MecMovies 4.0 : M5.8: Rigid Bar with Opposing Members

The figure shows the free body diagram of the rigid bar A B C D. The free body diagram cuts through the vertical bars 1 and 2. A load P directed vertically downwards is applied at D. The reaction forces on the bars 1 and 2 are F subscript 1 and F subscript 2 directed vertically upwards and vertically downwards respectively.

Draw a $FBD$ of the rigid bar.

Assume tension forces in members $(1)$ and $(2)$ .

Note: We make a practice of always assuming internal tension forces in the axial members even though we intuitively recognize that member

(2)

will be a compression member. The practice of always assuming tension in the axial members greatly simplifies the remainder of the problem and makes it much easier to derive the correct compatibility equation.

Write the equilibrium equation for moments about pin support $A$ :

$\sum_{} M_{A} = F_{1} (760 mm) - F_{2} (1, 520 mm) - P (1, 640 mm) = 0$