ARC 209 Structures PDF

Selection of the Structural Materials The choice of the structural material is another fundamental decision in the planning of a structure; it is an aesthetic as well as a technical decision. There are four principal structural materials namely: 1. Steel 2. Reinforced concrete 3. Maso...

Selection of the Structural Materials The choice of the structural material is another fundamental decision in the planning of a structure; it is an aesthetic as well as a technical decision. There are four principal structural materials namely: 1. Steel 2. Reinforced concrete 3. Masonry 4. Timber To select any of the above structural materials for a building type, several factors must be considered which are: 1. Scale: Which determines the spam of the building 2. Internal planning: which dictate the nature of the required spaces required. 3. External treatment For example: If a load bearing wall structure is to be adopted, this will favour the use of either masonry or timber. Reinforced concrete and steel would be the normal choices for skeleton frame structures. Skeleton frame structures can be constructed in steel, reinforced concrete or timber. The use of timber is rare for multi-storey buildings; however, the timber skeleton frame is most usually associated with single-storey enclosures. Properties of Materials Steel: Steel is material which has excellent structural properties. It has high strength in tension and compression and is therefore able to resist bending axial loads with equal facility, it is the strongest of the commonly used structural materials, being approximately twenty times stronger than timber and ten times stronger than concrete. It is therefore used to make the tall buildings and the enclosures with the longest spans. The Properties and Composition of Steel Steel is a ferrous metal (its principal constituent being iron) but it contains several other chemical elements which act as alloying agents, and which have a critical effect on its properties. The most important of these is carbon; steel is defined as ferrous metal with carbon content in the range of 0.02 to 2. Low-carbon steels are relatively soft and ductile while those with high carbon content are hard and brittle. Structure steel are mild steels, which have a carbon content of around 0.23,the other principal alloying agent is manganese which is maintained at around 1.6, sulphur and phosphorous are also present. According to BS 4360, four grades of structural steel are specified 40, 30, 50 and 55. The grade numbers refer to the tensile strength values [400,430,500 and 550 newton per square millimetre]. The properties of steel can be manipulated by heat treatment. Steel is high-strength materials which has equal strength in tension and compression. Technical performance of steel as a structural material: Steel is the strongest of the four commonly used structural materials but has a strength-to-weight ratio which is like that of timber [i.e., very high]. It is ideally suitable for skeleton frameworks, where the principal alternative in multi-storey buildings is reinforced concrete. Its high ratio of strength to weight also makes it suitable for lightweight frameworks such as are used in roof structure. In this application the principal alternative is timber. The Aesthetics of Steel The visual expression associated with steel structures contains some of the most powerful images of modern architecture.  The glass-clad framework; the use of slender precisely crafted structural components as visual elements and celebration of structural elements in form either long spans or very tall buildings are all different aspects of the expressive and impressive possibilities of steel. These aesthetic devices have been used from the beginning of the modern period and are still part of the twentieth-century architectural palette. They are often the primary reasons for the selecting of steel as the structural material for a building. Steel became available as a building material in the second half of the nineteenth century, following the development of economical processed, however, metal frameworks were gradually absorbed into the world of architecture. They were used mainly for “new” types of building such as department stores and multi-storey offices; the structural technology on which these buildings were based was that of a skeleton framework. The technology of the steel framework contributed to the aesthetic of architecture in 1920’s and 1930’s in two quite separate ways. First, it was crucial to the development of the glass-clad building (which was normally satisfactory), Second, it made use of structural elements as constituents of a modern visual vocabulary. Advantages of Steel 1. Strength: The high strength of steel and its high ratio of strength to weight makes it suitable for use in single multi-storey skeleton frames over a large range span and building heights. 2. Ratio of Strength to Weight: Steel frames are lighter than reinforced concrete frames of equivalent strength. This makes them more suitable than reinforced concrete frames for use in single-storey buildings and the roof structure of multi storey buildings. 3. Quality Control: Steel is manufactured under conditions of strict quality control and its properties can relied upon to be within narrow specified limits. 4. Appearance: Due to the strict quality control during its manufacture and the methods which are used in the final shaping of steel components, the finished structure has a distinctive appearance which is characterised by slender elements, smooth surfaces and straight sharp edges. 5. Prefabrication: Steel structures are assembled from prefabricated components which are produced off-site, and this allows their dimension and general quality to be carefully controlled. It also results in fast erection of the structure on site and enables a relatively simple erection process to be adopted, even on difficult sites. Disadvantages of Steel 1. Intractability: Steel is a very tough material which is difficult to work and shape in the solid form and this has a few consequences. It means that, in most steelwork design, it is necessary to specify elements from a standard range of components which are produced by steel manufacturers and to carry out minimum amount of modification to these. 2. Weight: The density of steel is high, and this makes individual components heavy. Elements such as beams and columns are difficult to move around on site and cranes are normally required for the assembly of steel structures. 3. Cost: The basic cost of a steel structure is normally greater than that of its timber or reinforced concrete equivalent. 4. Durability: Most steels are relatively unstable chemically and a corrosion-protection scheme is normally required for a steel structure. 5. Performance in Fire: Steel loses its ability to carry loads at a relatively low temperature [around 500C] and this means that whole a steel structures does not actually burn, it will collapse in fire unless it is kept cool. This is normally achieved by protecting the steelwork with suitably thick layer of fire-resistant insulating materials but sometimes more sophisticated methods such as water-cooling systems are used. The traditional fireproofing material was concrete – the elements of a steel frame were simply encased in concrete but much lighter materials which are easier to apply have been developed. The need to provide fireproofing for steelwork nevertheless increases the complexity of a steel-frame building and adds to the cost of the structure. Reinforced Concrete Concrete is an extremely versatile structural material, it compromises the addition of coarse aggregates (stones), fine aggregate (sand), cement and water in an appropriate ration. Concrete is moderately strong in compression but weak in tension, it has good resistance to fire and good durability. Concrete when used with steel in the form of small diameter reinforcing bars, it produces a composite material called “Reinforced Concrete “. This possesses tensile ad flexural strength as well as compressive strength. Reinforced concrete can be used to make any type of structural element. Concrete can be either cast directly into its final location in a structure, in which case it is said to be in situ concrete or used in the form of elements which are cast at some other location, usually a factory and simply assembled on site in which case it is referred to as Precast Concrete The Aesthetics of Reinforced Concrete In the late nineteenth century reinforced concrete was a “near” structural material capable of producing durable and fire-proof skeleton frameworks. It arrived on the architectural scene at a time when the precursors of the modern movement were exploring the possibilities of creating a new architectural language which would be appropriate for the twentieth-century world. Reinforced concrete has simple ability to be moulded into various irregular or curvilinear structural shapes; this was well exploited by the modern movement to show its structural aesthetic. The Constituents of Reinforced Concrete Reinforced concrete is a composite material whose constituents are concrete which forms the main bulk of the material and steel in form of reinforcing bars. Concrete itself is also a composite material being composed of cement and aggregate (fragments of stone) the properties of concrete depend on those of its constituents and on the proportions in which these are mixed. Concrete can be manufactured on the building site although in modern practice it is normal for even IN SITU concrete to be mixed in a separate factory and delivered to the site in liquid, ready-mixed form. Concrete is made by mixing appropriate quantities of cement and aggregate in the dry state and adding sufficient water to hydrate the cement. After the water is added a chemical reaction occurs which causes the concrete to become solid within a few hours in what is called the “initial set”. A considerable period is required before it develops its full strength however and this latter process is called the “final setting” or “hardening” of the concrete. The time required for final setting varies, depending mainly on the type of cement which is used but a typical concrete will have developed 80% of its full strength within three months of the initial set. The liquidity of fresh concrete is referred to as its “workability” and this properly affects the ease with which it can be “placed” and “compacted” to form from a dense solid. Plain concrete in the hardened state is a material which has moderate compressive strength (typical between 20N/mm2 and 60N/mm2 depending on the mix proportions) but very low tensile strength (usually about one Tenth of the compressive strength). When steel bars are incorporated into concrete the resulting composite material is called reinforced concrete.  Cement Cement is the binding agent in concrete which possesses the cohesive strength to hold the aggregate and reinforcement together into a solid, composite material. A considerable number of types of cement are used in building; most of these are varieties of Portland cement. All the cements which are used for structural concrete are dependent on water for the development of strength; when water is added to dry cement a complex series of chemical reactions take place in a process which is called hydration, and which causes the resulting paste to stiffen. The subsequent development of strength takes place in two stages: The initial settings [occurs quickly] usually within a few hours] and final settings [which occurs in a much longer period]. The hardening of the cement is simply a continuation of the hydration process which produced the initial set, and the cement must be kept wet if this is to proceed satisfactorily to compensate for water which may be lost due to evaporation. Aggregate Aggregate, being cheaper than cement, is used as a building agent in concrete and typically, will account for 75% to 80% of its volume. It also serves to control shrinkage and to also improve dimensional stability. Aggregate normally consists of small pieces of stone, of various sizes in the either natural occurring sand, gravel, or crushed rock fragments, other materials, such as crushed brick, blast furnace slag or reduced building materials, are sometimes used. Aggregate must be durable, of reasonable strength; chemically and physically stable; and free of constituents which react unfavourably with cement. The proportions which are present of the differently sized particles which occur in an aggregate are referred to as its grading and if the aggregate is to be effective as a building agent the grading must be such that a particular distribution of particle sizes occurs. The grading of aggregate also affects the workability of the concrete because a well graded aggregate allows a required workability to be achieved with a lower water-cement ratio than a poorly graded aggregate. Aggregate particles are classified into three categories according to their shape and are said to be rounded, irregular or angular. Naturally occurring aggregates tend to be rounded while crushed rock types are predominantly angular. The significance of this factor is that it affects the workability of the concrete; it also affects the cost of concrete because it affects the quantity of cement which must be provided to produce a given volume of concrete. Reinforcement The reinforcement which is used in concrete is normally in the form of steel bars, either of plain circular cross-section or with various surface treatments which increase the bond with the concrete, their various types of shape are square twisted bar, ribbed and twisted bar, stretched and twisted ribbed bar, and ribbed bars. The preferred reinforcement diameters are 6,8,10,12,16,20,25,32 and 40mm, the normal maximum length is 12m. Reinforcement is produced in both mild steel and high-yield steel. The latter allows much higher tensile stress to be specified for the reinforcement, its use is limited, however because the critical factor which determines the amount of stress which can be permitted in reinforcement is frequently the need to control the amount of strain which occurs to prevent cracking on the concrete. Advantages of Reinforced Concrete 1. Strength: of the four principal structural materials, reinforced concrete is one of the strongest. It performs well in skeleton frame type structures and is therefore best used in situations in which the properties of a frame are required. 2. Mouldability: The fact that reinforced concrete is available in semi-liquid form means that it can be cast in almost infinite variety of shapes. This properly together with its strength, characteristics means that virtually any form can be created relatively easily in reinforced concrete. 3. Durability: Reinforced concrete is a durable material which can be left exposed in relatively hostile environment. 4. Fire Resistance: Reinforced concrete performs well in fire; it is incombustible, and it retains its structural properties when exposed to high temperatures. 5. Cost: Reinforced concrete is relatively cheap and when used for frame structures will usually be cheaper than steel. It is, however, normally more expensive than masonry for load bearing wall structures. Disadvantages of Reinforced Concrete 1. Weight: Reinforced concrete structures are heavy. The material has a relatively low ratio of strength to weight, and a reinforced concrete frame is normally significantly heavier than an equivalent steel frame. 2. Construction: The construction of a reinforced structure is complicated and involves the erection of formwork, the precise arrangement of intricate patterns of reinforcement and the careful placing and compacting of the concrete itself. 3. Materials Storage: Another disadvantage of reinforced concrete is the requirement for sufficient space for storage of formwork and for assembly of reinforcement cages. This can be problematic if the building site is very tight and congested. 4. Strength: Although, as stated above, reinforced concrete is one of the four primary structural materials, it is nevertheless weaker than steel. The relative weakness also places restrictions on the spans for which reinforced concrete is suitable.  Timber Structures Timber is a structural material with a useful combination of physical property. Although its strength is not high (typical design stress values are in the range 5 to 250N/mm), timber is like steel, more or less equally strong in tension and compression. It can therefore withstand bending and can be used to make every kind of structural element. Due to the origins and nature of the material, timber is available normally in the form of slender, linear elements and this favours its use in framework arrangements. Timber is a lightweight material, capable of providing structural elements which are of a low dead weight, but which are nevertheless reasonably strong and tough. One of the problems associated with the structural use of timber is that its individual elements are relatively small. Timber and Structures Among the four principal structural materials, timber is one which is not directly associated with a major architectural style or movement in the western architectural tradition, although significant timber building traditions (for example, the “stick” and “shingle” styles of the north America) have occurred. Other architectural traditions, such as those of China or Japan have however produced significant timber styles. Although no major western architectural style is associated with timber, the contribution of the material to the development of western architecture has nevertheless been considerable. Its principal structural use has been as the horizontally spanning elements in post and beam structures in which the vertical elements were of masonry. Timber Skeleton Frame Structures In skeleton frame structures the volume of structural material which is present is considerably smaller than in load bearing wall structures and the structural loads are concentrated into slender beams and columns. Stress levels are therefore high and because the strength of timber is only moderate compared to material such as steel, it is frequently considered unsuitable for skeleton frames. The structures of the present day are significantly different developments in the technology of timber. Modern timber structures are lighter and are also more precisely crafted. Timber is a material which offers the designers of buildings a combination of properties which allows the creation of light weight structures which are simple to construct. It relatively low strength, the small sizes of the basic components and the difficulties associated with achieving good structural joints tend to limit the size of structure, which is possible, however most timber structures are small in scale with short spans and a small number of storeys. Currently, the most common application of timber in architecture is in domestic building where it is used as a primary structural material, either to form the entire structure for a building as in timber wall frame construction (also called timber frame construction) or to make the horizontal elements in loadbearing masonry structures. Trees may be classified into two types, i. Narrow-leaved trees ii. Broad-leaved trees. Narrow-leaved trees are coniferous and mostly evergreen- an exception is Larch; broad-leaved trees are mainly deciduous- an exception to this is holly. Many differences exist between the physiologies and anatomical structures of these two types of trees. The most significant of them is that the narrow-leaved species tend to grow much faster and produce timbers which are less dense and less strong than the broad-leaved species. Commercial timbers are subdivided into the two broad categories of softwoods and hardwoods, and these correspond approximately to the botanical classifications. The softwood is derived from the narrow-leaved species (coniferous) and the hardwoods from the broad-leaved species (deciduous). In general, the coniferous species from which softwoods are derived are fast- growing and produce timbers with a high proportion of spring wood. They are therefore less dense and less rigid than the hardwood timbers which are derived from slow-growing broad- leaved varieties. Advantages of Timber 1. Strength: Timber possesses tensile compressive and flexural strength and is therefore suitable for all types of structural elements. 2. Lightness: Timber is a lightweight material with a high ratio of strength to weight. It therefore produces lightweight structures with components which can be easily transported and handled on site. 3. Tractability: Timber can easily cut and shaped with simple tools and the erection of timber structures is therefore straightforward. Other components can be easily attached to timber with simple fasteners, such as nails or screws and this simplifies the detailing of timber buildings. 4. Durability: The constituents of timber are relatively stable chemically and the material does not suffer chemical degradation in environmental conditions (such as high humidity levels) which might prove detrimental to metals. It is, however, susceptible to insect infestation and fungal attack. 5. Appearance: Timber is a material which has a pleasing appearance which matures rather than deteriorates with age. It can therefore serve in the combined role of a structural material and a finishing material. 6. Performance in Fire: Timber is a combustible material but the rate at which it is consumed in a fire in relatively low and it does not lose its structural properties when it is subjected to high temperatures. Disadvantages of Timber 1. Lack of Strength: Although the strength to weight ratio of timber is high, its actual strength is low compared to other structural materials such as steel and reinforced concrete. This restricts the size of span which can be achieved and the number of storeys which can be constructed in all-timber structure. 2. Jointing Difficulty: Although the material is tractable it is difficult to make joints in timber which have a good structural performance. Joints which are with chemical fasteners, such as nails, screws and bolts, suffer from the problems of stress concentration. 3. Component Size: The size of individual timber planks or broads is obviously determined by the size of available trees. A consequence of the depletion of the world’s resources of timber is that lengths greater than around 6m and cross-sectional dimensions more than  300mm by 100mm are difficult to obtain in most of the species which are used for structural purposes. 4. Susceptibility to Rot and Decay: Timber components are susceptible to various kinds of infestation, notably by fungi and insects. The likelihood of fungal attack is minimised if the timber is kept dry and this can be achieved through suitable detailing of the structure. 5. Variability: Timber being a natural material, exhibits considerable variability in its properties such as strength, elasticity, durability and appearance can be high. The problem has been overcome by the adoption of grading system for commercial timber in which individual planks and boards are inspected and placed into categories according to their characteristics. Timber which is used in construction is specified by grade and this ensures that its properties can be relied upon to be within known limits.