3.1 Introduction
The shapes which are adopted for structural elements are affected, to a large extent, by the
nature of the materials from which they are made. The physical properties of materials
determine the types of internal force which they can carry and, therefore, the types of
element for which they are suitable. Unreinforced masonry, for example, may only
be used in situations where compressive stress is present. Reinforced concrete performs well
when loaded in compression or bending, but not particularly well in axial tension.
The processes by which materials are manufactured and then fashioned into
structural elements also play a role in determining the shapes of elements for which
they are suitable. These aspects of the influence of material properties on structural
geometry are now discussed in relation to the four principal structural materials of masonry,
timber, steel and reinforced concrete.
3.2 Masonry
Masonry is a composite material in which individual stones, bricks or blocks are bedded
in mortar to form columns, walls, arches or vaults Fig. 3.1. The range of different types of
masonry is large due to the variety of types of constituent. Bricks may be of fired clay, baked
earth, concrete, or a range of similar materials, and blocks, which are simply very large bricks,
can be similarly composed. Stone too is not one but a very wide range of materials, from
the relatively soft sedimentary rocks such as limestone to the very hard granites and other
igneous rocks. These ‘solid’ units can be used in conjunction with a variety of different
mortars to produce a range of masonry types. All have certain properties in common and
therefore produce similar types of structural element. Other materials such as dried mud,
pisé or even unreinforced concrete have similar properties and can be used to make similar
types of element.
The physical properties which these materials have in common are moderate
compressive strength, minimal tensile strength and relatively high density. The very low tensile
strength restricts the use of masonry to elements in which the principal internal force
is compressive, i.e. columns, walls and compressive form-active types see Section
4.2 such as arches, vaults and domes.
In post-and-beam forms of structure see Section 5.2 it is normal for only the vertical
elements to be of masonry. Notable exceptions are the Greek temples see Fig. 7.1, but in
these the spans of such horizontal elements as are made in stone are kept short by
subdivision of the interior space by rows of columns or walls. Even so, most of the
elements which span horizontally are in fact of timber and only the most obvious, those in the
exterior walls, are of stone. Where large horizontal spans are constructed in masonry
compressive form-active shapes must be adopted Fig. 3.1.
Where significant bending moment occurs in masonry elements, for example as a
consequence of side thrusts on walls from rafters or vaulted roof structures or from out-of-
plane wind pressure on external walls, the level of tensile bending stress is kept low by making
the second moment of area see Appendix 2 of
22
Chapter 3
Structural materials
the cross-section large. This can give rise to very thick walls and columns and, therefore, to
excessively large volumes of masonry unless some form of ‘improved’ cross-section see
Section 4.3 is used. Traditional versions of this are buttressed walls. Those of medieval Gothic
cathedrals or the voided and sculptured walls which support the large vaulted enclosures of
Roman antiquity see Figs 7.30 to 7.32 are among the most spectacular examples. In all of
these the volume of masonry is small in relation to the total effective thickness of the
wall concerned. The fin and diaphragm walls of recent tall single-storey masonry buildings Fig.
3.2 are twentieth-century equivalents. In the modern buildings the bending moments which
occur in the walls are caused principally by wind loading and not by the lateral thrusts
from roof structures. Even where ‘improved’ cross-sections are adopted the volume of
material in a masonry structure is usually large and produces walls and vaults which act as
23 Structural materials
Fig. 3.1 Chartres Cathedral,
France, twelfth and thirteenth centuries. The Gothic church
incorporates most of the various forms for which masonry is
suitable. Columns, walls and compressive form-active arches
and vaults are all visible here. Photo: Courtauld Institute
effective thermal, acoustic and weathertight barriers.
The fact that masonry structures are composed of very small basic units makes their construction
relatively straightforward. Subject to the structural constraints outlined above, complex geometries
can be produced relatively easily, without the need for sophisticated plant or techniques and
very large structures can be built by these simple means Fig. 3.3. The only significant
constructional drawback of masonry is that horizontal-span structures such as arches and
vaults require temporary support until complete.
Other attributes of masonry-type materials are that they are durable, and can be left exposed in
both the interiors and exteriors of buildings. They are also, in most locations, available locally in
some form and do not therefore require to be transported over long distances. In other words,
masonry is an environmentally friendly material the use of which must be expected to increase in
the future.
Structure and Architecture
24
Fig. 3.2 Where masonry will be subjected to significant
bending moment, as in the case of external walls exposed to wind loading, the overall thickness must be large
enough to ensure that the tensile bending stress is not greater than the compressive stress caused by the
gravitational load. The wall need not be solid, however, and a selection of techniques for achieving thickness
efficiently is shown here.
a
b c
Fig. 3.3 Town Walls, Igerman, Iran. This late mediaeval
brickwork structure demonstrates one of the advantages of masonry, which is that very large constructions with
complex geometries can be achieved by relatively simple building processes.
3.3 Timber