Determining load in centre support do I need the bit to the left?

I know a bit about bending moments. 

A 6m beam with a downward force of 1000N exerted, 3m from the right. 3m from left but with a support 2m from left. 

Would that be the same as a 4m beam supported at both sides with a downward force 1m from the left, 3m from the right and me trying to determine the load at he left support? Well, calling the centre support left and ignoring the one to the left of that?

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2 Answers

  • 1 month ago

    No, that is not the same.

  • Adam D
    Lv 7
    1 month ago

    It would not be the same.  Putting it simply, since the beam is continuous over that middle support, the additional length of beam and difference in stiffness it creates would cause the moments to be distributed differently. 

    The beam you're describing (removing the far left support) is statically determinate - you can solve the reactions with only equations of equilibrium.  The sum of vertical forces equal to zero, the sum of moments about any point equal to zero, this gives 2 equations, you have 2 unknowns (the reactions), then it's just algebra.

    The beam picture you've shown is indeterminate - there are too many unknowns to solve with just equilibrium alone, you need to use methods involving material compatibility.  You've still only got the 2 equilibrium equations, but you've now got 3 unknowns.  These methods would be a little tricky to teach oneself with only a basic knowledge of bending moments.

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