Force In 2D Elements
The Timoshenko convention is used for forces in 2D elements. This means
that a moment Mx is based on the stress in the x direction. With the
Timoshenko convention if a slab is in compression on the top face in
both the x and y 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
NxQxMx=∫−2t2tσxxdz=∫−2t2tσxzdz=∫−2t2tσxxzdzNyQyMy=∫−2t2tσyydz=∫−2t2tσyzdz=∫−2t2tσyyzdzNxyMxy=∫−2t2tσxydz=∫−2t2tσxyzdz Following from this a plate that has a positive in-plane stress in x/y
will have a positive force resultant and a positive bending stress in
x/y (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:
NxQxMx=σp,xxt=σxzt=σb,xx6t2NyQyMy=σp,yyt=σyzt=σb,yy6t2NxyMxy=σp,xyt=σb,xy6t2 where the superscripts p and b 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 can be calculated for each layer
ϵi=Biui Which relates back to the element displacements
ϵi=BiTiu 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}