In this article, we share the details of percentage of steel in beam as per IS 456 and IS 13920, minimum and maximum area of reinforcement in beam as per different codel provision.
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Table of Contents
Minimum & Maximum Percentage of Reinforcement in Beam
The indian and english standards of RCC Design limited the area of reinforcement in beam to overcome the chances of brittle failure of beam.
Brittle failure of beam is sudden failure such as crushing which not gives the warning before failure. The maximum and minimum percentage of steel provided in beam to avoid such failure and for flexural behavior’s of beam.
Let’s see the norm of indian and english standard for minimum and maximum percentage of steel in beam.
Also Read: General Structural Detailing Symbol
Percentage of Steel in Beam as per IS 456
IS 456 consist the criteria of design and detailing for Reinforced concrete structure which recommend the clause 26.5.1 for minimum and maximum area of reinforcement in beam.
Minimum Reinforcement
The minimum area of tension reinforcement shall not be less than that given by following formula.
As/bd = 0.85/Fy
Where,
- As = Minimum area of tension reinforcement
- b = Width of beam
- d = Effective depth of beam
- Fy = Characteristic Strength of reinforcement in N/mm2
Also Read: Ductile Detailing of Column as per IS 13920-2016
Maximum Reinforcement
The maximum area of tension reinforcement shall not exceed 0.04bD. Where, b = width of beam and D = Overall depth of beam.
0.04bD mean the maximum reinforcement in beam should be limited to 4% of overall cross sectional area of beam as per IS 456
Percentage of steel in beam as per IS 13920
IS 13920 consist the criteria for ductile design and detailing of reinforced concrete structure which recommend the clause 6.2 for minimum and maximum area of reinforcement in beam.
Minimum Reinforcement
The minimum longitudinal steel ratio required on any face at any section is:
Pmin = 0.24 ((Fck^(1/2))/Fy)
Where,
- Pmin = Minimum percentage of steel in beam
- Fck = Characteristic Strength of concrete in N/mm2
- Fy = Characteristic Strength of reinforcement in N/mm2
Also Read: Ductile Detailing of Beam as per IS 13920-2016
Maximum Reinforcement
The maximum percentage of steel in beam as per is 13920 is 2.5%. The longitudinal steel should not exceed on any face at any section by limiting value.
IS 13920 also recommended few other clause for minimum steel in beam at various face and section. As per the clause 6.2.3 and 6.2.4, the minimum steel in beam at various locations are,
- Longitudinal steel on bottom face of a beam framing into a column (at the face of the column) shall be at least half the steel on its top face at the same section.
- Longitudinal steel in beams at any section on top or bottom face shall be at least 1/4th of longitudinal steel provided at the top face of the beam at the face of the column; When the top longitudinal steel in the beam at two supporting column faces is different, the larger of the two shall be considered.
Also Read: Concrete Design Load Combinations as per ASCE-7 & ACI-318
Let’s Understand with example.
For example a beam having six location of steel requirement as shown in figure.
- The Positive steel at a joint face must be at least equal to half the negative steel at that face.
- Also the steel provided at each of the top and bottom face of the member at any section along its length shall be at least equal to 1/4th of the maximum negative moment steel provided at the face of either joint.
Also Read: What is Stiffness Modifiers – Value As Per IS 16700 & ACI-318
Steel at location-1 at least higher of,
- Steel provided as per requirement of Negative Bending Moment.
Steel at Location-2 is at least higher of,
- 1/4th of Ast-1
- Ast-2
- 1/4th of Ast-3
Steel at Location-4 is at least higher of,
- 1/4th of Ast-1
- 1/4th of Ast-3
- 1/2 of Ast-1
- Ast-4
Also Read: Stages in Structural Design of Building | Process of design