PFAST

Format

Example

Definitions

Comments

1. For a CFAST element, no material needs to be specified in the corresponding PFAST card - the stiffness of the element is directly specified in the PFAST card with KTi and KRi entries.
2. The diameter D will not be involved in the stiffness calculation directly. It is used along with GA and GB to find appropriate auxiliary points and related shell elements and grids. In this case, the stiffness contribution of the fastener depends not only on the stiffness values specified for KTi and KRi, but also the diameter D, because the location of the auxiliary points will be used to weight the contribution of the shell element grids to GA and GB of the fastener.
3. Element stiffness coordinate system. The three stiffness values KT1, KT2 and KT3 will be applied along the three axes of the element coordinate system. The unit vectors of the three axes are denoted as:

   $e_1$ , $e_2$ and $e_3$

   1. If MCID = -1, MFLAG will be ignored and $e_1$ will be defined as: (1)
      $${\mathbf{e}}_1=\frac{{\mathbf{x}}_B-{\mathbf{x}}_A}{\Vert {\mathbf{x}}_B-{\mathbf{x}}_A\Vert }$$
      $e_2$ is defined as being perpendicular to $e_1$ and lined up with the closest axis of the basic system. This is accomplished by taking the inner product of $e_1$ with the basic system unit vectors. The smallest will define the basic system direction which is closest to the plane perpendicular to $e_1\cdot e_2$ is then defined as the projection of the basic direction onto this perpendicular plane. For example, assume $m$ is the unit vector of the closest axis of the basic system. The direction of $e_2$ can be calculated as:(2)

      $${\tilde{\mathbf{e}}}_2=\mathbf{m}-(\mathbf{m}\cdot {\mathbf{e}}_1){\mathbf{e}}_1$$
      Unify this vector, then (3)

      $${\mathbf{e}}_2=\frac{{\tilde{\mathbf{e}}}_2}{\Vert {\tilde{\mathbf{e}}}_2\Vert }$$

      At last, $e_3$ can be calculated by the cross product of $e_1$ and $e_2$ as:

      $e_3=e_1\cdot e_2$

   2. If MCID ≥ 0 and MFLAG = 0, $e_1$ will be defined as: (4)
      $${\mathbf{e}}_1=\frac{{\mathbf{x}}_B-{\mathbf{x}}_A}{\Vert {\mathbf{x}}_B-{\mathbf{x}}_A\Vert }$$

      in which XA and XB are the coordinates of GA and GB.

      The T2 direction specified by MCID will be used to define the orientation vector $v$ of the fastener.

      Then, $e_3$ can be obtained as:(5)

      $${\mathbf{e}}_3=\frac{{\mathbf{e}}_1\times \mathbf{v}}{\Vert {\mathbf{e}}_1\times \mathbf{v}\Vert }$$

      At last, the $e_2$ can be easily calculated by the cross product of $e_3$ and $e_1$ as:

      $e_2=e_3\cdot e_1$

   3. If MCID ≥ 0 and MFLAG = 1, the unit vectors of the three axes defined by MCID will be used directly as $e_1$ , $e_2$ and $e_3$ . The element forces will be computed in the coordinate system. 3
   4. If MCID refers to a cylindrical or spherical coordinate system, the local origin used to locate the system is selected as:
      • if GA of the CFAST is specified, use GA as the local origin;
      • if GA is not specified but GS is specified, use GS as the local origin;
      • if neither GA nor GS is specified, use the point (XS, YS, ZS) as the local origin.
4. The final length of the CFAST element is defined by the distance between GA and GB. If the length is zero, the normal to shell patch A is used to define the axis of the fastener.
5. For the mass of the fastener, half of the value defined in the MASS entry is placed directly onto the translational degrees-of-freedom of GA and GB. Then they are distributed, via auxiliary points, to corresponding shell grids. As the result, while the mass will be represented correctly for general representation of the fastener in the vibrations of the structure, the moments of inertia relative to the local axes of the fastener will only be roughly approximated.