An Analysis of Structural Reasons for the Use of Bulbous Domes and their Prevalence in Iran: A Case Study of Shah Mosque in Mashhad

Bulbous domes are among discontinuous double-shell domes and has been developed in Iran since the middle of the 9th century AH/ the 15th century AD, concurrent with Timurid period. In this shape of domes, the dome profile was changed so that it began a return below the base of the arch, producing thereby a slight bulge that. The terminology for bulge in Iranian architecture is Avgun. Bulbous dome is considered as a part of Iranian architectural culture and the architects of this land have used this complex structure in a way that has been developing ever since its formation. It seems that this shape of dome requires a precise assessment in terms of design, mechanism of power transmission, and features of structural elements. The main purpose of this paper is to show the role of Avgun as an architectural structure in the bulbous domes, which supposed to have just an aesthetic function. In this study, the bulbous dome of Shah Mosque was simulated using Abaqus software. Then, its responses to the static and dynamic loads were tested. In order to show the Avgun role, this dome was compared with another dome, without Avgun, that possessed all similar architectural characteristics. The analysis revealed that Avgun is not just an aesthetic feature and that its prevalence had a structural reason too; So that Avgun reduces the amount of the dome's weight pressure on the dome chamber bases.