A doubly reinforced rectangular concrete beam has a width of 300 mm an...
To calculate the moment of resistance of the doubly reinforced rectangular concrete beam, we need to consider the following steps:
1. Determine the stress in the tension steel:
The stress in the tension steel can be calculated using the formula:
Stress in tension steel = (Tension force in steel) / (Area of tension steel)
Given that the area of tension steel is 2200 mm2 and the steel used is Fe250, which has a yield strength of 250 MPa, we can calculate the tension force in steel as:
Tension force in steel = (Area of tension steel) x (Yield strength of steel)
Substituting the values, we get:
Tension force in steel = 2200 mm2 x 250 MPa = 550000 N
Therefore, the stress in the tension steel is:
Stress in tension steel = 550000 N / 2200 mm2 = 250 MPa
2. Determine the stress in the compression steel:
The stress in the compression steel can be calculated using the formula:
Stress in compression steel = (Compression force in steel) / (Area of compression steel)
Given that the area of compression steel is 628 mm2 and the steel used is Fe250, which has a yield strength of 250 MPa, we can calculate the compression force in steel as:
Compression force in steel = (Area of compression steel) x (Yield strength of steel)
Substituting the values, we get:
Compression force in steel = 628 mm2 x 250 MPa = 157000 N
Therefore, the stress in the compression steel is:
Stress in compression steel = 157000 N / 628 mm2 = 250 MPa
3. Determine the stress block parameters:
As per IS: 456-2000, the stress block parameters for M20 grade concrete are:
- Depth of the stress block (a) = 0.48 x effective depth = 0.48 x 500 mm = 240 mm
- Lever arm (z) = 0.87 x effective depth = 0.87 x 500 mm = 435 mm
4. Determine the moment of resistance:
The moment of resistance (MR) can be calculated using the formula:
MR = (Area of tension steel) x (Stress in tension steel) x (Lever arm for tension steel) + (Area of compression steel) x (Stress in compression steel) x (Lever arm for compression steel)
Given that the effective cover for compression steel is 50 mm, the lever arm for compression steel is:
Lever arm for compression steel = effective depth - cover - (0.5 x diameter of compression steel)
Substituting the values, we get:
Lever arm for compression steel = 500 mm - 50 mm - (0.5 x √628) mm = 452.80 mm
Substituting the values into the formula, we get:
MR = (2200 mm2) x (250 MPa) x (435 mm) + (628 mm2) x (250 MPa) x (452.80 mm)
MR = 209.20 kNm
Therefore, the moment of resistance of the doubly reinforced rectangular concrete beam is 209.20 kNm, which corresponds to option (b).
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