Logo

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.

 

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 such as arches, vaults and domes.

 

In post-and-beam forms of structure 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).

 

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.

 

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 of 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 effective thermal, acoustic and weather tight barriers.

 

 

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.

 

 

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.

 

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