Actual structural materials

Every stiff physical object is a structure; the choice of suitable materials is immense. A slice of toast, a pair of shoes, flowers, aeroplanes and bicycles are all structures. However, for a building structure the choice of suitable materials is very limited. This is because the materials must be strong, stiff, durable and cheap. These are relative terms but building structures must be strong and stiff enough to carry the required loads without deflecting excessively; they must be sufficiently durable to last for the structure’s useful life and cheap enough to make the structure affordable. Because building structures consume con-siderable amounts of material they, unlike materials for musical instruments and racing cars, must be cheap which means plentiful.

Few materials comply with these requirements in any culture at any historical time.

The original traditional buildings were constructed from natural materials. These were vegetation (trees, grass, leaves, etc.), animals (skins and less commonly bones), rocks and stones (including caves) and, in the case of the Inuit people, ice and snow. Slowly man-made materials were evolved so mud-dried bricks and woven cloth were used and stones were shaped rather than used as found. Later, kiln-dried bricks and lime-based mortar and concrete were used. Even though bronze, first smelted about 4500 BC and iron first smelted about 2500 BC, are strong, stiff and durable they were far too expensive for use in building structures. Even as late as 1750 AD the use of iron nails was rare. Thus, for thousands of years building structures were constructed of timber, brick and stone.

This was changed by what is wrongly called the Industrial Revolution; a better word would be evolution (because it took about 150 years). In 1709, Abraham Darby discovered a method of smelting iron ore using coal (actually using coke, a product of coal). Previ-ously iron ore, which is plentiful, was smelted using charcoal which was neither cheap, and as the supply of trees ran out, nor plentiful. This crucial discovery meant that iron became a plentiful and cheap material. Therefore iron, or more correctly cast iron, could be used

Structural materials 147 for building structures. This was dramatically demonstrated by the erection, in 1779, of the Iron Bridge at Coalbrookdale in Shropshire. What is revolutionary about this bridge, which still stands, was not its size or method of construction but the fact that it is wholly constructed of iron. See also Section 11.2.

Fig. 5.8 Iron Bridge at Coalbrookdale

The evolving manufacturing and transport industries required a variety of new types of buildings and structures. These included mills, bridges, workshops, chimneys and railway buildings.

Fig. 5.9 Menai Straits Bridge and The Boat Store at Sheerness

Because the size of these structures and the magnitude of the loads were much greater than traditional buildings, there was pressure to produce both new types of structure and new struc-tural materials. After the availability of cast iron, wrought iron, due to Henry Cort in 1784 and later steel, due to the Bessemer process (1850) became cheap enough for building structures.

About 1811, Joseph Aspdin invented artificial cement made from Portland stone which allowed strong mortars and mass concrete to be made. In 1892, Françoise Hennebique patented the use of concrete reinforced with iron and steel, now known as reinforced concrete. Thus by about 1900 all the ‘modern’ building structural materials were available.

148 Building Structures: From Concepts to Design

Nowadays, building structures are constructed using concrete, both mass and rein-forced, timber, brick or block masonry and steel. So a combination of new materials, steel and concrete, and traditional materials, brick and timber are used. Cast iron, wrought iron and stone are now rarely used for building structures. Although there are constant efforts being made to find ‘new’ materials for building structures, none have been found mainly due to lack of cheapness. Many developments have taken place since 1900 but these have mainly been either new uses or methods of design and construction.

On the whole the behaviour of structures in the real world is too complicated to be modelled by structural theory; so theories are derived that are based on various simplifying assumptions. This provides theories that are simple enough (but not necessarily simple) to be used for structural design.

Although structural theory exists that can predict behaviour of structures built of non-linear elastic materials, computations are enormously simplified if non-linear elasticity is assumed. But is this a valid assumption for the limited range of materials used in building structures?

Fig. 5.10

The figure shows that only steel, and mild steel at that, closely approximates the idealised linear elastic/perfectly plastic behaviour. However, all exhibit some type of elastic behav-iour at the beginning of the load/deflection graph. Therefore, for design rather than research purposes, steel, concrete, timber and masonry are assumed to be linear elastic. This means that ‘ordinary’ structural theory can be used for the structural design of all the com-monly used materials. (It should be noted that Fig. 5.10 only shows the relative shape of the load/deflection graphs rather than the relative numerical values.)

The most important concept to grasp for an engineering understanding of structural materials is the load/deflection behaviour. For designing structures it is also necessary to know the strength of the materials. Of the four common structural materials steel is the strongest with concrete, masonry and timber very roughly the same strength. All the materials can vary considerably in strength depending on the process of manufacture, or in the case of timber, the species. Again steel is the stiffest material with concrete about one tenth, masonry about one twentieth and timber about one thirtieth as stiff as steel. Again these values, apart from steel, vary considerably.

Not all the materials are equally strong in tension and compression, that is pulled or squashed. Steel and timber are equally strong in tension but masonry and concrete although strong in compression are very weak in tension; so weak in fact that their tensile strength

Structural materials 149 is usually ignored in structural design. This difference in material behaviour has a great influence on the choice of structural form because if the loaded structure has to carry tensile forces then steel or timber must be used.

Another influential characteristic is the strength to weight ratio. The self-weight of structures constructed from steel or timber is usually not more than 15% of the total load carried, whereas the self-weight of masonry and concrete structures can be 40% of the total load carried. This is because steel and timber have high strength to weight ratios and masonry and concrete have low strength to weight ratios, so timber and steel are light-weight materials and masonry and concrete are heavylight-weight materials.