STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

It nevertheless suffered slight meridional

cracking but this could have been due to

thermal movement rather than primary load-

carrying stress. The walls of the hexagonal

drum which supports the dome are not

especially thick and the lack of outward

movement of these suggests that no significant

outward thrust occurred at the base of the

dome.

The dome of St Peter's in Rome (early seven-

teenth century) was designed by Michelangelo

Buonarotti and modified by Giacomo della

Porta. This too was a double-skin masonry

dome but it was much flatter than the

Florentine dome. Two iron chains were built

into it to prevent meridional cracking but

cracking which was extensive enough to

threaten the stability of the structure neverthe-

less occurred. The dome was stabilised in the

early eighteenth century, under the direction of

Giovanni Poleni, by the insertion of five

additional chains, at which time it was discov-

ered that one of the original chains had

broken. Poleni's measures were successful and

no further intervention has been required.

The architectural possibilities offered by the

double-skin dome were extended in the seven-

teenth century by the configuration adopted

by Christopher Wren in his design for the

dome of St Paul's Cathedral in London (Fig.

5.14). Wren was aware of the problems which

had occurred at St Peter's and adopted a very

lightweight double-skin structure of brickwork

masonry. The inner skin formed the inner

visible profile of the dome and was more-or-

less hemispherical. The outer structural skin

was a cone of brickwork which, with straight

sides, rose from the same springing as the

inner dome and provided direct support for

the masonry lantern which formed the climax

of the building's exterior. This outer structural

skin was bound with chains but the profile of

the cone was such that hoop stresses were

minimised. A lightweight secondary structure

of timber was built on top of the outer skin

and supported the visible external profile of

the dome. Thus did Wren create one of the

most spectacular examples of the architectural

scenic effect - one of the several examples of

ingenious architectural artifice which he

employed at St Paul's.^5

The brilliance of Wren's configuration was

that it allowed a dome to be constructed from

components of relatively light weight, which

could be supported on an underlying system of

slender columns and arches, but which never-

theless provided internal and external profiles

which were compatible with their respective

architectural schemes. A similar system was

adopted by Jules Hardouin-Mansart for the

Dome Church of Les lnvalides in Paris. This

type of arrangement was to be adopted for

virtually all subsequent large masonry domes

in the architecture of the West.

No significant developments in masonry

dome construction have in fact occurred since

the time of Wren and Mansart. The great

domes of the eighteenth century had similar

structural schemes and, in the nineteenth

century, masonry was displaced by metal and

reinforced concrete for long-span structures.

Post-and-beam structures

The most common architectural use of

masonry has always been as the vertical

elements in post-and-beam structures in which

timber or reinforced concrete was used for the

horizontally spanning elements. Buildings of

this type have been constructed from the very

beginning of the Western architectural trad-

ition. The majority are rectangular in plan with

flat or pitched roofs. The horizontal structural

elements are carried on sets of loadbearing

walls which are parallel to each other on plan

but the plan, for structural stability, must also

include some walls running at right angles to

these. The resulting buildings are therefore

multi-cellular. This generic plan and construc-

tional arrangement for loadbearing-wall build-

ings has remained unchanged for centuries.

The variety of architectural treatments which

have been possible has nevertheless been very

wide and the building type has been used in

5 A similar system of timber exterior supported on a

masonry core - in this case a single skin - had been

used at St Mark's in Venice in the eleventh century but

158 Wren may not have known of this.