How was the dome built in rome?

The Romans built domes in concrete. The

greatest example is the dome of the pantheon, a temple which has

been converted into a church. It is still the larges unreinforced

concrete dome in the world.

The dome was built on top of a rotunda (round

chamber) in concrete and was supported by a sophisticated system of

relieving arches. Its downward thrust rests on eight vaults in the

drum wall (which is 6.4 metres 21 ft. thick) which are supported by

eight piers. The structure had a symmetrical order. The height from

the floor to the oculus is the same as the diameter of the inner

circle of the rotunda, 43.3 metres (142 ft.). The height of both

the wall of the rotunda and the dome are the same of the radius of

the rotunda (half its dimension). Thus, the structure could contain

a sphere 43.3 metre in diameter and could fit into a cube with

43.33x43.33 m sides.

Despite its large size (43.43x2.67m) the dome

weight is only 5 metric tons. This was achieved with a number of

weight-saving strategies. Its thickness of 6.4 metres (21 ft.) at

the base narrows to 1.2 metres (3.9 ft.) around the oculus.

Successively less dense (and thus lighter) aggregates were used as

the dome went upwards. To make concrete, the Romans mixed a

volcanic rock called pozzolana with lime and added rubble aggregate

to make it solid. Tuff and/or travertine stone was used for the

lower layers, bricks for the middle ones and pumice and/or pottery

shreds for the higher ones. If the same concrete had been used

throughout, the downwards stress would have been 80% higher. Having

the empty space of the oculus instead of an apex also reduced

weight. This could be achieved only with Roman concrete. Modern

concrete is fluid. Roman concrete was less fluid and had to be

layered by hand. This allowed the use of different aggregates.

The interior of the dome has five rings of 28

evenly spaced coffers (sunken panels). This is a decorative

technique and this specific kind of layout is difficult to achieve.

In the case of the dome, it also reduces weight because of the

empty spaces in the squares. The arrangement of the coffers forms a

honeycomb structure, which has a high strength-to-weight ratio.

Thus, despite being hollow, it provides structural strength. The

weight of the dome around the oculus (which is 9.1 metre, 30 ft.,

in diameter) is carried by as ring arches which form it.