Felsenau Bridge

Robert Maillart를 계승하여 스위스를 대표하는 가장 유명한 교량 설계가인 Christian Menn의 부각은 prestressed concrete와 그 맥을 같이 한다. Ganter교, Sunniberg교 등 그를 대표하는 일련의 교량들을 언급하기 전에 오늘날 그의 명성을 낳게 한 계기가 되는 우리에겐 조금 낯선 PCS 박스 거더교를 우선 소개한다.

Aare 강 계곡부를 횡단하는 6차선의 교량 설계 공모전에 참가한 Christian Menn은 독일의 Ulrich Finsterwalder가 개발한 FCM(Free Cantilever Method, 외팔보 공법)을 적용한 17경간 1,116m의 Felsenau Bridge로 공모전에서 우승을 한다. 6차선, 교폭이 26.2m나 되는 광폭임에도 불구하고 횡방향 prestressing을 도입하여 1실(cell) 박스거더로 설계를 하는 당시로서는 상상하기 힘든 대담한 발상, 계획이었다. (치수 단면 보기)

주경간은 94m+144m+144m+94m, 측경간은 48m로 구성되어 있다. 주경간 교각이 12m 간격의 얇고 유연한 두 개의 기둥으로 되어 있는데, 이는 아마도 온도 및 건조수축시 종방향 거동을 효과적으로 제어할 수 있는 방안으로 보인다.

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. [The Art of Structural Design]

*Images from SOM(Skidmore Owings & Merrill Foundation)