Calculating Reactions in 4-Reaction Beam

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    Beam Reactions
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Discussion Overview

The discussion revolves around calculating the load taken by each reaction force in a beam with four reaction forces. Participants explore methods for determining these reactions, particularly in the context of statically indeterminate structures, and consider the implications of symmetry and elastic properties.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the method for calculating loads on a beam with four reaction forces, noting that taking moments about one point may not be sufficient, especially for non-symmetrical beams.
  • Another participant states that a beam with four reaction forces is statically indeterminate due to having more unknowns than equations, suggesting that assumptions about the elastic properties of the beam or its supports are necessary to find a solution.
  • A request for clarification on the assumptions regarding elastic properties is made, indicating a need for further detail on this aspect.
  • One participant provides an example involving a beam suspended from four identical springs, explaining that the relationship between force and extension can lead to equations for the forces, though they caution that the solution is complex.

Areas of Agreement / Disagreement

Participants express differing views on the methods for calculating reactions in statically indeterminate beams, with no consensus reached on a specific approach or solution.

Contextual Notes

The discussion highlights the limitations of the current understanding, particularly regarding the assumptions needed for elastic properties and the complexity of solving the equations involved.

bumfluff
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If you have a beam with 4 reaction forces, how do you calculate the load taken by each reaction? It occurs to me that you cannot simply take moments about one point so what is the method you can use to find the reactions? I made one example where the beam was symmetrical so I tried equating it to a beam supported at two points but it seems to me that the way I did this would not work for a beam where there is no symmetry.
 
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A beam with four (even with three) forces is said to be "statically indeterminate" because there are 4 unknowns and only two equations. In order to get an answer you have to make some assumption about the elastic properties of either the beam or the strings supporting it.
 
Would you be able to elaborate on the the assumptions of elastic properties that you would have to make?
 
As an example: If the bar is suspended from four identical springs, such that F=kx, where x is the extension in length of the spring, you can get an equation for the forces in terms of the distance the beam drops, and the angle it makes with the horizontal. This could be solved for the four forces. It is not easy.
 

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