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Fundamentals

Section Method

To observe the internal forces one has to cut the structure, however, this is obviously not possible. Therefore, we assume an imaginary cut such that the structure is divided into two parts. At the location of cut, we can then see the internal forces (or internal forces become external).

This technique is called Section Method. In general, there are six internal forces in a structure: An axial force (Nx) which acts along the longitudinal axis of the structural member and makes it stretch or squeeze, two shear forces (Vy and Vz) which act perpendicular to the longitudinal axis of the member, and three bending moments, or simply “moments” (Mx, My, and Mz) about the three axes of the structural member. Mx results in the twisting of the member, while My and Mz make it bend about the y and z axes, respectively. If the structure acts as a two-dimensional system, there are only three internal forces (Nx, Vy and Mz).

 

Section Method

 

If the moments My or Mz cause in the structural member to bend downwards (sag), the moment is assumed to be positive; otherwise it is negative.

Negative and Positive Moments

Negative and Positive Moments

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