Determining a statically indeterminate structure

[eurekameh]

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?

 

[PhanthomJay]

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.

 

[eurekameh]

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.

 

[PhanthomJay]

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.

 

[eurekameh]

Ah, thanks. I removed one of the members by accident when I tried solving for them.

 

[pongo38]

To clarify: Externally it is determinate and internally it is indeterminate.