STRUCTURAL DESIGN FOR ARCHITECTURE

Domes

The dome is a similar structural form to the

vault. In the history of the architectural treatment

of the dome, a similar gradual progression to

that which occurred with the vault may be

observed, that is a progression from fairly simple,

solid arrangements to gradually increased levels

of refinement (Fig. 5.9). The structural action of

the dome is slightly different from that of the

vault, however, because although, in theory a

perfectly intact dome exerts no horizontal thrust

on the supporting structure, which can be a

cylinder of masonry or even a ring of columns, in

practice, horizontal thrusts do frequently occur at

the supports. The precise behaviour depends on

the profile of the dome.

The easiest type of dome to construct is the

hemisphere because the constant radius of

curvature allows simple formwork to be used.

The hemisphere is not an ideal structural form,

however, because the lower parts of

hemispherical domes are subjected to horizon-

tal tensile stresses in the circumferential direc-

tion (hoop stresses), which can give rise to

meridional cracking (Fig. 5.10). The dome then

Fig. 5.9 Evolution of the form of the domed enclosure.
(a) The earliest domes were carried on very thick solid walls.
(b) At the Pantheon in Rome (2nd century CE) niches were
provided in the thick walls and the underside of the dome
was coffered to reduce the volume of material required.
Both of these devices were exploited in the architectural
treatment of the interior.
(c) The dome of the Hagia Sophia, in Istanbul (6th century
CE), and those of several other large Byzantine buildings
were carried on an arrangement of arches and pendentives
which transmitted the weight to four massive piers.

Fig. 5.10 Internal forces in a hemispherical dome. A

hemispherical dome which is intact exerts no lateral force

on the supporting structure. The lower parts of hemispher-

ical domes are subjected to horizontal, tensile hoop'

stresses, however, which can cause cracks to develop as

shown. If this occurs the individual segments of the dome

behave as arches and do exert horizontal force on the

supports. Most domes of masonry or unreinforced

concrete exhibit cracking caused by hoop stress and do

therefore require support structures which are capable of

resisting horizontal load. 153

Masonry structures

(a)

(b)

(c)