Design of Plinth Beam as per Indian Standard (IS code) Guideline

In this article, I have explained the purpose of providing the plinth beam and the Design parameter such as width, depth, span to depth ratio, width to depth ratio, Grade of concrete, Grade of steel, reinforcement detailing, and lap splices.

The main purposes to Design plinth beam are as follows;

  • Plinth Beam carries the load of the wall.
  • It ties all the columns of that level.
  • It Resists the lateral forces.
  • The plinth prevents the differential settlement of buildings.
  • It controls the cracks in the masonry wall.
  • It reduces the effective length of columns which prevents the columns from buckling.

Dimension Requirement for the Design of Plinth Beam

1. Width of Beam

As per IS 13920-2016, the minimum width for the Design of Plinth Beam shall not be less than 200 mm. The width of the beam shall not be more than the width of supporting beams such as columns and shear walls.

2. Depth of Beam

As per IS 13920-2016, The Design Depth of beams shall not be more than 1/4th of the Clearspan. It shall not be less than 200 mm. Depth of beam not only depends upon the dead load, live load, earthquake load, etc. but also depends upon the span of the beam.

3. Width to Depth Ratio

As per IS 13920-2016, the ration of width to depth shall be more than 0.3.

4. Span to Depth Ratio

The ratio of span to overall depth shall be in the range of 12 to 15.

 5. Width to Depth Ratio

As per IS 13920-2016, the ration of width to depth shall be more than 0.3.

6. Grade of Concrete

As per IS 456-2000, the Minimum Grade of concrete of Reinforced Concrete beam shall not be less than M20. Where M stands for Mix and 20 stands for Specified compressive strength of 150 mm cube after 28 days of curing and expressed in N/mm 2

As per IS 13920-2016, The Minimum Grade of a structural member shall be M20, but M25 for Buildings if the height of the building is more than 15 m for seismic zone III, IV, and V.

7. Grade of Steel

As per IS 456-2000, The Grade of steel for Stirrups Shall not be less than Fe 415. Steel Main Reinforcement used shall be of grade Fe 500 and Fe 550, that is; high strength deformed bars. Fe 500 stands for Fe means Iron and 500 means Characteristics tensile strength of steel bars is 500 in N/mm 2.

Reinforcement Detailing of Beam

Design of Beam

1. Top Reinforcement

  • A minimum of two numbers of Top reinforcement shall be provided.
  •  The diameter of Top reinforcement shall not be less than 12 mm.

2. Bottom Reinforcement

  • Minimum two number of Bottom reinforcement shall be provided.
  • The diameter of Bottom reinforcement shall not be less than 12 mm.

3. Transverse /Shear Reinforcement

As per IS 456-2000

  • The maximum spacing of shear reinforcement shall not be more than 0.75 d for vertical stirrups and d for inclined stirrups at 45 degrees, where d is the effective depth of the section. In no case shall be spacing exceed 300 mm.
  • Minimum shear reinforcement shall provide such that Asv/bSv > 0.75/0.87 Fy.

     Where,

                     Asv = Total cross-section area of stirrup legs effective in shear.

                     Sv = stirrup spacing along the length of the member.

                     b = breadth of the beam.

                    fy= characteristics strength of the stirrups reinforcement  in N/mm 2 which shall not be    taken greater than 415 N/mm 2

  • The minimum diameter of shear reinforcement shall not be less than 8 mm and having U link with 1350 hook with an extension of 6 times diameter (but not less than 65 mm) at each end.
  • The First stirrups shall be at a distance not more than 50 mm from the joint face.
  • Spacing between the stirrups shall not be more than 150 mm.

4. Minimum and Maximum Reinforcement for the Design of the Plinth beam as per IS 456-2000.

Tension Reinforcement

  • The Minimum area of tension reinforcement shall not be less than that of the given by the Following

                                As/ bd = 0.85/Fy

     Where;

                  As = minimum area of tension reinforcement.

                  b = breadth of beam or the breadth of the web of T-beam.

                  d = effective depth, and

                 fy= characteristic strength of reinforcement in N/mm 2.

  • The maximum area of tension reinforcement shall not be more than 0.04 bD.

Compression Reinforcement

  • The maximum area of compression reinforcement shall not be more than 0.04 bD.

5. Lap splices

  • Lap length shall not be provided within the junction of the Beam – column and Beam – Beam junction.
  • It shall not be within the distance of 2d from the face of the column, where d is the effective depth of the beam.
  • It shall not be provided within a quarter length of beam adjoining the location where the flexural yielding may occur under earthquake effects.
  • Not more than 50% of the area of a steel bar on either Top or bottom face shall be lapped at any section.
  • Lap length in beam shall not less than the development length of the largest longitudinal reinforcement bar in tension.

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