Bar element ================ Tag 'BARRE' A bar is a straight beam with a constant cross section containing only the degrees of freedom for traction-compression. The equation of motion, the stiffness matrix and the forces are therefore those of the beams (lines of constant cross section) relating to traction-compression. However, the mass matrix must take into account the degrees of freedom in the 3 directions of space, mainly so that in dynamics calculations the mass is taken into account in each direction (see [:external:ref:`V2.02.146 `]). Thus, if we note :math:`L` the length of the element, :math:`A` the area of its section, and the area of its section and :math:`\rho` its density, the elementary mass matrix is as follows (with the components in the order :math:`({\mathrm{DX}}_{1}{\mathrm{DY}}_{1}{\mathrm{DZ}}_{1}{\mathrm{DX}}_{2}{\mathrm{DY}}_{2}{\mathrm{DZ}}_{2})`) with :math:`m=\rho AL`: :math:`{M}^{\mathrm{elem}}=(\begin{array}{cccccc}m/3& 0& 0& m/6& 0& 0\\ 0& m/3& 0& 0& m/6& 0\\ 0& 0& m/3& 0& 0& m/6\\ m/6& 0& 0& m/3& 0& 0\\ 0& m/6& 0& 0& m/3& 0\\ 0& 0& m/6& 0& 0& m/3\end{array})` [:ref:`11-1 <11-1>`] The only geometric characteristic is the area of the cross section [:external:ref:`U4.42.01 §6 `].