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Force In 2D Elements

The Timoshenko convention is used for forces in 2D elements. This means that a moment MxM_{x} is based on the stress in the xx direction. With the Timoshenko convention if a slab is in compression on the top face in both the xx and yy directions the moments are both negative. Consequently starting from the assumption that tensile stress is positive, we have the following relationships for the forces and moments

Nx=t2t2σxxdzNy=t2t2σyydzNxy=t2t2σxydzQx=t2t2σxzdzQy=t2t2σyzdzMx=t2t2σxxzdzMy=t2t2σyyzdzMxy=t2t2σxyzdz\begin{aligned}N_{x} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{xx}dz}& N_{y} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{yy}dz}& N_{xy} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{xy}dz}\\ Q_{x} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{xz}dz} & Q_{y} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{yz}dz} \\ M_{x} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{xx}zdz} & M_{y} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{yy}zdz} & M_{xy} &= \int_{- \frac{t}{2}}^{\frac{t}{2}}{\sigma_{xy}zdz}\end{aligned}

Following from this a plate that has a positive in-plane stress in xx/yy will have a positive force resultant and a positive bending stress in xx/yy (i.e. positive stress at the top surface relative to the bottom surface) will have a positive moment.

When the structure is linear these simplify to:

Nx=σp,xxtNy=σp,yytNxy=σp,xytQx=σxztQy=σyztMx=σb,xxt26My=σb,yyt26Mxy=σb,xyt26\begin{aligned}N_{x} &= \sigma_{p,xx}t&N_{y} &= \sigma_{p,yy}t&N_{xy} &= \sigma_{p,xy}t\\ Q_{x} &= \sigma_{xz}t&Q_{y} &= \sigma_{yz}t\\ M_{x} &= \sigma_{b,xx}\frac{t^{2}}{6}&M_{y} &= \sigma_{b,yy}\frac{t^{2}}{6}&M_{xy} &= \sigma_{b,xy}\frac{t^{2}}{6}\end{aligned}

where the superscripts pp and bb refer to in-plane and bending stress terms.

When in-plane and bending thickness modifiers are user the in-plane forces are based on the in-plane thickness and the moments and shear forces are based on the bending thickness. Stresses are always based on the actual thickness of the element.

Force/moment in layered shell

Once the displacements are calculated the strains ϵi\epsilon_i can be calculated for each layer

ϵi=Biui\epsilon_i = B_i u_i

Which relates back to the element displacements

ϵi=BiTiu\epsilon_i = B_i T_i u

Stresses can then be calculated from

\begin{align} \sigma _i &= C_i \epsilon_i \\ &= C_i B_i T_i u \\ \end{align}

The stresses can then be integrated to get the force and moment

\begin{align} N_i &= \sigma _{i,p} {t_i} \\ M_i &= \frac{\sigma_{i,b} t_i^2}{6} \\ Q_i &= \sigma _{i,q} t_i \\ \end{align}

These can then be integrated across the element

\begin{align} N &= \sum_i{N_i} \\ M &= \sum_i{(M_i + N_i z_i)} \\ Q &= \sum_i{Q_i} \\ \end{align}