RCC Structures Stair

Introduction to Design of RCC StructuresRCC structures RCC (Reinforced Cement Concrete) is a construction technology which evolved with the evolution of different structural materials in the 18th century during the Industrial Revolution. Industrial Revolution brought in new technology which helped in the manufacture of various materials. The Architect Le Corbusier used RCC for various constructions. He believed that any shape and form was possible; if RCC is to be used. For example, Notre Dame Du Haut, Ronchamp, France This is an example of Le Corbusier Project where he used RCC like plastic. Notre Dame Du Haut, Ronchamp, France | RCC Structures What is RCC? RCC means Reinforced Cement Concrete, i.e., cement concrete reinforced with steel bars, steel plates, steel mesh etc to increase the tension withstanding capacity of the structure. Cement Concrete can take up immense compression but weak in tension whereas steel is good in withstanding both tension and compression. Here are some of the advantages of RCC construction: 1. 2. 3. 4. 5. 6. 7. Materials used in RCC construction are easily available. It is durable and long lasting. It is fire resisting and not attacked by termites. It is economical in ultimate cost. The reinforced concrete member can be cast to any shape because of the fluidity of concrete. Its monolithic character gives much rigidity to the structure. Cost of maintenance is nil. Here are some of its disadvantages: 1. 2. 3. 4. Scrap value of reinforced members is almost nil. Constant checking is required. Skilled labour is engaged in the work. The advantages of RCC outweigh its disadvantages. This is one construction technique that made construction very easy and brought a boom to the field of construction. Components of RCC structures We have already discussed and studied the design procedures for the “Components of RCC structures”. Design of RCC beams Design of RCC columns Design of RCC staircase Design of Foundations Design of Simply Supported Slabs Every component is designed according to the load it carries and its position in the structure. The study of the design of RCC components will help in understanding the basics of RCC design and the method of its implementation. We will study more about different construction techniques in our successive articles… Related posts: 1. Design of Staircase | RCC Structures RCC Structures RCC Structures are nothing but reinforced 2. 3. 4. concrete structures. RCC structure is composed of building components such as Footings, Columns, Beams, Slabs, Staircase etc.... Structures and Classification of Structures | Design of Steel Structures What are Structures? When any body is subjected to a system of loads and deformation takes place and the resistance is set up against the... Science of Structural Engineering | Design of Steel Structures Scope and Importance of Structural Engineering Structural Engineering deals with the mechanism of the structural system that is it deals with structural analysis and structural... Guide to Doubly Reinforced RCC Beam Design RCC Beams RCC beams are cast in cement concrete reinforced with steel bars. Beams take up compressive and add rigidity to the structure. Beams generally... 5. Methods of Structural Design | Design of Steel Structures Structural design is the science of studying the Mechanics of a structure. Designing the skeleton of a building determines the real strength of the structure.... Structures and Classification of Structures | Design of Steel Structures What are Structures? When any body is subjected to a system of loads and deformation takes place and the resistance is set up against the deformation, then, the body is known as structure. The structure are means of transferring forces and moments. The structures may be classified as statistically determinate structures and statistically indeterminate structures. When the equations of statistics are enough to determine all the forces acting on the structures, in the structures, then, the structures are known as statistically determinate structures. Design of Steel Structures When the equations of statistical equilibrium are not sufficient to determine all forces acting on the structure and in the structures, then the structures are known as Statistically Indeterminate Structures. The equations of consistent deformations are added to the equations of equilibrium inorder to analyse the Statistically Indeterminate Structures. One dimensional supporting system can also be termed as “Line supporting structures”. The high strength and ductility are the properties of the particular importance for the structural use of the steel. two dimensional and three dimensional supporting systems. RCC Structures and Steel Structures. The structural steel is used for the steel structures. tanks etc are the examples of shell structures. airplanes. then it is known as mixed system. it is known as basic system. ship bulls.Classification of Structures The structures are categorised by their supporting systems. Two Dimensional supporting systems can also be called as surface structures. They are built of sheets or plates. The curved surface may possess single or double curvature. When a supporting system is subjected to only one type of stresses. The framed structures are built by assemblies of elongated members. The material remains distributed along the surface. 1. aluminium structures. Three dimensional structures are large in all three dimensions and these structures may have any shape. railroads. The structure can also be divided into following three categories: 1. The ductility is the unique property by virtue of which it is able to deform substantially either in tension or compression before failure. cars. The material remains concentrated along a straight. Skeletal Structures 2. The surface maybe plain or curve. then. The surface structures is large in two dimensions and small in the third dimension. . curved or angular line. The framed structures are examples of surface structures made of line elements. timber Structures. A surface structure may also consist of line elements that form a continuous structure. The three dimensional framed structures are also Space Structures. The shell structures are rigid curved surface structures . 2. Solid Structures Studying the classification of structures will help us understand the flow and concept designing various steel structures of varied designs. The structures may be further classified depending on the material used as Plastic Structures. The three dimensional structures are also called Space Structures. The shell roof coverings for large building. A flexible cable is an example for such a “Line Supporting Structure”. The Line supporting structure is large in one dimension and small in other two dimensions. planes. When a system is subjected to simultaneously several types of stresses. Stressed Skin Structures 3. There are one dimensional. we will discuss how a Structure is to be analysed in terms of its structural components. aeronautical engineering and in all the specialized phases of engineering. Here we will discuss what is to be done when analysing the structure before or after its construction… Basically. The structural engineering plays an important and necessary part in mechanical engineering. civil engineering. Millennium Dome It not just deals with the various different fields of engineering but is also interrelated with Architectural Design. naval engineering. electrical engineering. Structural Design Structural Analysis Lets first discuss about Structural Analysis. People are usually curious to know what Structural Engineering is like. . Structural Analysis 2.Science of Structural Engineering | Design of Steel Structures Scope and Importance of Structural Engineering Structural Engineering deals with the mechanism of the structural system that is it deals with structural analysis and structural design. What is the work of a Structural Engineer??? What are Steel Structures? Structural Designing is basically split into two main steps: 1. Understanding the Interrelationship of Structural Engineering and Architectural Design is very essential for an engineer and an Architect… We have already discussed “Art of Structural Engineering” in our previous articles…. Structural Engineers are like puzzle masters. • The structural analysis deals with the development of suitable arrangement of structural elements for the structures to support the external loads or the various critical combinations of the loads which are likely to act on the structure. The finished design drawings with all pertinent or controlling dimensions for all the members. When the structural steel is used as the material for the structure. The structural design is governed with the standard specifications. whether the structures are built of plastic. reinforced concrete or steel.• The analysis also deals with the determination of internal forces in the various members. The methods of structural analysis and the principles involved in them remain independent of the materials used for all types of structures. • Structural Design • After we are done with the structural analysis. The external reactions are transmitted to the foundations. . proportions and shape of each member and its connecting details. legal. The structural design deals with the selection of proper material. They satisfy all the stress requirements and imposed by the most severe combination of the loads to which the structure is required to transmit or resist including its self-weight. proper sizes. It consists of various specializations like Structural Engineering. aluminium. business & practical skills. Geotechnical Engineering and Surveying. They are called “finished drawings”. parts and connections are made. The materials selected should be economical and safe. After the selection of materials. The finished design drawings are necessary for fabrication and construction. timber. magnitude and the direction of these stresses and the external reactions due to the worst possible combinations of the load. • • • • • • The Art and Science of Structural Design Engineering The practice of Civil Engineering is extremely diverse. The handbooks are used as working tools in structural design. the structural design is known as “Design of Steel Structures”. involving technical. Project Management. State of stress and critical combination of the stresses at various points which include the nature. The ability to combine skills in all these areas will contribute significantly to the success of a civil engineer. we head towards making our final Structural Drawings. The structural design in a limited sense also deals with the design of the various parts or members of a structure. our next step would be selection of appropriate materials to be used for construction. Akashi Kaikyo Bridge. A structural designer has to constantly keep abreast of the latest technological developments in the field of material sciences. Structural engineering is a field of engineering dealing with the analysis and design of structures that support or resist loads. bridges and even space stations. structural designers are called upon to provide their expertise to ensure the stability of the structures. A structural designer is capable of calculating different loads and stresses acting on a structure and designing the structure to resist there forces. Continuously learning and innovation are very integral part of this profession. the difficulties and conditions faced on site and the constraints put down by the Architect. understanding the needs and objectives of a client.World's Longest Suspension Bridge To provide an overview of factors which are relevant to the successful practice of structural engineering. Many new earthquake-resistant techniques are developed every year. marine structures. we have to look at the role of a structural engineer. Japanese Roads . tunnels. A good engineer would incorporate all the best practices in the industry. towers. For tall buildings. Japan . roads. Burj Dubai A good structural designer would maintain a balance between what his clients want. There are additional constraints introduced by the architect. so that the contractor can speed up construction. It is a very important job. During this long duration. be it a real estate developer or a contractor. In the field of structural design. It can no longer have sufficient strength due to aging. A structural designer might have limited options. The steel rods start weathering away due to oxidation. complexity has to be reduced and the design has to be made more simple in order to enable the work to go on at a faster pace. Maybe there could be a special request for using hidden beams so as to not impede the view. or to design a flat slab. Sometimes. You design a building to last for 80 years. concrete gets old and cracks. It is necessary to standardize sizes of beams and columns. He may be asked to work with too few columns. to save money. There could be a very large span cantilever beam or slab. is to minimize costs. There might be occasional earthquake and . It is the duty of a structural designer to design the structure in the most optimal way. and what can be considered safe. an engineer has to make a lot of compromises. so as to keep the costs of construction to a minimum.The biggest objective of any client. It allows for a complete 3dimensional analysis of the structure for different forces which might act on the structure.other natural phenomenon which put tremendous load on the structure. So. The Architect designs the building keeping all the functional aspects in mind that are to be incorporated into the design and then they are executed by a Civil or Structural Engineer. the structures have become extremely complex. all these points must be considered. and attracts the best in the field of Civil Engineering.. It is extremely rewarding and fascinating to design structures that can stand the test of time. These days. Etabs and Staad Pro. manual structural design has become very difficult. Structural designing is a very interesting and innovative field. And as this complexity continues to grow. Relationship between Structural and Architectural Design • Structural Engineering and Architecture are two different sciences that are inter-related. sound and stiff. using software like SAP2000. You have to identify and manage project risk. It helps to create ambient environment that is pleasing to eye. • Inter-relationship of Architectural Design and Structural Design . Structural Designing deals with the study of design of steel in a structure i.e. So. while designing the structure. Whereas Architectural Design deals with design of spaces meant for a particular function. most of the work is done on computers. the internal skeleton of the structure that helps to keep the structure durable. These dimensions are used for the structural design. the preliminary layout of the structure is made which is a part of Architectural Design. shape and arrangement of the various parts of the structure. loose materials for bins and bunkers and so on. The functional aspect includes the determination of location and arrangement of operating utilities. machinery or other features. A structure may be required to withstand any loads out of the various loads or combination of various loads. the various loads have to be taken into account such as: Dead load of the structure Live Load of the structure Wind Load Snow Load • • • • • • • • • • • • . All sections of the structures should have strength at least equal to the structural effects of design loads and forces that occur during the construction and use. vehicles on bridges. The structures are designed between two limits namely the structural safety and economy. These loads include overhead and other cranes. occupancy etc. This layout is useful in determining the type of the structure to be used. etc. stable and stiff. special equipment.• The structures and structural members are designed to meet the functional and structural aspects. Many structures are built to shelter humans and to protect them from the forces of nature. it is ensured that the building or the structure is structurally safe. water in hydraulic structure. Occupants of the buildings should feel safe with regard to their lives and properties. The approximate dimensions of the structures are established. For the purpose of determining the maximum stresses in any structure or member of a structure. durable and economical. The structures should be strong. The safety of many individuals at one time depends on the structural safety of the structure.. equipment. • Co-relation of Architectural Design and Structural Design • At first. The functional aspect includes the determination of the amount and arrangement of space necessary for the structure. The structures and structural members are designed to satisfy three basic principles of design. In the structural aspect. strong. A good design is one which is able to perform satisfactorily the services for which it is designed. incident to the proper functioning of the structures. Then the layout of the structure is then handed over to the structural engineer. All the loads should be taken into consideration which a building or a structure would be required to bear. Both the aspects are inter-related. in industrial buildings. the general size. in buildings. The functional aspect takes into consideration the purpose for which the building or the structure is designed. These components are reinforced with steel that give stability to the structure. More variety of construction is possible without much hassle. RCC Structures RCC Structures are nothing but reinforced concrete structures. certain checks are performed to make sure that the structure remains durable. Columns. Dog Legged Stair In this article. Beams. Slabs. Technology is advancing and so are the techniques of Architectural and Structural Design. Staircase etc. It is a combination of creativity and technicality. sound and stiff.• • Seismic Load Temperature effects Once the required calculations for the structure are done. Architectural Design and Structural Design are meaningless without each other. Staircase is one such important component in a RCC structure. we will discuss different types of staircases and study the RCC design of a dog-legged staircase… Stairs . RCC structure is composed of building components such as Footings. 1. The two flights in plan are not separated by a well. the stairs maybe classified into the following two types. Stairs spanning horizontally 2. Stairs spanning vertically Stairs spanning horizontally These stairs are supported at each side by walls. Design of Dog-legged Stairs Based on the direction along which a stair slab span. Since a stair is often the only means of communication between the various floors of a building. the succeeding flights rise in opposite directions. In a residential house. Loads • • • • Dead load of a step = ½ x T x R x 25 Dead load of waist slab = b x t x 25 Live load = LL (KN/m2) Floor finish = assume 0. In a public building. the staircase may be provided near the main entrance. the location of the stair requires good and careful consideration.5 KN/m Stairs spanning Longitudinally In this. the stairs must be from the main entrance itself and located centrally.Stairs consist of steps arranged in a series for purpose of giving access to different floors of a building. Stringer beams or at one side by wall or at the other side by a beam. A landing is provided corresponding to the level at which the direction of the flight changes. . stairs spanning longitudinally. Various types of Staircases • • • • Straight stairs Dog-legged stairs Open newel stair Geometrical stair RCC design of a Dog-legged staircase In this type of staircase. to provide quick accessibility to the principal apartments. the beam is supported ay top and at the bottom of flights. All staircases should be adequately lighted and properly ventilated. 5 KN/m For the efficient design of an RCC stair. RCC Slabs whose thickness ranges from 10 to 50 centimetres are most often used for the construction of floors and ceilings.Loads • • • • • Self weight of a step = 1 x R/2 x 25 Self weight of waist slab = 1 x t x 25 Self weight of plan = 1 x t x 25[(R2 + T2)/T] Live load = LL (KN/m2) Floor finish = assume 0. is used to construct the ground floor of a building. The load calculations will help us determine. • • RCC Slab Construction • In many domestic and industrial buildings a thick concrete slab. These are the essential steps that are to be followed for the RCC Stair Design. Thin concrete slabs are also used for exterior paving purpose. The strength bearing capacity of a staircase is determined on the amount of steel and concrete used. supported on foundations or directly on the sub soil. . It is a structural element of modern buildings. The ratio of steel to concrete has to be as per standards. Steel in the staircase will take the tension imposed on it and the concrete takes up the compression. how much strength is required to carry the load. we have to first analyse the various loads that are going to be imposed on the stair. Slabs are supported on Columns and Beams. Reinforced Cement Concrete Slab • A Reinforced Concrete Slab is the one of the most important component in a building. For reinforced concrete slabs. • • • • .c. and the underside is modulated: • Corrugated. While making structural drawings of the reinforced concrete slab. A waffle slab. so that when the concrete sets it completely envelops the reinforcement. They may be pre-stressed (in the factory). A ribbed slab. thinner. Formwork is a box-like setup in which concrete is poured for the construction of slabs. post-stressed (on site). These slabs could be cantilevered or Simply Supported Slabs. Construction • • A concrete slab can be cast in two ways: It could either be prefabricated or cast in situ. the slabs are abbreviated to “r. Care should be taken to see that the supporting structure is built to the correct dimensions to avoid trouble with the fitting of slabs over the supporting structure.c. reinforcing steel bars are placed within the formwork and then the concrete is poured. • Design of various types of slabs and their reinforcement For a suspended slab.slab” or simply “r. The corrugations run across the short dimension. In all cases the top surface remains flat. giving added strength in both directions. there are a number of designs to improve the strength-to-weight ratio. from side to side.”. In situ concrete slabs are built on the building site using formwork. or unstressed. or plastic bar chairs are used to hold the reinforcing steel bars away from the bottom and sides of the form-work.• In high rises buildings and skyscrapers. A two way slab has structural strength in two directions. Prefabricated concrete slabs are cast in a factory and then transported to the site ready to be lowered into place between steel or concrete beams. This improves strength and prevents the slab bending under its own weight. pre-cast concrete slabs are slung between the steel frames to form the floors and ceilings on each level. giving considerable extra strength on one direction. usually where the concrete is poured into a corrugated steel tray. Plastic tipped metal. • • Reinforcement design • • A one way slab has structural strength in shortest direction. If one span is much longer than the other. or left there permanently. a ground slab surrounded by brick or block foundation walls. On commercial building sites today. the modification factors for the areas of tensions and compression steel are as given in the figure 2 and 4 of the code. As a slab is usually a slender member the restriction on the span-depth ratio becomes more important and this can often control the depth of slab required in terms of the span – effective depth ratio is given by.• Formwork differs with the kind of slab. If the slab is square and the restraint is similar along the four sides. then more than one-half of the load will be carried in the shorter direction and lesser load will be imposed on the longer direction. Solid Slab spanning in two directions • When a slab is supported on all four of its sides. • • Span – Effective Depth ratios • Excessive deflections of slabs will cause damage to the ceiling. it effectively spans in both directions. a large portion of the load will be carried in the shorter direction and the slab may as well be designed as spanning in only one direction. and it is sometimes more economical to design the slab on this basis. wooden planks are very common. or steel. floor finishes and other architectural details. the form-work may consist only of sidewalls pushed into the ground whereas for a suspended slab. To avoid this. For a ground slab. On low-budget sites. The moment of bending in each direction will depend on the ratio of the two spans and the conditions of restraint at each support. After the concrete has set the wood may be removed. • • • . for instance when laying a concrete garden path. In some cases formwork is not necessary – for instance. • Minimum effective depth = span/ (basic ratio x modification factor) The modification factor is based on the area of tension steel in the shorter span when a slab is singly reinforced at mid-span. plastic and steel are more common as they save labour. plastic. where the walls act as the sides of the tray and hardcore acts as the base. If the slab is rectangular. limits are set on the span-depth ratios. These limits are exactly the same as those for beams. then the load will span equally in both directions. Moments in each direction of span are generally calculated using co-efficient which are tabulated in the code. the form-work is shaped like a tray. Materials used for the formwork • The formwork is commonly built from wooden planks and boards. often supported by a temporary scaffold until the concrete sets. whose lengths do not exceed three times the least lateral dimension. • RCC Column A column forms a very important component of a structure. The span-effective depths are based on the shorter span and the percentage of the reinforcement in that direction. A column may be classified based on different criteria such as: . we are going to discuss in detail the basis of classification of columns and different types of reinforcement required for a certain type of column. It should be realized that the failure of a column results in the collapse of the structure. may be made of plain concrete. In this article. Columns support beams which in turn support walls and slabs.• The slab is reinforced with the bars in both directions parallel to the spans with the steel for the shorter span placed farthest from the natural acis to five the greater effective depth. Supporting the slabs is the main function of the columns… Such slabs are called Simply Supported Slabs. The design of a column should therefore receive importance. Compression members. Simply supported slabs could be either one way slab or a two-way slab. Reinforced Cement Concrete Column Plan and Section A column is defined as a compression member. It depends on the dimensions of the slab. the effective length of which exceeds three times the least lateral dimension. l = unsupported length of column in ‘mm’ D = lateral dimensions of column Types of Reinforcements for columns and their requirements Longitudinal Reinforcement • Minimum area of cross-section of longitudinal bars must be at least 0. Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross crosssection area of the column.8% of gross section area of the column. Based on type of loading • • • Axially loaded column A column subjected to axial load and uni-axial bending A column subjected to axial load and biaxial bending 4. • • • • . Based on slenderness ratio • • Short column. <12 Long column. Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm. > 12 3. The bars should not be less than 12mm in diameter.1. Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column. Based on shape • • • • Rectangle Square Circular Polygon 2. Based on pattern of lateral reinforcement • • Tied columns Spiral columns Minimum eccentricity Emin = l/500 + D/30 or 20mm Where. The pitch should not exceed (if helical reinforcement is allowed). • • 75mm 1/6th of the core diameter of the column Pitch should not be less than. The diameter of the lateral ties should not be less than 1/4 th of the diameter of the largest longitudinal bar and in no case less than 6mm. The pitch of lateral ties should not exceed • • • Least lateral dimension 16 x diameter of longitudinal bars (small) 300mm Helical Reinforcement The diameter of helical bars should not be less than 1/4 th the diameter of largest longitudinal and not less than 6mm.Transverse reinforcement • • It maybe in the form of lateral ties or spirals. • • 25mm 3 x diameter of helical bar Pitch should not exceed (if helical reinforcement is not allowed) Least lateral dimension • • 16 x diameter of longitudinal bar (smaller) 300mm . Shear reinforcement diagram (beam longitudinal section) Here are the steps for the design of Shear Reinforcement in a beam: Step one Nominal shear stress Tv = Vu/bd Where. Vu = shear force due to design load b = width of the beam d = depth of the beam Step two Percentage of steel Percent steel = Ast/bd x 100 Step three .Design of Shear Reinforcement in a beam The beam is failed by the diagonal tension in which the cracks start from support and extend upto a distance equal to effective depth and making an angle more or less than 45 degrees. Ast = 3 x 3. d/(Vus) Where. However.Find the shear stress in concrete (Tc) for the above percentage of steel as per IS:456:2000 Step four If. Design the shear reinforcement for M20 grade concrete.87fy. Tv > Tc Sv = 0. Asv.75d Step five If. Here are the steps for the design of Shear Reinforcement in a beam: Width of the beam = b = 300mm Shear force = Vu = 40KN Effective depth = d = 500mm Area of steel. Sv = spacing of stirrups Asv = Area of stirrups In any case.87fy) Where. nominal stirrups are provided and their spacing is determined by.47 mm2 Step one Nominal shear stress Tv = Vu/bd . Vus = strength of shear reinforcement Example of the Design of Shear reinforcement in a beam Design of Shear reinforcement in a beam A reinforced cement concrete beam 300mm wide and 500mm effective depth is subjected to a shear force of 40KN at the ends. Tv < Tc No shear reinforcement is required. Asv/b. the spacing should not be more than 0. The beam is provided with 6 bars of 20mm diameter of which 3 bars are cranked at 45 degrees.Sv= 0.14/4 x 20 x 20 = 942.4/(0. 54 mm2 Sv = (0.48)/(0.Asv)/0.87 fy) Assuming 6mm diameter.87fy.52 N/mm2 Therefore. . As per IS : 456 : 2000 Asv/bsv = 0.87 x 250 x 56. Tv < Tc No shear reinforcement required.47 x 100)/ (300×500) = 0. 2-legged stirrups@100mm c/c.63 – 0.48 + (0.75-0.75d = 0.14 x 6 x 6)/4 = 56.4/(0.75 x 500 = 375mm Provide 6mm diameter. 2 – legged stirrups Asv = (2 x 3.26N/mm2 Step two Percentage of steel Percent steel = Ast/bd x 100 Percent steel = (942.Tv = 40 x 1000/(300 x 500) = 0. Maximum spacing = 0.54)/(0.47mm say 100mm As per IS:456:2000.56-0. Step four Provide minimum shear reinforcement.4×300) = 102.4b Sv = (0.5) (0.63% Step three As per IS: 456: 2000 Tc = 0.5) Tc= 0. e. which then transfer the force to adjacent structural compression members. The loads carried by a beam are transferred to columns. In Light frame construction the joists rest on the beam.Maximum spacing = 0. loads due to an earthquake or wind). or girders. walls..75 x 230 = 172mm Provide 6mm diameter 2-legged stirrups @ 100mmc/c Shear reinforcement in a Beam Guide to Doubly Reinforced RCC Beam Design RCC Beams RCC beams are cast in cement concrete reinforced with steel bars.75d = 0. Beams generally carry vertical gravitational forces but can also be used to carry horizontal loads (i. . Beams take up compressive and add rigidity to the structure. a doubly reinforced beam is provided. Doubly reinforced beam • Beams reinforced with steel in compression and tension zones are called doubly reinforced beams. we are going to discuss types of beam construction and RCC design of Doubly reinforced beam… RCC beam construction is of two types: • • Singly reinforced beam Doubly reinforced beam Singly reinforced beam A singly reinforced beam is a beam provided with longitudinal reinforcement in the tension zone only. • • • Besides. Usually. This type of beam will be found necessary when due to head room consideration or architectural consideration the depth of the beam is restricted. by making the beam over-reinforced on the tension side. By increasing the quantity of steel in the tension zone. in order to further increase the moment of resistance of a beam section of unlimited dimensions. the moment of resistance cannot be increased indefinitely. to resist the bending moment. this doubly reinforced beam is also used in the following circumstances: . the moment of resistance can be increased by not more than 25% over the balanced moment of resistance. The beam with its limited depth. Hence. may not have enough moment of resistance.Doubly Reinforced Beam In this article. if reinforced on the tension side only. then doubly reinforced beam is required to be designed for additional moment.36.b.87.Xumax)/(0. 70] . Mu – Mulim = fsc.fck.Ast1. The member may be subjected to a shock or impact or accidental lateral thrust.• The external live loads may alternate i. The loading may be eccentric and the eccentricity of the load may change from one side of the axis to another side.87fy) Step 2 If factored moment Mu > Mulim. For example: • • A pile may be lifted in such a manner that the tension and compression zones may alternate.fsc/0. • Design procedure for doubly reinforced beam Step 1 Determine the limiting moment of resistance for the given c/s(Mulim) using the equation for singly reinforced beam Mulim = 0.87fy Step 4 Total tension steel Ast.fy.d [1 – 0.e.Asc (d – d’) Step 3 Additional area of tension steel Ast2 Ast2 =Asc.42Xumax] Or Balanced section Ast1 = (0. may occur on either face of the member. Ast = Ast1 + Ast2 [fsc value from page no. we discussed in detail about the “Theory of Built-up Beams“. The permissible bending stress (sigma bc) is calculated. In this article.1 (Steel tables). Step four From the steel table. The strongest Rolled steel beam that will allow for necessary thickness of power plates at top and bottom. Step six The area of power plate required is found out by trial and error method are by. Step three The required section modulus (Z) for the given beam section is calculated. The geometrical proportion of beam sections are noted. Step five When the depth of the beam is noted. Here are the simple steps that are to be followed for the design of Built-up Beams. then the usually the practice is to select from ISI handbook No. . Step two Value of yield stress (fy) for structural steel is to be assumed. Step one The effective span and load required to be carried by the built-up beam are known. we will move a step ahead and understand the concept of the design of Built-up beams. Maximum bending moment and shear force in built-up beams are calculated. I = Ibeam + 2(Ap)(h/2)(h/2) For finding the area. divide the whole equation by h/2. Step seven Width of cover plate and thickness are decided with the restriction of outstands.Guide to Design of Built-up Beams Design Procedure of the Built-up Beams design In my earlier articles. a trial section for a beam is adopted having the modulus of section (Z) about 25% to 50% in excess of that requirement. a specific type of foundation is adopted. Depending on whether the soil is hard soil or soft soil. cal = (sigma bc)cal x (gross area of tension flange/Net area of tension flange) = Value should not exceed the permissible bending stress (sigma bc) or (sigma bt) = 0/66fy Step nine Check for shear and Deflection Types of Foundations | Design of RCC Structures Foundations Foundation of a structure is like the roots of a tree without which the tree cannot stand. The construction of any structure. Before designing the foundation. be it a residence or a skyscraper. the type of soil is determined. starts with the laying of foundations.Step Eight Check for bending stress/Actual bending compressive stress (sigma bc)calculated = (maximum bending moment/Gross MI) x Distance of extreme fibre in compression from Neutral Axis = (M/Ixx)x y1 Actual bending tension stress sigma bt. Shallow Foundations versus Deep Foundations . On the contrary if the foundation base is absolutely rigid. This can be achieved by gradually decreasing the thickness of the base towards the edges so that the base is only as much thick as it is regarded to resist the induced moments and shears. strap and strip footings Column Footing • In this type of foundation the base of the column is sufficiently enlarged to act as the individual support. Piles are the most commonly used Deep Foundations used in skyscrapers… Types of Shallow foundations Footings Footings are structural members used to support columns and walls and to transmit their load to the underlying soils. Mats or rafts Combined footings. The bearing capacity of soil plays a major role in deciding the type of foundation. • . Shallow foundations are commonly used in smaller structures such as residences and small buildings whose floor height is limited to 10m whereas Deep Foundations are used in Skyscrapers…. If the depth of the foundation is greater than its width the foundation is classified as a deep foundation. The choice of material to be used in the construction of foundations also depends on the weight of the structure on the ground. The widened base not only provides stability but is useful in distributing the load on sufficient area of the soil. Foundations are broadly classified into shallow foundations and deep foundations. the pressure distribution will not be uniform but may follow such pattern. the pressure distribution under the foundation will be uniform. In our designs it is usual to assume a flexible base and hence to regard the pressure distribution to be uniform. The safe bearing capacity of soil should be 180N/mm2 to 200N/mm2. If really the soil is homogeneous and the base of the foundation is flexible. Column footings are usually used in the foundations of residences and buildings where the soil is hard enough has has sufficient bearing capacity. The depth of the foundation means the difference of level between the ground surface and the base of the foundation.Foundations are made in various materials… They could be reinforced cement concrete foundations or brick foundations or stone rubble masonry foundations etc. • Pressure distribution under a Foundation • The law of distribution of pressure under a foundation depends on the homogeneity of the soil and flexibility of the base. • • In our next article. • When a soil is yielding soil. This is a very important point in reinforced concrete structures due to the rigid connection between the different components of the structure. Laying of Column Footing Reinforcement The strength of the foundation determines the life of the structure. That is the reason why we have to be very cautious with the design of foundations because our entire structure rests on the foundation. It is necessary that a foundation shall be designed so that if at all a settlement should occur. Guide to Foundation Design | Column Footings Foundation Design Foundation is the base of any structure. type of structure and its load. As we discussed in the earlier article. Reinforced Concrete Footings . we will discuss the procedure of designing an isolated foundation and also justify the foundation design rules mentioned above. design of foundation depends on the type of soil. Without a firm foundation. In this article. the settlement of all the footings must be more or less the same. In other words.General rules of Foundation Design While designing a foundation the following points must be borne in mind. it will be uniform. the foundations are basically divided into Shallow Foundations and Deep Foundations. we are going discuss the step by step guide to Column Footing Design…. a certain amount of settlement must be reduced as much as possible by bringing down the pressure intensities. On that basis. the structure cannot stand. moments and forces and the induced reactions and to ensure that any settlement which may occur shall be as uniform as possible and the safe bearing capacity of soil is not exceeded. Design Procedure of Column Footings Here is a step-by-step guide to Column Footing Design: Column Footing Plan and Section Step 1 Area required for footing Square = B = (w+w1)/P0 . pillar or wall which i enlarged with projecting courses so as to distribute load. Footings shall be designed to sustain the applied loads. and the angle of slope or depth and location of steps should be such that the design requirements are satisfied at every section. the effective cross-section in compression shall be limited by the area above the neutral plane.Footing comprises of the lower end of a column. In sloped or stepped footings. Po = safe bearing capacity of soil w1 = self weight of footing w = self weight of footing For Rectangle = b/d = B/D A=bxd Net upward pressure on the footing q/p = W/A Step 2 Bending Moment Critical section for maximum bending moment is taken at the face of the column For a square footing. . Q = moment of required factor Depth from shear consideration Check for one way shear Check for two way shear or punching shear Critical shear for one way shear is considered at a distance ‘d’ from face of the column. Mxx = q x B/8 (L – a)2 Mxx = q x L/8 (B – b)2 Myy = q x B/8 (L – a)2 Step 3 To fix the depth of the footing shall be greater of the following: Depth from bending moment consideration d =?(M/Qb) where.Where. 16?fck Area of steel. Tc ——. SF.{for a rectangle Tv = V/4((b+d)d) = k .{for a square Tv k = 0. V = q [ B2 – (b + d)2] SF. Tc Tc = 0.16?fck Step 4 Check for two way shear Critical section for two way shear is considered at a distance at a distance d/2 from all the faces of the column. Tv = k . [? = ratio of sides of the column = 0. Ast = M/(?stjd) . V = qB [ ½(B – b) d] Nominal shear stress.5 + ? > 1 Tc . Tv = V/2((a+d)(b+d)d) ——.Shear force. V = q [L x B – (a + d)(b + d)] Nominal shear stress. 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