Stiffeners (Khashkhāshi) in the Design of Iranian Detached Double-Shell Onion Domes: Rethinking the Definition based on the Study of Structural Form

One of the most innovative structural technologies in Iranian Islamic architecture is the detached double-shell onion dome consisting of the outer shell, inner shell, drum, and usually stiffeners. Stiffeners, called khashkhāshi in Persian, are brick structural elements constructed between the two shells. Design varieties, complicated constructional features, and the inaccessibility of the space between the two shells, have hindered a comprehensive study and documentation of these elements. This, together with the lack of a comprehensive definition in the literature, has directed the aim of this article towards understanding the essence and the structural role of this unique element by surveying an extensive range of structural forms of kashkhāshi’s, eight of which chosen here to represent the distinct designs and groupings of these elements. Required data is collected through surveys and direct observations of selected examples, the archival study of their historical and restoration documents, and examining the limited secondary scholarship. The dimensions, numbers, profile shapes, and locations of this selection are then compared using descriptive-analytical methods, descriptive-classification techniques, and three-dimensional modellings of samples. A comprehensive and grounded definition for the khashkhāshi is proposed as a result. Consequently, these elements are classified under three groups:
Those comprised of relatively thin and short walls located within the swelling part of the outer shell, representing a minimal design approach. This group stiffens the outer shell against deformation and bending through increasing its thickness;
Those extending along the profile of the outer shell to improve its bending resistance;
The horizontal extension of the first group, containing thin and tall walls of khashkhāshi playing three distinct functional roles: increasing the bending stiffness of the outer-shell, reducing the drum's buckling, and improving the bending stiffness of the inner-shell.