Structural engineers have spent much of the last 14 centuries in awe of the Hagia Sophia church built by the Romans on a hill overlooking the Turkish capital Istanbul - then known as Byzantium, eastern capital of the Roman Empire. Emperor Justinian and his designers Anthemius and Isidorus won their place in engineering history with their bold use of semi-domes flanking an innovative main dome structure to increase the column-free space below. Between them the dome and semi-domes create the vast 75m by 69m free span central nave which forms the focal point of the building's interior.
Earlier structures like St Peter's in Rome incorporated a dome, but using semi-domes to increase the column-free space was relatively new when work began on the Hagia Sophia in 532.
The main dome is supported by four arches which in turn rest on 21m high piers built at the corners of a 30m square at the centre of the nave. These arches are reinforced and connected by concave masonry triangles known as pendentives - again thought to have been among the first of their kind.
Domes had previously been built on cylindrical structures with relatively few openings in their walls, but pendentives created the feeling of a structure floating in space when viewed from below.
The semi-domes jutting from the eastern and western ends of the structure rest on three arches arranged in a semi-circle at each end of the nave. While the semi-domes absorb lateral forces from the main dome at the eastern and western ends of the building, they have created other problems as their weight is making them topple slowly towards the centre of the structure.
Strangely, the original side walls had little in the way of lateral support. As a result the main dome appears to have pushed them outwards over the centuries despite extra bolstering from buttresses.
The building still standing today is not exactly the same structure built by Emperor Justinian. Speed of construction appears to have undermined the stability of the finished product, according to some commentators. This is thought to have contributed to the collapse of the main dome and eastern semi-dome in 538 after an earthquake in 537.
When it was rebuilt, Byzantine engineers constructed a higher, stronger central dome which stood until the 14th century when another earthquake prompted further reconstruction work.