A train of four wheel loads 30 kN, 10 kN, 20 kN and 40 kN (distance between each sequential wheel is 1 m) is passing in the given order leading with 30 kN, from right to left on a simply supported girder of 10 m span. Which of these wheel loads should be placed at the section 2m from the left side support, to produce Maximum Bending Moment at that section?
A train of four wheel loads 30 kN, 10 kN, 20 kN and 40 kN (distance between each sequential wheel is 1 m) is passing in the given order leading with 30 kN, from right to left on a simply supported girder of 10 m span. Which of these wheel loads should be placed at the section 2m from the left side support, to produce Maximum Bending Moment at that section? Correct Answer 30 kN
Explanation
Given wheel loads are 30 kN, 10 kN, 20 kN, and 40 kN
30 kN is the leading load
Remember load is moving from right to left
It is asked at a section 2 m from the left support
Given the length of the span = 10 m
Spacing between point loads = 1 m
Let C be the point at 2 m from the left support
To get a maximum bending moment at any load on section C
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We determine the average load on the portion AC and BC
|
Loading Conditions |
Average load on AC |
Average Load on BC |
|
|
When all loads are on span CB |
0 |
(30 + 10 + 20 + 40)/8 =12.5 |
12.5 > 0 |
|
When 30 kN crosses C and is on AB |
30/2 = 15 |
(10 + 20 + 40)/8 = 8.75 |
8.75 < 15 |
As we can see when 30 kN load crosses section C average load on the CB portion is less than the AC portion
∴ 30 kN is the load that must be placed at 2 m from left support.