The Timber Skeleton

Abstract

On April 15th, 2019 a devastating fire burned down Notre Dame de Paris and left the cathedral without its central spire and ancient gable roof, better known as The Forest. The required reconstruction initiated a global debate between architects, engineers, and other stakeholders. The discussion was put to an end by the French authorities, who proposed an identical reconstruction within a timeframe of 5 years. The decision forced the construction crew into a difficult job of finding identical construction materials, eventually leading to environmental and humanitarian suffering in Ghana, where a tropical underwater forest was felled. Besides, the time pressure put the quality of the reconstruction at risk.

In the presented thesis, an alternative design for Notre Dame’s roof is proposed. First, the authorities’ reconstruction plans are analyzed to see which practical challenges will have to be tackled. An alternative reconstruction strategy is then developed for the choir and nave of the cathedral. Inspiration is drawn from Notre Dame’s initial construction in the 12th and 13th centuries, during which time pressure also played a key role in the design. The Master Builders’ clever construction strategies like integrated support structures and utilization during construction are indispensable parts of Gothic engineering and are used to tackle current problems.

The proposed alternative construction process consists out of three phases: Fast Fix, Field Factory, and Former Finishing. The resulting structure, The Timber Skeleton – named after Professor Jacques Heyman’s book: The Stone Skeleton -, consists out of stabilizing cassettes (Fast Fix), expandable roof segments (Field Factory), covered by a roof cladding with the same external appearance as before the fire (Former Finishing). Notre Dame’s attic is imagined to function as a publicly accessible workshop, dedicated to the constant maintenance of the cathedral and all its Gothic ornamentation. As an alternative to the difficulties the authorities are facing in finding proper timber, modern construction materials and prefabrication strategies are investigated. They are utilized to facilitate a relatively quick construction without obstructing other on-site restoration works. Laminated veneer lumber with cross-plies is used to design openworked members with structural expression.

For the particular geometry of the rafters, the boundaries of current analytical engineering tools have been explored. Different verification strategies have been assessed and compared to analyze the structural functioning and feasibility of The Timber Skeleton. A conventional stress analysis is eventually used for the structural verifications of The Timber Skeleton and its most important joints. Recommendations are given for further development of the design, in which mainly the moment rigid connections of the rafters will require numerical and physical testing. A Master Builder’s mentality of prototyping and trial and error will be needed to realize the proposed design.

The research shows how an alternative reconstruction strategy for Notre Dame de Paris could result in a good functioning structural design that pays homage to Gothic engineering principles. The design functions as an innovative example for how modern production processes could help to arrive at a contemporary interpretation of The Forest that functionally and structurally better fulfills the contemporary requirements.