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Akbari Z, Neyestani J, Hejebri nobari A, Nasiri M R. (2023). The Process of Changes in the Appearance and Structural Elements of Bulbous Domes During the Timurid and Safavid Period in Central Asia and Iran. Parseh J. Archaeol. Stud.. 7(23), 243-264. doi:10.30699/PJAS.7.23.243
URL: http://journal.richt.ir/mbp/article-1-597-en.html

Abstract
Bulbous domes are among discontinuous double-shell domes and were built in Iran in the early nineth century A.H. Previous domes were shaped pointed and there was no bulge coming out beyond their base diameters. But in the Timurid period, dome profile was changed so that it began a return below the base of the arch, producing thereby a slight bulge. The main question is that what are the reasons for the changes in the appearance and structural elements between two shells after the initial formation of these domes in Central Asia and then their evolution in Iran. This research is written with the aim of finding the hidden aspects in the development of bulbous domes based on both historical documents, and architectural remains, and also examines the reasons of the changes created in these kinds of domes during four centuries in the context of cultural Iran. This study has a descriptive-analytical approach for exploring case studies, which are selected based on primary and secondary sources and also field research. The appearance changes of bulbous domes include removing flutes from the outer shell, adjusting the height of the drum and creating more bulge at the place where the outer shell joins the drum. In this article, the structural changes to improve the strength of the domes, the change in the number and proportions of the stiffeners to prevent the damage of the dome, as well as the shape and arrangement of the wooden struts to increase domes flexibility against seismic shocks, have been examined.
Keywords: Bulbous dome, narrow ribs, stiffeners, wooden struts, Timurid-Safavid architecture.

Introduction
Since the most obvious feature of Timurid architecture is its splendor, the architects of this period paid special attention to the dome than other architectural elements. In this era, domes were raised on high drums, and in order to achieve desired height on the exterior, architects usually built another shell on the top of the internal one. In such a way the dome profile is changed, so that it began a return below the base of the arch, producing thereby a slight bulge. Therefore, the bulbous dome is one of the innovations of the Central Asian architects during the Timurid period. The first cases of the bulbous domes in Central Asia have flutes on the outer shell; in addition, they have an oval-shaped structure. Construction of this domes continued with a few changes during the reign of Timur’s successors in Great Khorasan and within the current borders of Iran. To put it more clearly, after the Timurid period and during the rule of Safavids in Iran, construction of these domes continued with few changes. This study is dedicated to the analysis of the process of changes and also reasons for these changes in the appearance and structural elements of the bulbous domes according to the case studied in Central Asia and Iran during the Timurid and Safavid eras. The main question is, what changes have occurred in the appearance as well as structural elements, and as a result, the stability of these domes in the Timurid and Safavid periods in Central Asia and Iran, and what were the reasons for them.
Theoretical method of this research is based on the cultural history approach. Therefore, each case is viewed as a culture. Concerning this, three domes were selected from different regions of Central Asia (Turkestan, Samarkand, and Herat) and seven cases from Iran (Khorasan and Isfahan). Three main criteria for selecting the case studies can be listed: 1. Geographical and historical context; 2. Similarities of the cases during the Timurid period in Central Asia and the Safavid period in Iran, and; 3. Authenticity of the domes.  

Discussion
All the studied domes in Central Asia have a lofty cylindrical drum. The height of the drums was adjusted in Iran during the Timurid era, in Khorasan and also in the central regions of Iran. It is worth to mention that, construction of high drums in Central Asia, during the reign of Timur, and in Iran, during the reign of Shah Abbas I, can be seen as an expression of the visual effects of architecture as a manifestation of the power and reflection of the perfectionist personality of those Kings. 
The most important exterior characteristic of the studied domes is in the external shell (Avgon), where the drum and the external shell connected to each other. Therefore, in the bulbous domes, part of the weight load of the outer shell at the junction of the dome and drum is directed inward and less pressure is applied to the base of the monument.
Flutes move from their common place at the top of the external shell to the bottom, and transfer forces and lateral loads to the inside, and enhance the resistance of the dome against the thrust forces.
In Central Asia’s domes, we couldn’t find narrow ribs on the surface of internal shell. This is despite the fact that in most of the studied cases from the Safavid period, narrow ribs were used to transfer the weight of the stiffeners to the lower parts and base of the monument. It seems that, utilize of the narrow ribs in the Safavid domes was the invention of the Iranian elite architects to give cohesion to the double-shell domes, especially bulbous ones.
From the earliest bulbous domes, we can see stiffeners between two shells. These elements have different shapes. Establishing stiffeners, especially in the Avgon part, greatly reduce the destruction of the dome during earthquakes and shows the architects’ awareness of stiffener’s structural role and function.
The wooden struts are arranged somewhat circularly to help connect the stiffeners to increase flexibility against earthquake shocks.

Conclusion 
Changing process of bulbous domes in Iran includes removing the flutes from the external shell, adjusting the height of the drum and creating an avgon with more bulge at the junction of the external shell and drum. Domes with flutes in the Central Asia were a main characteristic of monuments. It seems that use of flutes, in addition to structural reasons, also had an aesthetic aspect; because, vertical and parallel concave and convex lines on the external shell make the domes look higher and add to the glory of the whole monument. Moreover, flutes help in transferring the thrust forces entered into the building and have a function like an avgon at the place of joining the dome’s drum. It seems that removing the flutes from domes in Iranian architecture of Timurid period should be considered as a result of the lack of a single political power in different regions of the country. Among other fundamental changes, we can notice domes with more bulge. During the Safavid period, architects learned and experienced the basic stages of bulbous domes and gained sufficient skill with regard to the morphological elements of dome and its strength. In this way, for aesthetic purposes, structural solutions such as creating narrow ribs on the internal shell of the dome and deepening the stiffeners in the avgon part have been used to maintain the integrity of the external shell. One of the reasons for the construction of narrow ribs is to transfer the compressive force of the stiffeners to the lower parts and bases of the dome and to strengthen their stability. In some cases, ribs were not used to connect stiffeners to the internal shell due to small diameter of the dome span. The changes made in the stiffeners from the Timurid period to the end of the Safavid period should be considered more related to their number, shape and proportions, which can be attributed not only to the creativity of local architects, but also to the architects’ complete awareness of the role and function of the stiffeners. These structural elements prevent the destruction of domes due to thrust forces (wind and earthquake). Wooden struts also make it possible to create a better connection between the stiffeners and external shell, as well as the connection between the components of the dome.