# 2.3: Generalized Forces and Bending Moments in Plates

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In plates there are three in-plane components of the stress tensor \(\sigma_{\alpha \beta}\{\sigma_{xx}, \sigma_{yy}, \sigma_{xy}\}\). Replacing \(\sigma_{xx}\) by \(\sigma_{\alpha \beta}\) or \(\sigma_{z\alpha}\) in Equations (2.2.16-2.2.18) the generalized forces and couples are defined

\[M_{\alpha \beta} = \int_{-\frac{h}{2}}^{\frac{h}{2}} \sigma_{\alpha \beta}z dz \; [\mathrm{Nm/m}] = [\mathrm{N}] \label{2.3.1}\]

\[N_{\alpha \beta} = \int_{-\frac{h}{2}}^{\frac{h}{2}} \sigma_{\alpha \beta} dz \; [\mathrm{Nm/m}]\]

\[V_{\alpha} = \int_{-\frac{h}{2}}^{\frac{h}{2}} \sigma_{z\alpha} dz \; [N/m] \label{2.3.3}\]

Note that in the plate theory the integration is performed over the thickness of the plate rather than the entire surface. Therefore the dimensions of the quantities defined by Equations \ref{2.3.1}-\ref{2.3.3} are “per unit length”.