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Engineering Statics: Open and Interactive

Section 8.1 Internal Forces

In Subsection 3.3.3 you were introduced to axial loadings, which were either tension or compression, or possibly zero. This section will explain two other internal forces found in two-dimensional systems, the internal shear and internal bending moment.
Internal forces are present at every point within a rigid body, but they always occur in equal-and-opposite pairs which cancel each other out, so they’re not obvious. They’re there however, and when an object is cut (in your imagination) into two parts the internal forces become visible and can be determined.
You are familiar with straight, two-force members which only exist in equilibrium if equal and opposite forces act on either end. Now imagine that we cut the member at some point along its length. To maintain equilibrium, forces must exist at the cut, equal and opposite to the external forces. These forces are internal forces.


In this interactive you may change the external forces on this two-force member from tension to compression, and then cut the beam into sections. You will see that each piece must have an internal force to balance the external force. If you move the cut point, you’ll see that the value of the internal force is constant at every location.
Figure 8.1.1. Internal forces in a straight two-force member.
Now let’s examine the two-force member shown in Figure 8.1.2. This time, the member is L shaped, not straight, but the external forces must still share the same line of action to maintain equilibrium. If you cut across the object, you will obtain two rigid bodies which must also be in equilibrium. However, adding an equal and opposite horizontal force at the cut won’t produce static equilibrium because the two forces form a couple which causes the piece to rotate. This means that something is missing!
L-shaped beam turned 45 degrees with force applied horizontally to each end. If the beam is split on one leg, a horizontal force alone does not result in equilibrium; a moment is necessary.
Figure 8.1.2. A horizontal force alone does not create equilibrium.
Two-dimensional rigid bodies have three degrees-of-freedom and require three equilibrium equations to satisfy static equilibrium in order to prevent translation in the \(x\) direction, the \(y\) direction, and to prevent rotation about the \(z\) axis.
Assuming the material is rigid, the connection between the two halves must resist both translation and rotation, so we can model this connection as a fixed support and replace the removed half of the link with a force reaction and a couple-moment reaction as shown in the free-body diagrams of Figure 8.1.3. This internal loading is actually a simplification of a more complex loading distributed across the section plane. The couple \(\vec{M}\) represents the net rotational effect of the force system on the surface of the cut.
The L-shaped beam from previous figure shown with resultant force and moment achieve equilibrium.
Figure 8.1.3. The internal forces are represented as an equal and opposite force \(\vec{F}\) and a bending moment \(\vec{M}\)
The horizontal force can also be resolved into orthogonal components parallel and perpendicular to the cut. These components have special names in the context of internal forces.
The L-shaped beam from previous figure shown with resultant force broken into normal and shear components, as well as the bending moment.
Figure 8.1.4. The internal forces are represented as a normal force \(\vec{N}\text{,}\) a shear force \(\vec{V}\text{,}\) and bending moment \(\vec{M}\)
The internal force component perpendicular to the cut is called the normal force. This is the same internal tension or compression force that we assumed to be the only significant internal load for trusses. If the object has an axis, and the cut is perpendicular to it, the normal force may also be properly called an axial force.
The internal force component parallel to the cut is called the shear force. The word shear refers to the shearing that occurs between adjacent planes due to this force. You can get a feel for shearing adjacent planes by sliding two pieces of paper together.
The internal couple-moment is called the bending moment because it tends to bend the material by rotating the cut surface.
The shear force is often simply referred to as shear, and the bending moment as moment; together with the normal or axial force the three together are referred to as the “internal forces”. The symbol \(\vec{V}\) is commonly chosen for the shear force, and \(\vec{A}\text{,}\) \(\vec{P}\) or \(\vec{N}\) for the normal force and \(\vec{M}\) for the bending moment.


This interactive demonstrates how the internal loads at a cut may be represented.
Figure 8.1.5. Internal Loading in a L shaped member.

Thinking Deeper 8.1.6. Deformation.

The controlling design parameter for most engineering systems is deformation. Thankfully, due to a property called elasticity, most materials will bend, stretch, and compress, long before they ultimately break. For example, when designing the floor in a new building, the floor is often limited to deflecting less than the length of the span in inches, divided by 360. Any more deformation than this would be considered disconcerting to the building residents and also start damaging surface materials like drywall. For example, for a \(\ft{20}\) span, the deflection would need to be less than
\begin{equation*} \delta = \dfrac{\ft{20} \cdot \dfrac{\inch{12}}{\ft{1}}}{360}=\inch{0.667}\text{.} \end{equation*}
To meet this deformation limit, we need to consider the magnitude and location of applied loads, the size and shape of the floor beams, and the material the floor beams are made from. As deflection is an internal property of the flooring materials, the first step is to determine the internal forces that arise from the externally applied loads, using the methods of this chapter.