In the race to build the tallest cathedral in the 13th century, the builders of Saint-Pierre de Beauvais pushed the technology to the limits. In 1284, only twelve years after completion, part of the choir vault collapsed, along with a few flying buttresses. It is now believed that the collapse was caused by resonant vibrations caused by high winds. As the floor plan shows, the original design included a nave that was never built. Thus, the absence of the shouldering support of the nave contributes to the structural weakness of the cathedral.
The north transept now has four large wood and steel lateral trusses at different heights, installed during the 1990s to keep the transept from collapsing. These temporary measures will be in place until more permanent solutions can be determined. Various studies are underway to determine with more assurance what can be done to preserve this magnificent structure.
Source : Wikipedia