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pnd910
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Homework Statement
The attached solution was given as an answer to a simple truss problem in my sophomore level Statics class. We were to solve for the forces in each member of the truss, using the method of sections for FE and BC, and by joints for the rest. The Hibbeler 12th edition we use indicates that external reactions should be solved for from the free body diagram of the whole truss before using the method of sections. The problem is not from the text. The truss is supported by a pin at point A, with two reactions (Ax,Ay), and has a roller at point C with one reaction (Cy) and a roller at D with one reaction (Dy). There is a 60 Kip force (in x) at point F, and no other external forces. Our (first year) professor solved for Ax=0 using the whole truss free body diagram, then solved Ay=0 using the section free body diagram, since the whole truss free body diagram was indeterminate (3 unknowns, 2 equations left). We ended up with a roller whose reaction force is 80 Kip down at point C, which is not physically possible, since it is not attached to the roller surface (wouldn't be a roller if it was, right). I am sure this solution is wrong, and that his method of solution "created" the 80 Kip force downward at point C, but he says it is valid.
Homework Equations
Sum of forces =0, sum of moments =0
The Attempt at a Solution
I solved for the forces the way we were shown in class, got the "correct" answers, and noted on my homework that the "correct answers" were not correct, that the truss was statically indeterminate and could not be solved for (assuming no deformation in the truss), and that I do not believe that the reaction at C is down, or that FA is in compression, or that FE pushes back harder than it is pushed on. My concerns were not addressed except to say that it probably means the roller weighs 80 Kip, or that we need 80 Kip at C for equilibrium. The truss was in equilibrium to begin with. It is not urgent, but if I am missing something here, I'd like to know what it is. Could someone either confirm or deny his solution, and why? Thanks