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Reinforced concrete is a term used to define concrete that incorporates reinforcement grids, fibers, plates or bars. Reinforcements chosen are of different varieties depending on the desired quality and other physical characteristics, like strength and durability. They range from iron and steel to composites of various forms, as well as inorganic and organic fibers. Concrete is by itself strong in compression, but at the same time weak in tension. The addition of reinforcement fundamentally increases the strength in the tension. The concrete failure strain in tension is very low, hence, the reinforcement is supposed to put the cracked regions together. For a ductile, strong and durable design construction, the reinforcement is supposed to posses the following characteristics: high tensile strain; thermal compatibility; good bonding nature to the concrete; high strength; and durability in the concrete surrounding. In most of these cases, reinforcements that have been commonly used are steel bars (AISC 2010, pp. 360-9). Reinforcements have been added to concrete to beef up the strength.
Forces in Concrete and Reinforced Concrete
What is known as ‘concrete’ is a mixture of stone chips, aggregates of fine sand and a binder material which is commonly cement (Naaman1985, p. 2125). The addition and mixing with water leads to the hydration of cement ultimately forming microscopic opaque lattices that encapsulate and lock the entire mixture into a structure that is very rigid. A very typical concrete is characterized with very high resistance to stresses of compression. All the same, any significant tension, due to bending forces for instance, breaks the microscopic rigid structure, leading to cracking and the detachment within the concrete elements (Ganesh and Pavan 2004, pp. 123-45). It is for this reason that a typical non-reinforced concrete should be well anchored to deter tension development.
Therefore, concrete requires a material with high tension, like steel, to make a strong composite material in construction. This composite material is the reinforced concrete which offers resistance to compression and bending, as well as other direct actions of tension. Thus, a reinforced concrete material, where the concrete part acts as resistance to compression and the steel acts as resistance to tension, can be formed into almost any size and shape for the building and construction industry (Chopra 2001, pp.34-67).
The bond and support in concrete has some specification codes. Since the actual bond stress changes along the bar length supported in tension zone, international specification codes apply a development length concept instead of bond stress. The main requisite for safety against failure in bonding is to offer enough extension of the bar length past the point where the steel material is expected to develop yield stress, and this length should be the same as the development length. All the same, if the real available length is not enough for the full development, special support should be offered like hooks or cogs, or mechanical end plates. A similar concept should be used to lap splice length stated in the codes where overlapping provided in between two close bars for the purposes of maintaining the needed stress continuity in the splice field.
Reinforced Concrete Frame Design
Reinforced concrete structural designs are established through several approaches: the ultimate strength approach and the working stress approach. However, figure 1 gives details and features of a building that was photographed, and that particularly used the ultimate stress method in the design process. Most manufacturing companies from which the photograph was taken make use of the ultimate stress method because of the advantages of using considerably less concrete and rebar. Again, the calculations of design are also easy to work with. On the other hand, the model of working stress design makes an assumption that the concrete beam will bend under the induced moments, while the relationship between the concrete in compression and the rebar in tension remains a constant (Ganesh and Pavan 2004, pp. 123-45). Unlike in the working stress model, the ultimate strength design puts the rebar in absolute yield so the relationship of strain between concrete and reinforcement is overlooked and a rectangular reinforced concrete compression block that is stressed at design strength is outlined.
Figure 1 Reinforced Concrete Frame Analysis
B – Concrete beam compression width
B’ – Concrete beam wed width
Fc – Allowable concrete working stress
Fc’ – Design ultimate concrete strength
Ts – Slab thickness
D – Concrete beam depth from reinforced bar (rebar) centroid up to the extreme compression fiber
X – Concrete compression depth from reaching the extreme compression fiber from the neutral axis
A – Ultimate compression block developed by the yielding concrete
C – Total compression load
T – Total tension load
D-X/3 – Moment lever arm for the working stress design
D-A/2 – Moment lever arm for ultimate stress design
(4,000 psi concrete and 60,000 psi rebar)
Reinforced Bar Area
In making reinforced concrete bar, one of the main considerations is cover. Cover is taken as the shortest distance from the nearest concrete face to the encased reinforcement support (Griffiths 2000, pp. 2747-2753). The cover offers protection from corrosion for the reinforcement and permits the bars to connect well with the concrete. Builders use the cover to facilitate the concrete flow around the reinforcement. A cover from 3/8" to 3" is recommended based on factors of bar size and weather exposure. It is important to avoid the use of thin covers, as they can result in the formation of cracks which are parallel and, at the same time, over the bars. The danger posed by this cracking is accelerated corrosion and weakening of bond. High strength types of steels that are applied in post and pre-tensioning require a very special consideration (AISC 2010, pp. 360-9). These kinds of steels are subject to stress corrosion and, therefore, the steel rusts more quickly than the normal strength steels under the high tension. High steel steels are very brittle and, therefore, any sharp defect on the surface will make the steel snap much in a similar manner in which glass breaks along lines of weakness.
The buildings were constructed to meet special requirements, because they are located in areas that experience acid rains within a very busy industrial region. The effects that were taken into consideration matched those of deicing salt and seawater that are very corrosive and need a cover with a special waterproofing concrete or a cover on the order of 3". The building houses high vibrating production machines which present a vibrating force impact, and, thus, this was a consideration in determining the strength of the load bearing walls and floor foundation. For instance, a composite safety factor for 60,000 psi reinforcement bar of U=1.4D + 1.7L is recommended in a basic ultimate strength design. In this case, U is the load for ultimate design whereas D is the “dead load” which takes into account dirt and concrete together with other dynamic fixed loads. The live load in the building is has the value L. the live load includes all equipment and machinery housed in the building. Lastly, 1.4 and 1.7 are the minimum dead load and minimum live load factors of safety respectively.
As it can be seen from “U=1.4D + 1.7L,” the dead load safety factor is much smaller compared to the live load safety factor value. The reason behind this is the fact that dead loads can be determined with rational certainty and are not dynamic. The configurations of live loads are mere approximations at best, usually constantly and dynamically changing. The live loads will induce fatigue and vibration. Machines can at times be over-loaded, or the surface on which they are mounted may change over time. Thus, the bigger live load factor of safety is set to cater for such indeterminate factors. Other inclusions that may be taken into considerations are wind (W) and earthquake (E) loads. E has been given the value 1.1W. Therefore, U = 0.75(1.4D+1.7L+1.7W). The most prevalent industrial design loads are normally 250 psf. All the same, the industrial loads that concentrate dynamic places in this case on the tire footprints were forklifts and cranes.
a) Reinforced Concrete stone block load bearing walls: Side View
b) General view of the reinforced concrete building.
c) Interior side of the reinforced concrete building
Factors Considered in Reinforced Concrete Design Process
The mechanism of the composite action of the reinforcement and the concrete by itself is very important. The design of a reinforced concrete beam requires a good mastery of the mechanism in order to produce a high quality design. The reinforcement in a Reinforced Concrete (RC) structure like the steel bar has to be subjected under the same deformation or strain as the surrounding concrete for the purposes of preventing slip, discontinuity, or the detachment of both materials that are under the load (Nilson 2003, pp.80-90). The maintenance of the composite action needs the transfer of the load between the steel and concrete. The direct stress that is usually transferred to the bar interface from the concrete in order to change the tensile stress in the rebar along the entire length (Griffiths 2000, pp. 2747-2753). The transfer of the load is attained through bonding and anchorage and, at the same, time idealized as a continuous stress region that emerges from the surrounding of the interface between concrete and steel
Steel Structural Design
Steel is a very common material in building that is applicable throughout the industry of construction. Steel is mainly used in the formation of a skeleton for a structure or building, and, most importantly, the structure that holds all other building elements together. Structural Steel consists of the structural elements that are used to build up the frame essential in supporting the loads of design like columns, trusses, plate, braces, beams and fasteners. Steel structural designs in constructions have many advantages over reinforced concrete, as highlighted above. For one, steel is friendly to the environment, because it is absolutely recyclable. According to the American Iron and Steel Institute, steel stands out as the most recycled material in the United States (AISC 2010, pp. 360-9). It has, thus, been helpful in reducing the burden on modern landfills. Unlike reinforced, concrete steel does not require time for curing and is definitely at full strength immediately. Steel is a multipurpose material with a high strength and less weight. Moreover, steel has uniform quality and has proven durability with low life cycle costs. Such qualities make steel as the main construction material of choice.
a) Arrows point at the structural steel trusses resting on concrete load bearing walls
b) Steel structural design with steel beams supported by steel trusses
Steel has been very useful in the building and construction industry due to its mechanical properties, simplicity of design, speed and ease of construction, availability in different practical and useful shapes, and due to the fact that it is economically feasible. The structures photographed in this category were basically frame structures of a single story building. It comprises steel roof truss and web steel joist anchored by columns of steel and masonry walls as shown in the sketches below.
Figure 2 Steel Roof Construction on steel columns (see Table 2-b)
Figure 3 Steel Roof construction on masonry walls
The design of the steel structure in Figure 2 follows a specific design process. The structural process must apply a concept of forces resolution in an analytical or graphical form. In order to achieve a strong and stable design, the introduction of resolving a force into its various components in both, the vertical and horizontal directions is a paramount concept. The size of these components must be equal to the effect of the single force, originally acting on the members in the design. The absolute definition of a force requires the specification of its direction and magnitude. The nature of the steel truss provides the balancing of the different forces acting to keep the design in an equilibrium position in the final structural design. Therefore, the structural measurements must be accurate and correlated well to ensure a balanced and stable design. An elementary concept of analytical or graphical representation through the drawing of the initial force to scale in the suitable direction is very important. Both representations are equal. The analysis of the steel trusses takes into account the equilibrium of these components of the initial force that is acting on each and every joint.
Forces Acting on Steel
Some of the structural principles related to steel take into account these forces. The main types of the forces that apply in steel structures are: Tension (T), Compression (C), Shear (S), Bending (B) and Torsion (To). They are the forces that could be resisted by elements and simple systems in different ways.
Figure 4 Compression and Tensional forces on a steel truss (see figure 2)
C – Compression force exerted by a uniformly distributed load on the truss and steel purlin
T- Tension force (Reaction at the main support resisting the load)
If several forces converge at a certain point, they can be resolved individually into two components that are not parallel. The total effect is established simply through the summation of the force components in both directions independently. If such a system of force is in equilibrium, the total value of the components of this force in every direction should be equal to zero. This is a basic principle of forces that can be used to the force system acting at a particular joint within a truss. Since the separate members of a truss are subject to direct forces alone, the direction of these forces should be similar to the physical sense of the member itself. The equilibrium of a given joint can, thus, be considered through the treatment of force system, acting in directions matching up the member converging at the joint and the resolution of the forces into horizontal and vertical components (Srinivasa and Seshadri 2005, pp. 43-50).
The resolution of the forces can be applied over and over again at each individual joint within the truss and can be conducted either graphically or analytically. The initial step is the calculation of the reactions from the supports. Support reactions are independent of the configuration of the truss. A simple approach involves the calculation of the support reactions in a similar manner as is the case with a simple beam through the use of moments about one of the end supports. The roof truss in Figure 2 can be analyzed in an arrangement as shown below. In this case, all the angles of the truss, µ, are 45° while the length of each truss member is given one unit. A simple design can be determined by taking moments about Joint 4 as follows: given reaction at joint 1 is R1,
R1x 1.5 - 10 x 1 - 20 Cos µ = 0
Hence, R1 =13.3kN
Figure 5 Resolution of forces in a steel truss
The calculation for the Figure 5 above does not depend on the shape of the truss. If it was a beam, the calculations would be just the same. The form of the truss is taken into account when establishing the value of all the forces in the specific members. The system of force acting on each particular joint is then taken into account. Making reference to Figure 5, and taking into account the Joint 1, the forces which converge at Joint 1 can be determined in their exact components. Through the application of the condition that the absolute total of all horizontal and vertical components should total to zero, the values for each individual component can be attained. Suppose the angle of member 1-2, the diagonal member (with regard to the horizontal member 1-3) is µ:
Resolution of forces vertically will apply as follows
Reaction – Force in 1-2 diagonal member’ Sin µ = 0
Therefore, F1-2 = 18.84 kN
Resolution of forces vertically will be determined as follows
AB Cos µ - AC = 0
Thus, AC = 13.3 kN
The design and calculation of the forces that act on the steel truss must follow a consistent sign convention to make a good design. Inherent in all these calculations is a convention that the forces in vertical are positive in an upward manner and the horizontal forces are positive towards the left (Singh et al 2005, pp. 438-444). This is an elementary knowledge that is important in determining a good structural design. It is merely a convention, and any convention can be applied as long as it is followed throughout the design process. This is of paramount importance, otherwise, the design will be flawed. By taking a closer look at the direction of the forces, it is easy to determine that for the loading situations shown, 1-2 is subject to forces of compression and 1-3 is subject to forces of tension.
In any design, it is most convenient to work from a point where there are only two forces that are not known (Hart and Wong 2000, pp. 23-37). As a general strategy, it is very easy and advisable to begin the analysis at a joint where there are only two forces which are not already determined. In the case above, joint 1 is most appropriate as a starting point and the forces at that particular joint can be determined. Having knowledge about the forces in members 1-3 and 1-2, it is evident that either joint 3 or 2 can now be analyzed. Following that procedure through the truss, all the forces in the members can be determined. This operation normally consumes a lot of time, but yields a very sound structure (Chopra 2001, pp.34-67). However, there are simpler and quicker ways of doing it.
Material Pragmatism: Steel Structures versus Reinforced Concrete Structures
There are very important aspects and build ability features which account for the selection of either reinforced concrete or steel in constructing the frames for these purposes chosen, as seen in different photographs (Compare Table 1a-c and Table 2a-b ).
Steel and reinforced concrete are commonly used interchangeably in construction. Most reinforcement is mainly steel and, thus, these two materials work well when joined together. Steel on its own is very advantages as it has quite a lot of advantages starting with its reusability and in the fabrication of high strength designs. Reinforced concrete is a concrete the tensile strength of which has been boosted with reinforcement (Parviz 1991, pp. 129-134). The reinforcement material that is commonly used is steel bar. The reinforcement makes the construction of many concrete buildings possible, as they provide an additional tensile strength. Reinforced concrete can contain a variety of components and structures, including beams, columns, frames, walls, foundations and slabs (see Table 1a-c). A lot of emphasis on reinforced concrete is put on the systems of floor. The design and implementation of the most successful floor system is the way to constructing optimal structures and buildings (Hart and Wong 2000, pp. 23-37). Every small change in the design and make of a floor system can have a very huge impact on the cost of materials, schedule of construction, cost of operation, levels of occupancy, ultimate strength and the final use of a building structure.
Just like steel structures, reinforced concrete design must meet certain requirements to achieve a balanced structural design. The ratio of concrete and the reinforcement must meet a desired qualification to balance out the compression and tensional forces (Srinivasa and Seshadri 2005, pp. 43-50). Steel structures, on the other hand, must ensure the resultant force acting on a given frame equals to zero. In that sense, the steel structural design will be in an equilibrium and stable state. It is an undeniable fact that both, steel and concrete are good construction materials. However, steel produces a much stronger and useful design compared to concrete. To match the characteristics of steel, it would be correct to say that concrete will require reinforcements to come up with a good structural framework in any construction where it is to be used.