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The frame shown in the figure is loaded at S with a force of 2000 N. The reactions at T are denoted by Tx and Ty, while the reaction at W is Wy. Neglect the weight of the members.

A-frame: vertical member PQT 5.4 m tall pinned at T, inclined member PRW to a roller at W 4.8 m from T, horizontal member QRS with a 2000 N downward load at S, 3.6 m from the line of PT

Which one of the following options for the magnitudes of the forces (in N), Tx, Ty and Wy, is CORRECT?

  • A.\(T_{x} = 0, T_{y} = 1000\) and \(W_{y} = 1000\)
  • B.\(T_{x} = 0, T_{y} = 1500\) and \(W_{y} = 500\)
  • C.\(T_{x} = 0, T_{y} = 800\) and \(W_{y} = 1200\)
  • D.\(T_{x} = 0, T_{y} = 500\) and \(W_{y} = 1500\)

Answer: D

The reactions are \(T_{x} = 0\), \(T_{y} = 500\) N and \(W_{y} = 1500\) N.

Treat the whole frame as one free body. W is a roller, so it gives only a vertical reaction; the only other horizontal force could come from the pin at T, and there is no horizontal load, so \(T_{x} = 0\).

Take moments about T. The 2000 N load at S acts 3.6 m to the right of the line PT, and W is 4.8 m from T:

\[ \begin{aligned} W_{y} \times 4.8 &= 2000 \times 3.6 \Rightarrow W_{y} = 1500\ \text{N} \\ T_{y} &= 2000 - 1500 = 500\ \text{N} \end{aligned} \]

The internal pins at P, Q and R do not affect the support reactions of the frame as a whole. Equal shares of 1000 N would need the load midway between T and W (2.4 m), and the 1500/500 split the other way round puts the load nearer T than it is.

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