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In solid mechanics, a bending moment is the reaction induced in a structural element when an external force or moment is applied to the element, causing the element to bend. [ 1 ] [ 2 ] The most common or simplest structural element subjected to bending moments is the beam .
The sign of the bending moment is taken as positive when the torque vector associated with the bending moment on the right hand side of the section is in the positive direction, that is, a positive value of produces compressive stress at the bottom surface.
Shear and Bending moment diagram for a simply supported beam with a concentrated load at mid-span. Shear force and bending moment diagrams are analytical tools used in conjunction with structural analysis to help perform structural design by determining the value of shear forces and bending moments at a given point of a structural element such as a beam.
As a consequence the three traction components that vary from point to point in a cross-section can be replaced with a set of resultant forces and resultant moments. These are the stress resultants (also called membrane forces, shear forces, and bending moment) that may be used to determine the detailed stress state in the structural element. A ...
where , are the coordinates of a point on the cross section at which the stress is to be determined as shown to the right, and are the bending moments about the y and z centroid axes, and are the second moments of area (distinct from moments of inertia) about the y and z axes, and is the product of moments of area. Using this equation it is ...
Loads imposed on structures are supported by means of forces transmitted through structural elements. These forces can manifest themselves as tension (axial force), compression (axial force), shear, and bending, or flexure (a bending moment is a force multiplied by a distance, or lever arm, hence producing a turning effect or torque).
In the beam equation, the variable I represents the second moment of area or moment of inertia: it is the sum, along the axis, of dA·r 2, where r is the distance from the neutral axis and dA is a small patch of area. It measures not only the total area of the beam section, but the square of each patch's distance from the axis.
is the second moment of area (area moment of inertia), is the area cross section. For slender columns, the critical buckling stress is usually lower than the yield stress. In contrast, a stocky column can have a critical buckling stress higher than the yield, i.e. it yields prior to buckling.