External Force: Definition & Examples

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  • 0:03 What Are External Forces?
  • 1:06 Effects of External Forces
  • 2:18 External Forces vs.…
  • 2:59 Lesson Summary
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Lesson Transcript
Instructor: Hassan Alsaud

Earned my B.S. in Civil Engineering back in 2011. Have two years of experience in oil and gas fields and two year as a graduate research assistant. Earned my Master degree in Engineering from Tennessee State University in 2016.

In this lesson, we are going to learn about external forces and their effects on the body to which they are applied. We are also going to distinguish external forces from internal forces.

What Are External Forces?

In our everyday lives, we are exposed to many external physical forces as we perform our activities. These forces vary in magnitude and direction, and hence, vary in their effects. When you feel a nice spring morning breeze, for instance, the feeling differs from the effect of wind loads on high rise buildings.

In structural analysis, external forces are forces applied to the structure from the outside of the structure. Examples of external forces include dead loads, such as the weight of the structure itself and the non-structural materials it supports, and live loads, which include moving loads, such as occupants, goods, and furniture, as well as wind loads, seismic loads, and impact loads, among others.

A reaction force at points where the structural member is supported resulting from external forces is also an external force. In addition, despite the lack of connection between two or more objects, forces such as gravitational and electromagnetic forces can also be classified as external forces.

Effects of External Forces

Effects of external forces applied to a structure include deflection, internal stress, rotation, settlement, and overturn. Deflection occurs when an external force is applied at a structural element perpendicularly to the axis of the member. In addition, internal stress, whether normal or shear, occurs due to an external load applied on structural members. Furthermore, rotation occurs when an external load is applied to a structure supported by a roller or a hinge at one of its ends. Finally, overturn is a result of active stress on retaining walls.

Each of the load types has different effects on the structure. For instance, dead loads and live loads cause bending, and hence they cause internal stress. Lateral loads, or wind loads, on the other hand, cause, in addition to bending, overturn.

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