The calculation of CLT panels is based on the laminated shell theory, which is a general way of calculating any kind of laminated shells, regardless on the material of the components. However, in the civil engineering practice the most popular application area is definitely the timber shells. Keep in mind this, in FD user interface the CL(T) feature strictly connects to timber panels, including the possibility for both analysis (internal forces/stresses) and design options. Moreover, to the make the best of the applied mechanical theory, general composite laminated shells are also implemented, with any kind of constituent material. As the design of these general composite shells are not supplemented by the standard, only analysis results are available of them.

Definition on User interface

For the definition of CLT panels the currently existing Timber plate and Timber wall tools are used:


All the tool window options (Define, Properties, Hole, Edge connection, Physical extent, Panels) are valid and applied for both original and CLT timber panels.


On the General tab the Eccentricity on General tab is also modified (improved). So far it was only a display option, without any effect on the calculation. This limitation has no real technical obstacles, thus it will be the choice of the User with a new checkbox here.

On the Material tab of properties dialog, we can set the mechanical model of the plate. The original orthotropic shell model is supplemented with two new types, so User can select from the following options:

  1. Orthotropic shell
  2. Cross laminated timber
  3. General laminated composite

For the sake of uniformity, Display stiffness is available for all three kind of mechanical models. This matrix is calculated to the middle surface of the element (independently on the eccentricity of the General tab), it should be noted in the tooltip of this button.


Independently on the selected type, the General, Border and Panel tabs are the same for all the three options, only the Material tab changes according to the selection.

Orthotropic shell

Within this feature, there is not change on the Material tab of orthotropic timber shell (picture taken after the separation of creep factors and library displaying change to tree view item)


Cross laminated timber (CLT)

CLT panels consist of individual timber layers. As there are several typical panels, they need a new a library, which can be managed from the Material tab, similarly to the orthotropic shells. For the analysis and design the following parameters are necessary:

  • mechanical properties: Ex, Ey, Gxy, Gxz, Gyz (Grolling), nuxy, Unit mass, Alpha
  • limit stresses: fb0k, fb90k, ft0k, ft90, fc0k, fc90k, fxyk, fvk, fRk

It is assumed by default that the grain runs in x direction. The directions by the layers for the CLT panels are: 0 or 90 degrees. As the constituent layers are not general timber materials (in the specification of producers it is stated that the timber material properties are taken according to EN338, but they clearly use different values), a new timber library should be created as constituent materials for CLT panels. According to these specifications even the E0,mean value can be different for C24 types, the library should differentiate them by producers. This library needs to store the following data:


There are some values which not specified by the Eurocode, these can be assumed from the existing ones according to the following (if producer does not give exact values for them):

  • Grolling = Gmean / 10
  • nu0.90 = 0.2
  • fRk = fvk / 2
  • fxyk = fvk

The Material tab of the CLT plate/wall with the existing library (left) elements and necessary application data (right):


The library elements are grouped by the producers, our aim is to implement many manufacturers’ product by default, it would mutually beneficial for both the producers and for FEM-Design. We can edit the new CLT library elements with the New/Modify buttons:


The library elements are basically consisting of a list of Timber materials (Timber library elements). User can add a new layer by clicking to an empty Material cell (or change one by clicking on the filled row’s cell Material). By this, the new special Timber library pops up. After selecting one (e.g. C24), the row is automatically filled with the Mechanical properties and Limit stresses, only the Thickness and Alpha angle should be defined by the User (the mechanical properties and limit stresses are NOT modifiable in the table). If Limit stresses is selected at the top, the table is changing according to the following:


The preview and Information sections are automatically updating themselves after inserting/modifying/deleting a layer. On the preview picture, layers with the same Alpha are filled with the same colour, moreover layers with 0 angle are displayed with sign of grain direction.

General laminated composite

As these composite shells are general and can be made of any material, they are strictly for analysis purpose only, they are not supported with design calculations (User get displacement, internal force and stress results for them). As in case of CLT, the General, Border and Panel tabs are the same, only the Material tab is different:


Because of this generality, a new library is needed for these laminated composite shells, which has similar appearance as CLT library, with slight modifications:


In this case, only mechanical properties are needed for a library element (as no design for them), and these values should be manually defined by the User (instead of selecting an existing material from the Concrete/Steel/Timber library).

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