Calculating Re-acting Forces of 10m Beam w/ 3 Forces & Bending Moment Diagram

In summary, the problem involves a 10 meter beam with 3 forces and 2 reacting forces acting on it to keep it in equilibrium. The re-acting forces are located 2 meters from either end of the beam. The forces on the beam are a 10kN force at 3 meters from the left end, a 20KN force at 10 meters from the left end, and a distributed load of 0.5 kN/m over the full length of the beam. The re-acting forces are calculated to be 40KN each. However, there may be an error in the calculations for the bending moment at each meter. Double checking the signs of the moment calculations and considering the UDL as 0
  • #1
smell5
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Question : A beam 10 meters long, with 3 forces acting on it and 2 re-acting forces on it to keep it in equilibrium. The re-acting forces are at 2 meters from either end. The forces onto the beam are at 3 meters from the left end of 10kN, 10 meters from the left end of 20KN and a distributed load over the full 10 meter beam of 5KN.

a) work out the 2 re-acting forces
b) Calculate the bending moment at 1meter intervals along the beam
c) Draw a bending moment diagram of the beam


Right so there's the question, my issue I believe; is calculating the re-acting forces, I have tried numerous ways, and believe that both reacting force will be 40KN each.

As taking moments from R1, (10 x 1) + (50 x 3) + (20 x 8) = R2 x 8
R2 = 40

As the UDL = 50KN at 5 meters, R1 = 80 - 40 =40KN

Then I drew a sheer diagram which seemed fine.

Then I calculated each meter individually expecting the 10th meter to equal zero.

However I get M10 = 0 - (10 x 5 x5) + (40 x 8 ) - (10 x 7 ) + (40 x 2) = 60
Obviously not right!

Could someone please give me some guidance whether you believe it's the re-acting forces or whether it's my calculations for working out each meter.

Any Help Would Be Appreciated.
 
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  • #2
The applied loads are not symmetrical, so the reacting loads are not symmetrical.
Double check the signs of your moment calculations.
The way the problem statement is given, a load of 5kN is distributed oven the length of the beam. I read this as a UDL of 5 kN / 10 m = 0.5 kN / m
 

1. How do you calculate the re-acting forces of a 10m beam with 3 forces?

The re-acting forces of a 10m beam with 3 forces can be calculated by using the principle of equilibrium, which states that the sum of all forces acting on an object must equal zero. This means that the upward forces must balance the downward forces, and the leftward forces must balance the rightward forces. By setting up equations and solving for the unknown forces, the re-acting forces can be determined.

2. What is a bending moment diagram and why is it important?

A bending moment diagram is a graphical representation of the variation of bending moment along the length of a beam. It shows the distribution of forces acting on the beam and helps to visualize the internal stresses and deflections of the beam. It is important because it can aid in designing and analyzing structures to ensure they can withstand the applied forces without failure.

3. How do you determine the bending moment at a specific point on the beam?

The bending moment at a specific point on the beam can be determined by taking the sum of all the moments on one side of the point and setting it equal to the sum of all the moments on the other side of the point. This equation can then be solved for the bending moment at that point.

4. What is the difference between a positive and negative bending moment?

A positive bending moment occurs when the top of the beam is in compression and the bottom is in tension, causing the beam to bend downwards. A negative bending moment occurs when the top of the beam is in tension and the bottom is in compression, causing the beam to bend upwards. The direction of the bending moment depends on the direction of the external forces and the support conditions of the beam.

5. How does the magnitude of the re-acting forces affect the bending moment diagram?

The magnitude of the re-acting forces can greatly influence the shape and magnitude of the bending moment diagram. A larger re-acting force will result in a larger bending moment, while a smaller re-acting force will result in a smaller bending moment. This can also affect the maximum bending moment and the location of the point of maximum bending moment on the beam.

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