Design of Concrete Structures: Solving a Tricky Frame Question from Past Exam

[Bill92]

Hi the following question is from a previous exam for the subject Design of concrete structures, just wondering if I can get any pointers as I was not able to find the reactions.

 

[cake of doom]

What was the process you went through to try to determine Ra and Rd? The exercise is not that difficult so I think you may be tripping yourself up somewhere.

 

[Bill92]

I summed the moments at point A making the UDL into an EPL to find By, then summed the forces in the y direction to find Ay. Pretty sure that part is right but I was unable to determine the x reactions at A and B as well as the moment reaction at point A.

 

[cake of doom]

If I was to say (50*4)+(80*4)-(Vd*8) = 0 .'. Vd = ? Would that point you in the write direction? Or have i misunderstood your process?

 

[PhanthomJay]

Careful, not that simple, let's not forget the fixed end moment at A, without which this frame would collapse. It must enter into the equations when summing moments. It is best to break apart each member in a free body diagrams of each. Note that internal pins cannot support or transfer any moments, they only can support or transfer forces. Note also that CD is a 2-force member, and as such, it can support vertical forces only directed along the axis of CD. Bill, I believe you have calculated By and Ay correctly, although you don't state your results, so I can't be sure. Now find Ax and the FEM at A, and of course you know what Dx and Dy are, right?