SWISS ENGINEERS
CHRISTIAN MENN

“[S]uccessful design of a perfect structure can never be performed only on the basis of general rules concerning structural system, dimensions and proportions alone, as long as the design lacks in originality and individuality.”

Christian Menn (born 1927) entered the Eidgenössische Technische Hochschule (Federal Institute of Technology) in 1946 and two years later began taking classes from Lardy. Following graduation, he entered the Swiss military service. In 1951 he contracted tuberculosis and spent fourteen months in a sanatorium where he began theoretical studies on the structural analysis of spiral staircases. Upon his recovery, he accepted an offer to assist Professor Lardy, with whom he spent the next three years (1953–56) at the ETH. He, along with two others, acted as the usual teaching assistant for Lardy’s courses and helped Lardy with his research and consulting. In addition, he completed his dissertation and in 1956 received the doctoral degree with Lardy as his principal advisor.

Menn’s earliest bridges were relatively long-span deck-stiffened arches in the tradition of Maillart, but eventually the revolutionary new material—prestressed concrete—began to open up a new vision of design for him. He saw that prestressing could actually replace the arch itself. The deck of his arch bridge could then become the main supporting member without the arch. Moreover, during construction, no scaffolding would be needed to support the arch and no curved formwork would be required to shape it.

The Felsenau Bridge is an example of such a design, which dispenses with the arch form and carries the roadway solely by a curved hollow-box beam that has been prestressed. The effect is a structure of exceptional lightness, a stripped-down form of transparency that satisfies all three of Menn’s main criteria for design: efficiency, economy, and aesthetics.

One of the uncommon features of the Felsenau design is the single box for a wide roadway. Previously, prestressed segmental bridges had two boxes, one for each half of the bridge. The single box with wide cantilever slabs requires less material than would two boxes; however, those two transverse cantilevers could have caused deflection problems had Menn not decided to use partial prestressing. The slabs are transversely prestressed for dead load only, and additional non-prestressed steel resists the live load. In this way the permanent loads are balanced, and there is little tendency for vertical deflection caused by creep under prestress-induced bending. At the same time, some cracking is tolerated under full live load, as it would be in a normally reinforced concrete structure. Compared to a double box, the single box has the cost-saving advantage of halving the forming operations and allowing the cantilever slabs to be built at a later time on simple scaffolding supported from the completed box girder.

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