Determining a statically indeterminate structure

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Discussion Overview

The discussion revolves around determining whether a given structure is statically indeterminate or determinate. Participants explore the implications of external and internal forces, member redundancy, and the application of equilibrium equations in the context of structural analysis.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant believes the structure is statically determinate, citing three unknowns and three equations, while noting that external loads are known.
  • Another participant argues that although the reactions are statically determinate, the presence of a redundant member makes the structure internally statically indeterminate.
  • Some participants express confusion about how the structure can be considered indeterminate when they can solve for all member forces without issue.
  • A later reply suggests that removing certain members can lead to a stable, internally statically determinate structure, questioning the initial analysis.
  • One participant mentions a formula for determining degrees of indeterminacy, applying it to conclude that the truss is statically indeterminate to the first degree.
  • Another participant acknowledges an error in their previous calculations, having removed a member by accident.
  • A clarification is provided that externally the structure is determinate, but internally it is indeterminate.

Areas of Agreement / Disagreement

Participants do not reach a consensus; multiple competing views regarding the structure's indeterminacy remain, particularly between external and internal considerations.

Contextual Notes

Participants reference specific formulas and methods for assessing structural indeterminacy, indicating the discussion may hinge on varying interpretations of structural analysis principles.

eurekameh
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Is this structure statically indeterminate? I'm thinking it actually statically determinate, since there are three unknowns (Ax, Bx, By) and three equations. The external loading Px, Py are known. I also solved for the forces in the members of the truss and they seem to work out, but my instructor solved it thinking it is statically indeterminate. Does anyone know why?
 
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The reactions are statically determinate from the equilibrium equations, but looks like there's a reduntant member in there, making it internally statically indeterminate. To the first degree.
 
How are you determining this by just looking at the structure?
I'm not understanding how it's indeterminate when I can solve for all of the forces without a problem.
 
eurekameh said:
How are you determining this by just looking at the structure?
I'm not understanding how it's indeterminate when I can solve for all of the forces without a problem.
When you look at the structure, you can remove either a bottom chord, a top chord, or one of the diagonals, and the truss is still stable and now internally statically determinate and solvable by the equilibrium equations. I am not sure how you solved for your member forces with all members in there.

Ther are some formulas for determining degrees of indeterminancy that you must be careful when using. Like m + R - 2J , where m is the number of members, R is the number of support force components, and J is the number of joints. In this example, 6 + 3 - 8 = 1, so the truss is statically indeterminate to the first degree.
 
Last edited:
Ah, thanks. I removed one of the members by accident when I tried solving for them.
 
To clarify: Externally it is determinate and internally it is indeterminate.
 

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