Das »glücksrad« des Baseler Münsters. Baugeschichte und baukonstruktion eines hochmittelalterlichen radfensters

The preserved original fragments of the northern rose window of Basel's cathedral serve as the basis for a detailed analysis of this unique rota. Being the only known example of a large-sized wooden window tracery, its details and its structure show interesting similarities to contemporary stone structures. The so-called Glücksrad of the northern transept, built in the 1220's, has a diameter of about 5,2 m. The structure is carved entirely out of oak wood and is set , in a stone embrasure. It survived the great Basel earthquake of 1356, but was considerably deformed. In a conservation campaign in the late 18th century, only minor repairs of the structure were made. The additions show a sensitive approach towards the original medieval construction. In the late 19th century, the wooden rota was replaced by a stone copy. The original wooden fragments were preserved, thus enabling a detailed analysis and survey of the surviving structural details. The details show a remarkable analogy to the stone structure of contemporary rose windows and to the first Gothic tracery windows that were developed in the Ile-de-France at the same time. Of special interest is the combination of wrought-iron dowels and bolts with the wooden framework, as well as the insertion of the wooden structure into the ashlars of the stone embrasure. The close examination of the joints enables a reconstruction of the assembly: apparently, the rota was inserted after the masonry of the transept was completed. After the insertion of the arcades and the posts, the installation of the inner oculus locked the complete structure in its embrasure. A survey of cracks, deformations, and details of the repairs gives an insight into the structural principles and deficiencies. Different models of the transfer of vertical loads and wind loads are developed and discussed. The open oculus in the rota's centre enabled the absorption of deformations such as the considerable lowering of the gable's stone courses onto the rota during the great earthquake of 1356. Grooves and recesses in the wooden fragments allow a reconstruction of the medieval wrought-iron window bars and of the structure of the glazing. They can be compaired with historic records on the rota.