Abstract
Iranian architects, to stabilize historical structures, opted for reducing weight and lightening the construction instead of adding complexity. Aesthetic beauty was also important to them, and in this regard, they devised solutions that, in addition to reducing weight, also preserved the beauty and functionality of the structure. Lightweighting in Iranian architecture initially was a simple structural solution, but later architects were able to combine this structural solution with architectural approaches. While lightening the load, they created beautiful architectural spaces. The mausoleum of Mohammad Khodabandeh in Soltaniyeh, as one of the largest funerary buildings of the Islamic era in Iran, is a complex structure in which combined lightweighting techniques such as boxing (sanduqeh kardan), interlocking (kenobandi), and bracing (ghayd kardan) have been employed. It seems that this combined structural and architectural approach can be considered one of the most effective factors in the stability of the Dome of Soltaniyeh. This research seeks to answer the question: How were lightweighting solutions in historical structures developed in Iranian architecture, from a simple structural solution to the integration of structure and architecture for beauty while considering structural requirements? And how were these cases applied in the Dome of Soltaniyeh? This research is fundamental in its objective and has been conducted using a descriptive-analytical method. Information and data were collected through library studies in lightweighting and its types, along with field surveys in the case study. The research results showed that the use of techniques such as decreasing thickness from the base to the apex of the dome, the use of a double-shelled dome, as well as the presence of internal and external iwans, led to a reduction in the weight of the structure alongside the creation of unique architectural spaces.
Keywords: Structural weight reduction, Historic building, Lightweighting of historic buildings, Vaulted structure, Soltaniyeh Dome.

Introduction
Lightweighting as a method for stabilizing historical structures in Iran has been a significant challenge for architects. They aimed to achieve not only a reduction in the structure's mass but also to ensure its apparent and structural stability. Therefore, alongside focusing on structural solutions for lightweighting, they also paid attention to aesthetic factors and visual concepts. For example, 'panam bandi' (creating intermediate layers) was used not only for thermal and acoustic insulation and to provide human-scaled spaces but also for lightening the structure. In the space of dome chambers, the dome itself, as the covering, is the first and most important element in creating this space. In the construction of domes, there were always specific reasons, such as creating a covering for large spans without columns. However, structural necessities and spatial needs, like reducing the effects of heat and cold and the efficient use of materials, could also be among the other reasons. Lightweighting was a structural solution, and architects utilized this technique in various parts, from wall construction to roofing. Architects strived to use architectural solutions in combination with lightweighting, and this lightweighting led to the formation of new architectural forms. The aim of this research is to present various lightweighting solutions based on the study of Iranian architectural examples, with an emphasis on the case study of the Mausoleum of Khodabandeh, known as the Dome of Soltaniyeh. This is to demonstrate how a structural solution was transformed in combination with architectural space in Iranian architecture. The main research question can be stated as: How have Iranian architects been able to transform lightweighting, which is a purely structural matter, into a completely architectural solution for creating beautiful architectural spaces? The architecture of the Dome of Soltaniyeh is a suitable example for this study, as its structural stability has been achieved through this very method.

Discussion
A comparative study of lightweighting techniques in Iranian architecture and those employed in the construction of the Soltaniyeh Dome demonstrates that although many of these techniques initially served purely structural purposes, in the case of the Soltaniyeh Dome, they evolved into a sophisticated combination of structural engineering and architectural design. Traditional Iranian architects, throughout history, developed lightweighting strategies primarily to reduce the dead load of massive structures, thus enhancing their stability and resistance to natural disasters such as earthquakes. These methods included the reduction of cross-sectional areas, the use of lighter materials at higher elevations, the introduction of double-shelled domes, and the application of vaulting techniques such as ribbing and coffering.
In the Soltaniyeh Dome, while these classical techniques were present, the innovative aspect lies in the architects' deliberate integration of lightweighting methods with the creation of functional and aesthetically pleasing spaces. On the first and second floors, the incorporation of recessed niches and small iwans overlooking the inner spaces not only contributed to the reduction of the wall masses but also introduced dynamic spatial experiences for users. These spaces, while maintaining the structural integrity of the dome, also reflected the aesthetic values of Iranian-Islamic architecture, where the balance between engineering and beauty was meticulously achieved.
Moreover, in the upper levels, the use of external iwans around the structure provided further opportunities for lightweighting by removing unnecessary mass while simultaneously enhancing the building’s visual articulation and environmental performance, such as improved ventilation and light access. This strategy illustrates a unique architectural approach where structural necessity was seamlessly blended with spatial innovation.
The Soltaniyeh Dome, therefore, serves as a pivotal example of how lightweighting techniques transcended their initial structural purpose to become an intrinsic part of the architectural language. It showcases how Iranian architects moved beyond mere engineering solutions, using lightweighting not just for structural efficiency but also to enrich the spatial, visual, and functional qualities of monumental buildings. This holistic approach represents a milestone in the evolution of architectural and construction practices in Iran, setting a precedent for subsequent developments in Islamic architecture.
 
Conclusion
The principle of lightweighting holds significant importance not only in architectural construction but also in various discussions such as restoration and maintenance. This research has two main objectives. Firstly, to study and examine the various lightweighting solutions in different structural forms: flat, vaulted, and specifically domed. In the next stage, the aim was to understand the skillful combination of lightweighting solutions by re-examining the different components and elements of the Dome of Soltaniyeh. This includes how the main weight of the structure has been significantly reduced despite the presence of eight arches, and how the dome's form and the number of its shells have addressed structural issues, while simultaneously creating beautiful architectural spaces through the construction of vaults and shelves, and elegant iwans. Furthermore, the research explores how niches and arches are created in both load-bearing and non-load-bearing elements in a way that, while creating visual appeal, also contributes to the lightweighting of piers and load-bearing walls. The use of different methods significantly impacts the reduction of the structure's weight, especially when several methods are used simultaneously. In the Dome of Soltaniyeh, multiple lightweighting methods have been employed concurrently, from the dome's covering to the internal and external walls and facades. These lightweight techniques include:

     The construction of internal and external iwans on the first and second floors not only created architectural space but also significantly contributed to reducing the overall weight of the building. Reducing the weight of the building plays a crucial role in increasing its resistance to applied forces.