CORNELL UNIVERSITY - STRUCTURAL SYSTEMS II
PROFESSOR - MARK CRUVELLIER
TEACHING ASSISTANTS - MADELEINE EGGERS & JEREMY BILLOTI
PROJECT BRIEF - STUDY AND RE-CREATE STRUCTURAL SYSTEM MODEL OF EXISTING ARCHITECTURAL PROJECT
COX RAYNER ARCHITECTS & ARUP GROUP ENGINEERS
The Kurilpa Bridge is the world’s largest tensegrity bridge located in Brisbane, Australia and was awarded ‘World Transport Building of the Year’ at the 2011 World Architecture Festival. The bridge is a $63 million dollar project which connects Kurilpa Point to the central business district across the Brisbane River, fulfilling the need for a pedestrian and cyclist only bridge.
The bridge was designed by Cox Rayner Architects in coordination with Arup engineers in order to resolve the complex tensegrity system of the bridge. The bridge is just over 1500 ft long and its largest span is close to 420 ft; in total the structural system contains 4.2 miles of steel cable and about 550 tons of structural steel. The bridge itself consists of a 21ft wide deck covered by a canopy, both of which are held in place by tensegrity elements. Construction of the bridge was so accurate that the completed construction was within 0.5 inches of its planned design.
The masts resist lateral loading through the use of stay cables, which run from the tops of the masts down to brackets connected to the bridge deck girders. Each mast has at least two stay cables, oriented in opposing directions to achieve equilibrium. The mast stays not only keep the slanted columns from falling over, but they also resist against the forces applied by the cables that connect to the spars.
Horizontal spars, spanning perpendicular to the length of the bridge, are supported by means of tension cables connected to the tips of the masts and to tie-points on the bridge deck. The cables are connected to the ends of the masts and spars by a pinned connection created by a tension fork and flat steel plate. Once again the use of a pinned connections, in this case a literal pin, eliminates the moment forces, allowing for a purely axial loading on the member. The tension fork, with the cable fixed to it by means of a turnbuckle, is pinned to the steel plate, creating a true pinned connection.
The profile of the masts and cable in elevation reflect the bending moment diagram. The bridge deck is supported inward of the ends of the tensegrity portion, essentially creating two cantilevers at the ends. Therefore the greatest bending stresses are at the two supports, which is reflected in the fact that the masts at those locations are both larger and taller. Where the bending stresses are less, the two ends and the middle, the masts are smaller in diameter and shorter.
The use of tensegrity as the main structural system for the Kurilpa Bridge is to create a floating bridge deck that spans the Brisbane River with minimal impact. While a more conventional suspension style bridge could achieve the same effect of a thin, floating strip that spans the river, the use of tensegrity creates a more sculptural piece in the city. The density of the cables, masts, and spars begins to define an architectural space above the walkway, that “encloses” and emphasizes space.