Masonry structures
5.2 The architecture of masonry factors which affect the decision to
5.2.2 The technical performance of masonry as a structural material
5.2.2.1 Introduction
The decision by an architect to use masonry as the structural material for a particular building is based on a knowledge of its properties and capabilities in relation to those of the alternative structural materials. The proposed form of the building, in relation to those for which the material is best suited, will normally have a major bearing on this. Any design decision has consequences and one of these is that the constraints and limitations of the material must thereafter be accepted. The adoption of masonry will normally result in the selection of a loadbear- ing-wall, panel-type structure (see Section 1.3.1). The advantages and disadvantages of masonry in relation to the other structural materials are summarised below.
5.2.2.2 Advantages Strength
Masonry obviously has sufficient strength to perform as a structural material. It has moder- ate compressive strength but very low tensile strength and therefore has a limited capacity to resist bending. It performs best in situations in which compression predominates, as in the walls and piers of buildings of moderate height. It is also suitable for use in compres- sive form-active structures such as arches, domes and vaults.
Durability
Masonry is a durable material both physically and chemically. Except in the most exposed situations externally, and often even then, it can be left free of finishing materials. This both simplifies the detailing of buildings and provides a carcass which is virtually mainten- ance free.
Low cost
Due to the fact that relatively little energy is consumed in their production and due to their good availability at most locations, the basic cost of masonry units and the cost of trans- porting them to particular sites are low. Masonry structures can be built using simple, traditional techniques which do not require complicated plant or machinery. The construc- tion process is therefore also relatively cheap; low cost is one of the principal advantages of masonry construction.
Appearance
Most masonry has a pleasing appearance which matures rather than deteriorates with age. A considerable variety of colours and surface textures is usually available and may be used to create a range of architectural effects.
Design flexibility
The fact that large structures can be assembled from small basic units allows complicated geometries to be achieved relatively easily with masonry and the material therefore offers considerable scope for imaginative design, subject always to the technical constraints (the
loadbearing wall, the arch, the vault and the dome). Another consequence of the method of construction is that other materials can be incorporated into masonry so as to augment its properties. The placing of steel reinforce- ment in the bedding planes to give masonry flexural strength is an example of this.
Fire resistance
Masonry performs well in fire; it is non- combustible and retains its structural proper- ties at high temperatures.
Acoustic performance
Masonry walls form good acoustic barriers.
Thermal performance
Masonry walls provide a reasonable level of thermal insulation. Their high thermal mass is a further advantage which allows the creation of enclosures with good levels of passive environmental control.
5.2.2.3 Disadvantages Lack of tensile strength
The lack of tensile and therefore of flexural strength is masonry's principal structural disadvantage and has restricted its use, in modem times, to loadbearing-wall-type struc- tures. The selection of masonry therefore normally implies that the constraints of this type of structure be accepted in the planning of the building. The most obvious of these is the requirement that a plan consisting of parallel loadbearing walls be adopted and that, in multi-storey buildings, the plan be more-or- less the same at all levels.
The lack of tensile strength also makes diffi- cult the construction of high-strength struc- tural connections between masonry and other structural elements. This has tended to restrict the use of masonry in modern times to multi- cellular buildings in which none of the interior spaces is large.
Weight
In comparison to timber, masonry is relatively heavy and while this has advantages (it is
responsible for the good performance of masonry as an acoustic barrier) it also has disadvantages. In particular, it results in high dead loads being imposed on supporting structures, such as foundations. It also affects the cost of transport to the site.
Porosity
Most masonry is porous and while this may not be a serious disadvantage so far as its use as a structural material is concerned, it does affect the structural design. In particular, where the material is used for external walls, it complicates the detailed design, which must be such as to prevent water from penetrating the building.
5.2.2.4 Conclusion
To sum up, the principal advantage of masonry is that it possesses a good combination of properties rather than that it performs
outstandingly well with respect to any particu- lar criterion. Its good appearance and durabil- ity, its reasonably high compressive strength, its low cost, its thermal and acoustic proper- ties and its performance in respect of fire make it an ideal material for structural walls and this is, of course, its principal use. In modem practice masonry structures are usually based on post-and-beam arrangements and the particular properties of the material affect the forms which are adopted. The general prin- ciples which are followed in the planning of masonry buildings, and which take these into account, are outlined in Section 5.3.
The treatment of masonry which is given here is confined almost entirely to the use of brick and block manufactured components. These are by far the most commonly used constituents in the developed world, but masonry is also constructed in a variety of other materials such as natural stone and mud. Although these materials are not dealt with here specifically, the general principles of loadbearing masonry which are outlined in this chapter apply equally to construction in all brittle building materials.