How Do You Draw a Force Diagram for a Structure?

Draw a force diagram by isolating the structure, replacing its supports with reaction forces, and showing every external load as a labelled arrow. The arrow’s direction represents the force direction, while its length may represent magnitude if the diagram is drawn to scale.

Method

1. Isolate the structure. Sketch only the structural system being analysed. Remove surrounding objects while retaining their effects as forces.
2. Add applied loads. Draw point loads at their exact positions and show distributed loads across the appropriate length. Include the structure’s weight at its centre of gravity if it is relevant.
3. Replace supports with reactions. The reaction forces depend on how each support restricts movement.
4. Label forces clearly. State known magnitudes and assign symbols such as $F$, $W$, $R_x$, and $R_y$. Include dimensions so that moments can be calculated.
5. Check equilibrium. For a stationary planar structure, the external forces and moments must satisfy:

$$\sum F_x = 0, \qquad \sum F_y = 0, \qquad \sum M = 0$$

For example, a simply supported beam carrying a downward load should show the load at its application point and upward reactions at the two supports. After calculating the reactions, the structure can be divided into sections to identify tension, compression, and other internal forces.

A common misconception is that a force diagram is just a sketch of the structure. It must instead show all relevant forces, their directions, points of application, and distances.

Exam Technique

For B3.2 Structural systems application and selection, examiners expect correct support reactions, labelled force arrows, and evidence that equilibrium conditions can be applied. Never add a reaction that the support cannot physically provide, and always include dimensions when calculating moments.