The Hagia Sophia Dome is one of the most striking features of this building in Istanbul. Now a World Heritage Site and museum owned by the Turkish government, the building was originally constructed as Christian church and opened in 537. After Muslims took control of the city the building became a mosque in 1453. It continued to serve as a mosque until 1931. Four years later, the building opened as a museum.
The massive dome is the most outstanding architectural feature of the Hagia Sophia. The sheer size of the structure would make it a remarkable undertaking to build, even in present day. The diameter of the main Dome is 108 feet, and its peak is 180 feet above the floor. The height was increased during the first rebuild as part of the redistribution of the weight. The dome was build almost 1,500 years ago, when the builders did not even have access to steel. It is said by many to be the finest achievement in Byzantine architecture as the dome incorporated design features that had never been used before.
The size and weight of the dome have resulted in a number of structural failings. The first collapse happened when the cupola fell in 558 following an earthquake. The dome was rebuilt within five years. Reinforcing ribs were put in place to spread the weight more evenly. Because of the ribs the dome has often been compared to the inside of a raised umbrella. However, the ribs were not sufficient to prevent further collapses. Now only the two sections at the north and south of the dome remain from the original construction. Another weight saving strategy was to insert windows all around the base of the dome. It is the light flowing through these windows that is responsible for giving the illusion that the dome is floating