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What is a Bending Moment?

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  • Written By: Malcolm Tatum
  • Edited By: Bronwyn Harris
  • Last Modified Date: 25 October 2016
  • Copyright Protected:
    2003-2016
    Conjecture Corporation
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A bending moment is a term used to describe the force or torque that is exerted on a material and leads to the event of bending or flexure within that material. Determining the bending moment is crucial to determining how much pressure a given object can withstand without experiencing any type of sagging or breakage. For this reason, structural engineers often look closely at the performance of different materials when designing a building, dam, or other structure, using the data to determine which materials to use and in what measure to achieve the desired effect.

One of the easiest ways to understand the bending moment is to consider the performance of a clothing rod in a closet. The rod is designed to maintain its integrity as long as the total weight of the clothing and hangers remains under a certain amount. As the weight of clothing that is suspended from the rod via wire hangers is increased, additional stress is placed on the device. Over time, the constant weight bearing could cause the rod to begin sagging in the middle; the point at which the weight becomes more than the rod can withstand is a bending moment.

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The same general concept applies to other types of devices that could reach a bending moment. Joists that supply floors and beams that provide the framework for roofs have a point at which the amount of weight carried will cause the structure to sag and possibly break, if subjected to that weight for a prolonged period of time. The framework of a weight-bearing wall in the design of a home or a commercial building will also experience a bending moment under specific circumstances.

Plotting points at which a bending moment is likely to occur is key to the engineering process. Those points are usually captured on what is known as a bending moment diagram, effectively identifying points at which changes in external pressure from any direction may shift and cause the beam to begin sagging or collapsing in some manner. Often, identifying these points makes it possible to refine the blueprints for the construction of a building, so the amount of pressure experienced from each direction remains in sync, minimizing the potential for bending.

Identifying the bending moment associated with different materials makes it easier to design products that are highly likely to hold up to the stress and pressure placed upon them. Because of the ability to determine the amount of bending or flexure that occurs at different stages, it is possible for architects, engineers and even construction professionals to choose materials that are highly likely of holding up under certain amounts of pressure or tension. This makes it easier to select materials that will comply with local building codes and ultimately result in the construction of buildings that are safe and capable of remaining structurally sound for decades.

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