and Durability
The tower, girders and beams are produced from 350 W steel. The deck is fabricated from an innovative new material: the sandwich plate system, consisting of two steel plates with an elastomer core. The SPS is light weight, stiff, and has a built-in fire protection system. Galvanized bridge wires with high breaking strength, high Modulus of Elasticity, and good tension fatigue performance will be used for the cables. To prevent corrosion problems, the bridge should be repainted regularly. A cathodic protection system is installed, to be activated when necessary.

A New Landmark
The bridge does not only serve as a means to cross avenue du Parc. When Frederick Law Olmstead designed Mont Royal Park in 1876, he intended it for the working people, the majority of whom resided on St. Laurent street, east of avenue du Parc. By crossing the divide that removes the mountain from the Main, the bridge brings the mountain-park back to the people. Its inviting steps serve as a gathering place for Tamtams dancers, park goers and cross country skiers alike, playing an essential role in activities of all four seasons. With its organic design, the bridge emanates a breath of fresh air onto the Montreal urban landscape. At night, the illuminated bridge is enshrouded in mystery, offering a tentalizing play of shadow and light. Emerging from the darkness, it distantly echoes the thriving downtown core. With different lighting, it can be easily integrated into the various festivities that Montreal hosts, transforming into a new landmark in this historic city.

"crossing the divide" - a pedestrian bridge

McGill University

Faculty Advisor:
Peter Sijpkes

Amount: $2,000.00

Both iconic and fundamental in the worlds of design and construction, bridges bring together engineering and architecture in a unique conjunction: they provide the very essential example of form existing for function. The simple footbridge is one of the earliest known structures, accomplishing the primary function of any horizontal structure: spanning. Their design, both structural and architectural, explicitly and implicitly, complies with this simple requirement. Originally constructed, perhaps, from fallen logs or branches, the development of the bridge has, more directly than any other structure, followed the development of materials themselves. Simply moving from one side to the other of a stream, river, ravine, or street, has, in modern times, been elevated to an art form in itself. Bridge design is one of the most pure areas for testing architectural ideas. Reduced to one programmatic requirement, the bridge cannot hide its structural requirement; it must, instead, be celebrated and exploited, both architecturally and structurally.

Students are challenged to design a single span pedestrian bridge, on a site of the designers’ choosing. The structure must be primarily steel, but otherwise, the material palette is open.