A rectangular beam 100 mm deep and 75 mm wide in simply supported over...
Problem Statement
A rectangular beam 100 mm deep and 75 mm wide is simply supported over a span of 6 m. It carries a uniformly distributed load of 4000 N/m over the whole span. Calculate the maximum bending stress developed in the section at:
- Supports
- 1.5 m from the support
- Mid-span of the beam
Solution
Given data:
- Depth of beam, d = 100 mm
- Width of beam, b = 75 mm
- Span of beam, L = 6 m
- Uniformly distributed load, w = 4000 N/m
Step 1: Calculation of Maximum Bending Moment (M)
The maximum bending moment occurs at the mid-span of the beam and is given by the formula:
M = wL^2/8
Substituting the given values, we get:
M = (4000 x 6^2)/8 = 135000 N-mm
Step 2: Calculation of Maximum Bending Stress (σ)
The maximum bending stress occurs at the top and bottom fibers of the beam and is given by the formula:
σ = My/I
- M = Maximum bending moment
- y = Distance of fiber from the neutral axis
- I = Moment of inertia of the beam section
The moment of inertia of a rectangular beam section about its neutral axis is given by:
I = bd^3/12
Substituting the given values, we get:
I = (75 x 100^3)/12 = 62500000 mm^4
Now, let's calculate the maximum bending stress at different points:
(i) At supports
At the supports, the bending moment is zero. Therefore, the bending stress is also zero.
(ii) 1.5 m from the support
At a distance of 1.5 m from the support, the bending moment is:
M = w (L/2 - 1.5)^2/2
Substituting the given values, we get:
M = (4000 x 2.25)/2 = 4500 N-mm
The distance of the top and bottom fibers from the neutral axis is half the depth of the beam, i.e., y = d/2 = 50 mm. Therefore, the maximum bending stress is:
σ = My/I = (4500 x 50)/62500000 = 0.036 N/mm^2
(iii) At mid-span of the beam
At mid-span, the bending moment is maximum and is equal to M = 135000 N-mm. The distance of the top and bottom fibers from the neutral axis is half the depth of