DESIGN OF RC BEAMS

DESIGN OF RC BEAMS

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  DESIGN OF RCBEAMS PRESENTED BY AGALIYA B V BASINENI UDAY KUMAR
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  Basic Steps ToDesign A Beam  Assume the section size, grade of steel and concrete according to the structure to be built,  Calculate the total UDL, point load and other loads if applicable,  Calculate the factored moment,  Revise the size and grade of materials if required,  Calculate the area of steel required,  Provide the reinforcement detail. Before going to start the design of beam, let's understand the following three concepts which are useful for design calculation.
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  BEAMS  Beam isa horizontal structure member used to carry vertical load, shear load and sometime horizontal load. Beam type  There are two types of RCC beam, Singly reinforced beam and doubly reinforced beam. Singly reinforced beam  If the factored moment (Mu) is less than the limiting moment ( Mulim ) then the beam is designed as a singly reinforced beam. ( Mu<Mulim )  We mostly use the singly reinforced beam in the building if the stresses are less.  The bottom reinforcements are designed to resist the tensile load.  Top reinforcements are also provided in a singly reinforced beam but it is designed to hold the stirrups in position and not designed to be carried the compression load.
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  Doubly reinforced beam When the factored moment (Mu) is greater than the limiting moment (Mulim) then the beam is designed as a doubly reinforced beam. (Mu > Mulim)  The bottom reinforcements are designed to resist the tensile load and top reinforcements are designed to resist the compressive load.  The doubly reinforced beam is most suitable where there is a higher chance of earthquake or stress reversal. also, if an increase in depth is limited for the beam.
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  Neutral axis  Neutralaxis separates the compression and tension zones in the beam. it is denoted by xu. formula to find xu is following,  xu=0.87⋅fy⋅Ast0.36⋅fck⋅bxu=0.87⋅fy⋅Ast0.36⋅fck⋅b  To avoid the brittle failure of concrete, keep the maximum depth of the neutral axis (xumax) always less than the neutral axis (xu). xu ⊁ xumax  The value of xumax for different grade of steel can be obtained from  IS 456, p- 70 or SP. 16, p-9.
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  TYPES OF BEAMSECTION Types of beam section There are three types of beam section, Balanced section In the balanced section, xu = xumax Mu = Mulim The strain in concrete and steel will reach their limiting values simultaneously.
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  Under reinforced section The steel will fail first with showing the warning if under reinforced section is used in the beam. the failure called ductile failure.  Every designer prefers to design the section according to the under the reinforced section. xu < xumax Mu < Mulim Over reinforced section  The concrete will fail first without showing any sign. the sudden failure will occur if the over reinforced section is used. the failure will be called brittle failure. xu > xumax Mu > Mulim
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  DETAILS PROCEDURE OFDESIGN OF BEAM Step 1: In the first step, calculate the intensity of the load which is expected to act on the beam. This can be found out by adding the transferrable loads from the slab to the beam, and the self-weight of the beam. Also find out the clear span of the beam to be designed, from the provided drawings. Step 2: In the next step, find out the effective span of the beam (pg 34 of IS456). In case of a simply supported beam, the effective span is found out, should be the least of the following two values-  Clear span plus effective depth  Centre to center distance of the support.
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   In thecase of Cantilever Beam, the clear span(overhang portion) is mostly adopted as the effective span to be used for the purpose of Design. After finding out the effective span of the beam, find out the bending moment and shear force, from the loads obtained through step 1. Step 3: In this step, find out the trial dimensions of the beam. In the case of a simply supported beam, the trial depth is taken as l/12 to l/15, where l is the effective span of the beam. The breadth of the beam taken as is half the depth of the beam. Step 4: Perform the Depth check step. The depth check formulae provide the minimum required depth of that beam and can be found out from any Design book. The provided depth should always be equal to or greater than minimum depth obtained. If not, the section should be redesigned by using different span to depth ratios.
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  Step 5: Calculatethe amount of reinforcement required. As the section Designed should be an under reinforced one, so that formulae required for under reinforced section should be adopted. After putting the required value in the formulae, a quadratic equation is going to be formed, which when solved, provides the amount of reinforcement used, depending upon the bending moment and the dimensions of the beam. After finding out the amount of reinforcement used, it is then checked against the minimum reinforcement required for the section. It is also checked against the maximum reinforcement that should be used, which is generally 4% of the total cross-sectional area of the beam. If it does satisfy against this two of it, the section should be redesigned. Step 6: In the next step, calculate the cross-sectional area of a single steel bar, that is going to be provided in the beam as per their diameter. Then by dividing the amount of reinforcement used obtained in the previous step, by the cross-sectional area of a single bar, the number of the bars required for bending can be found out easily. The design for bending is completed
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  Step 7: Theshear design starts. At the beginning of this step, find out the nominal shear stress and permissible shear strength depending upon the dimensions, and the percentage of tensile reinforcement. If the nominal shear strength exceeds the permissible shear stress, shear reinforcement is to be designed. It is also verified that the nominal shear stress should not exceed the maximum shear strength, either the section is redesigned. Step 8: As per shear reinforcement formulae, the spacing of shear reinforcement is found out. The obtained spacing should not be more than—  0.75d, where “d” is the depth of the section  300mm.  It is also checked against the minimum spacing required.
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  Step 9: TheServiceability check is done. The Serviceability check includes, check for deflection and cracking (pg 37 of IS 456). The development length (pg 42 of IS4546) is also found out from the given formulae. Step 10: This is the last step of the Complete Design process, in which, detailed design data are provided and a cross-section of the beam showing the reinforcement detailing is also provided.
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