DESIGNWORKS Vol.12

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Topic伝統建築を支える現代の技術‐平成の大改修‐Major renovation during heisei era: Supporting traditional architecture with modern technology平成10年から始まった国宝唐招提寺金堂の保存修理事業が21年の秋に落慶を迎えて完了した。明治の修理から100年を経た今回の平成修理は、材料の経年劣化に対する部材補修というよりも、柱の内倒れの原因を解いてその補強を求めるものであった。あわせて耐震性能についても検討を加えるなど、平成の保存修理は、構造上の問題を解決することを主な目的として計画され、構造技術者の参画が求められた。柱の内倒れは大きな軒屋根重量によるもので、水平抵抗力の乏しい骨組みを有する伝統的な社寺建築に共通の特徴である。大虹梁で高く持ち上げられた内陣空間をもつ唐招提寺金堂では特に顕著であり、明らかに構造架構の仕組みに起因するものである。そこで、今回の構造補強が唐招提寺金堂の1200年の歴史の中でどのように位置づけられるのかを確認するため、創建時からこれまでの構造架構の変遷を現代の解析技術で見直すことから始めている。創建時のシンプルな屋根架構は900年を経た元禄時代に大きく改変された。軒先の垂下を減じるために、屋根を高くして屋根内に補強部材を配し、内倒れ対策として、内陣に方杖を現すことまでも行われた。明治の改修は、この方杖を撤去するために、屋根裏での補強材をさらに増加させ、洋小屋トラlarge koryo beam, and was clearly the result of the structural frame mechanism. Accordingly, in order to confi rm the role that the new structural reinforcement would have in terms of the 1200-year history of the Toshodaiji Temple main hall, the process began with the use of modern analysis technologies to review the changes in the structural frame since the temple was originally constructed. The simple roof frame underwent a major transformation in the Genroku period (1688-1704), 900 years after the temple was constructed. In order to eliminate the problem of drooping eaves, the roof height was increased and stiffened members were placed on the inside of the roof. Angle braces were even placed in plain sight in the inner sanctum to prevent inward collapse. In the restoration conducted during the Meiji era, the number of stiffening members inside the roof space was increased in order to allow removal of these angle braces, and a roof truss was added. However, these measures were unable to resolve the problem of inward collapse. Throughout repair history, the temple was subjected to successive renovation, and for this reason structural reinforcement is not simply a matter of adding stiffened members to an existing frame. It is important to clearly separate the reinforcement system from the traditional frame system of the original temple, in order to preserve the temple's authenticity and out of consideration for future repairs. In the Toshodaiji Temple main hall restoration, it was decided to incorporate an angle brace mechanism in the space inside the roof, in order to allow the horizontal forces in the temple inner direction that would produce inward collapse to counteract one another. In addition, earthquake-resisting performance was provided by horizontal trusses placed in the ceilings of the aisles (irikawa tenjo). The restoration of the Toshodaiji Temple main hall promoted a widespread recognition of the effectiveness of structural analysis, extending far beyond the relatively small circle of people involved in the construction of temples. As a result, there are now great expectations for the role of structural engineers in future restoration of cultural properties. (Tadashi Nagase)スを組み合わせたが、内倒れを解決することは出来なかった。構造補強は、このように歴史的な改修変遷を経ているだけに、現存する架構に補強部材を追加するだけの単純なものではない。補強システムを伝統的な建物本来の架構システムと明解に分離することで、｢ほんもの｣としてのAuthenticityを保持して、将来の改修にも配慮している。今回の補強では、内倒れを生じさせる建物内側に向かう水平力を互いに相殺する方杖機構を屋根裏に組み入れることとした。あわせて入り側天井に水平トラスを設けることで耐震性に配慮している。唐招提寺金堂の保存修理を通じて、構造解析の有効性が建築関係者の枠を越えて広く認識され、今後の文化財保存修理において構造技術者の役割が大きく期待される状況が作りあげられている。(長瀬 正)保存修理:奈良県教育委員会国宝唐招提寺金堂保存修理事業委員会構造解析:竹中工務店 長瀬正・佐分利和宏保存・修理・施工:奈良県Preservation and repair: Nara Prefectural Board of EducationRestoration and repair committee of the Main Hall of ToshodaijiStructure analysis: Takenaka Corporation, Tadashi Nagase, Kazuhiro SaburiConstruction works: Nara Prefectural Board of Education 29The restoration of the main hall of Toshodaiji Temple that began in 1998 was completed in the autumn of 2009. This new restoration comes 100 years after the last major repair work, conducted during the Meiji era. This time, the work was done not to repair aging members but rather to determine the cause of the inward collapse of the columns and provide reinforcement. A study of earthquake-resisting performance was also conducted, and as the main purpose of this restoration was to resolve structural problems, the participation of structural engineers was needed. The weight of the roof and eaves was determined to be the cause of the inward collapse of the columns. The meager horizontal resisting force of the frame structure is a common feature of traditional temple architecture. It was particularly conspicuous in the main hall of Toshodaiji Temple, which has an inner sanctum space that is raised high by a 30Topic