DESIGN OF RC BEAMS

BASICS STEP FOR DESIGN AND IS CODES

1. DESIGN OF RC BEAMS PRESENTED BY AGALIYA B V BASINENI UDAY KUMAR

2. Basic Steps To Design 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. 

3. BEAMS Beam is a 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 (  M ulim   ) then the beam is designed as a singly reinforced beam . ( M u < M ulim   ) 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.

4. Doubly reinforced beam When the factored moment (M u ) is greater than the limiting moment ( M ulim ) then the beam is designed as a doubly reinforced beam.  ( M u  >  M ulim ) 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.

5. Neutral axis  Neutral axis separates the compression and tension zones in the beam. it is denoted by  x u . formula to find  x u  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 ( x umax ) always less than the neutral axis ( x u ). x u  ⊁  x umax The value of  x umax  for different grade of steel can be obtained from  IS 456, p- 70 or SP. 16, p-9. 

6. TYPES OF BEAM SECTION Types of beam section There are three types of beam section, Balanced section  In the balanced section,     x u  =  x umax       M u  =  M ulim The strain in concrete and steel will reach their limiting values simultaneously.

7. 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 . x u  <  x umax   M u  <  M ulim 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.     x u  >  x umax     M u  >  M ulim

8. DETAILS PROCEDURE OF DESIGN 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.

9. In the case 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.

10. Step 5:   Calculate the 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

11. Step 7 : The shear 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.

12. Step 9:  The Serviceability 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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