NCEES Practice Problem 112 (Building)

[kennyb04]

Has anyone worked problem 112 from the NCEES practice morning exam and got the correct answer?  They are showing the Force in Column AD due to the 10k load as 12k.  Assuming the reaction is 12k and there is load in the brace I don't understand how the load in Column AD could be 12k.  Are they possibly assuming tension only braces so there is no component of load in the brace next to the compression column and and the column takes the entire 12k load?

 

[StructuralVFL]

Sum the moments about the base in order to get the reactions.  for clarity, let's call the top right joint point E. To determine the force in the brace member, analyze the joint at point A (lower left hand corner or the braced frame).  Sum the moments in the x direction and you will get 5/sqrt(61)[AE]=0, which works out to be AE=0.  This shows that the force in member AE is zero, meaning the column is carrying the entire 12k axial load.

Does that make sense?

 

[kennyb04]

It is shown as a pin on both base connections.  So shouldn't 5/sqrt(61)[AE] = x reaction?  Assuming a 10 k load, the reaction should be 5 or something around 5.  And if there is a component of load on the brace than the column can't equal the reaction force.  Maybe it was assumed that one reaction was a roller and took no component of load (or I misread it as both pinned).

 

[StructuralVFL]

I suppose you're right in that we're supposed to assume one is a roller, otherwise the frame is indeterminate if both are pinned (4 unknown equations, 3 equations). Hopefully we don't have any questions that leave room for ambiguity on the upcoming test.

 

[kennyb04]

Ya, I modeled in FEM as well and did not get their force (because of the indeterminate with 4 unknowns).  I will just focus on the general concept of combining a portion of load from two directions and hopefully the set-up on the actual exam will be more clear.