Passerelle Simone de Beavoir A lenticular pedestrian bridge adds to the engineering legacy of Paris
engineering | rfr - paris footbridge by henry bardsley
lenticular truss double curves french engineering rivers foundations
RFR ingénieurs, an multidisciplinary firm founded by Peter Rice, Martin Francis and Ian Ritchie in 1982, specialises in extreme and complex structures. Its use of contemporary material technology and construction meth- ods continues a tradition of Parisian technical modernity. One of RFR’s recent projects (1999- 2006) is a pedestrian bridge that crosses the Seine, linking two railyard redevelopments in the east of Paris: housing and a park at Bercy and tertiary industry at Tolbiac. The footbridge crosses the Seine in single span of 190 metres. There are three spans all together, one crossing the river and two cross- ing the quay side roads, in total 270 metres long. The formal structure is a pectinate len- ticular truss; its component geometries are reminiscent of some existing bridge forms of Paris, particularly the dominant arch shape and early experiments in suspension bridges by Navier. Also, opposing curves are char- acteristic of RFR’s work. The unitary central vesica, barged down the Rhine, by Vlissingen, Cap Gris Nez, and up the Seine, was pulled into position in one night, without falsework. Three deck ribbons follow two opposing
curves, the central deck rising with one curve whilst the flanking decks fall with the other. The truss is deep only where depth is useful — a covered belvedere on the river in the central lens gives shelter from the rain, and there is dept at the end points of support to accommo- date the different quay levels that characterise the morphology of Paris. The structure and the deck are geometrically separate. Two parallel vertical trusses, 5me- tres apart, place half the pedestrian surfaces outside the structure, with an open view to the river. This configuration makes the bridge quite slender transversely, which is compen- sated for by continuity with lateral spans. The shear strength of the truss is provided by the parabolic geometry of the booms for uniform loads and radiating obelisks for par- tial loads. The obelisks give bending restraint to the stiff compression boom. This system, invented for the specific context of the Paris Seine landscape, is called a pectinate truss. The compliant bending stiffness of the ele- ments provides shear stiffness without com- promising the differentiation of the tension and compression booms. The deflections of
the apparently slender structural depth are controlled by the moment continuity at the supports. The structure is intentionally flexible, able to react to wind and pedestrian move- ment. Movement, a reminder of high intrinsic strength and economy of material and energy, dampens vibrations, along with tuned mass dampers which limit vibration for safety and comfort. The deck has a designed porosity to avoid the very unusual Scanlan coefficients which are characteristic of this topology in section. At the abutments a pair of slotted verti- cal tension plates and bilinear struts transfer forces from the truss to the foundations. The foundations are reinforced concrete barettes, anchored in the underlying limestone. Tension foundations using piles, whether prestressed or with corrosion resistant reinforcement, are not presently recognised by French national codes, so pre-stressed proprietary ground anchors have been used. These are placed in a way that allows them to be monitored, and eventually replaced.
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