# Fictitious shell (Geometry)

**Contents**

In *3D Structure* module a fictitious shell can be defined by clicking on . Fictitious shell is ideal to model a section with composite material, where the designer may calculate appropriate stiffness and fill the D, K and H stiffness values. Fictitious shell is also a powerful tool by the calculation of a section with complex geometry where the parameters are specified by the manufacturer of the structural element.

**Definition steps**

- If needed, set a proper position for the
**working plane**. - Start
*Fictitious shell*command from tabmenu and choose*Define*. In the tool-window the type of the edges (Rigid or Hinged) can be chosen.

- Set the properties of the new plate at
*Default settings*. The User can define the stiffness matrices (Membrane stiffness matrix, Flexural stiffness matrix and Shear stiffness matrix) and physical properties (Unit mass, distances of the planes where thermal load acts from the center plane, coefficient of thermal expansion).

Stiffness of orthotropic shell element for fictitious shell can be calculated as follow:

These matrices belong to an orthotropic, homogenous, constant thickness shells according to Mindlin theory. |

The H matrix is calculated as ρ*G*h (see in Profiled plate), where the ρ is the shear modification factor, the G is the shear modulus and the h is the thickness of the fictitious shell. For constant thickness the shear modification value is 5/6 = 1/1.2, this gives the equivalency of the energy of the shear forces and shear stresses. |

As it is mentioned above, if the D, K and H stiffness values are calculated and filled in, then it is possible to model the sections with complex geometry and composite materials with new fictitious shell object. A few sections are shown below which can be handled by the help of fictitious shell.

Corrugated steel + Concrete floor

Corrugated steel + Timber floor