FBD Feedback Structure - mkraska/meclib GitHub Wiki

Feedback Structure

Back to Feedback for Free Body Diagrams

• Free Body Diagram
  • Correct isolation of the system? All elements inside the system must be activated (visible), all adjacent elements of the environment must be deactivated (hidden, grey). Use fb_system() to check.
  • Distributed loads ("q") must be deactivated and replaced by one or two resultants. Use Feedback for Line Loads for check and feedback.
    • Value (expression as name),
    • Direction,
    • Location (must go through CG of the distributed load)
  • Supports must be replaced by appropriate reactions. Available feedback functions:
    • Feedback for "fix1", Feedback for "fix12", Feedback for "fix123", Feedback for "fix13",
    • Feedback for unidirectional supports, generic feedback for "bar", "dashpot", "springt", "springc", "fix1", "rope"),
    • fb_bar, fb_bar_name more specific feedback for bars
      • Correct Name (based on support names, bar or rope names)
      • Correct type (moment, force) and number
      • Correct direction
  • No spurious forces or moments (students might create a force outside the system and forgot to delete). Use fb_reactions() to check if the number of forces and moments is correct.
• Force and moment balance (equations of equillibrium)
  • Expression for resultant forces and moments match the free body diagram, see Sums of Forces and Moments
  • Reference point for moment balance appropriately selected (such that two reactions go through that point)
• Solution for the unknown reactions, given as list of equations
  • Are the balances satisfied? Apply ev(balance, solution)
  • No tricky additional equations (solution might contain all zero reactions and setting the external load to zero).
  • No spurious equations for other variables not being part of the problem (use fb_vars()).

The input scaffold of the question should not give away too much of the solution.

• Name and number of the reactions
  • Don't use them in the input description of their values. Use matrix input with equations A_x=2*F
• Choice of reference point for moment balance
  • Let the student indicate the reference point using "crosshair"

FBD Example Question