Why is the shear force Vz for a rib directly above the support smaller than the one close to it?


In the case of a rib member, the shear force Vz consists of member internal forces, which relate to the eccentrically connected rectangular member, and of surface internal forces, which are integrated over the defined integration width.

A decreasing value of the shear force Vz directly above the support (see Figure 01) usually results from the distribution of the surface internal forces (basic internal force vx or vy). See the distribution of basic internal forces v‑y in Figure 02. A "zero crossing" of the surface internal forces results above the support. This "zero crossing" results from smoothing of the surface internal forces within the surface and is also reflected accordingly in the member internal forces of the rib member.

To avoid the "problem," it is possible to divide the continuous surface above the support into a surface before and behind the support.

During the integration of the surface internal forces, the smoothed distribution of the surface internal forces "within the surface" is applied. If you enter a surface before and behind the support, you will obtain a result value exactly on the location next to the nodal support without zero crossing. In Figure 04, it corresponds to +112.63 kN/m or -112.63 kN/m.


Shear force Vz Shear force distribution Rib T-beam Downstand beam Upstand beam


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