Orthotropic slabs
There are two ways to define an orthotropic meshed slab in Tekla Structural Designer: you either specify relative stiffness factors, or you enter material properties manually.
Both methods (as described in more
detail below) require you to select the Orthotropic option in the slab
properties. This option actually has two practical uses:
- It enables different stiffnesses to be set for the two orthogonal (X and Y) directions of slabs thus making them orthotropic rather than isotropic (same stiffness in both directions). This can be used to model slab elements that exhibit orthotropic material properties and are assumed to behave predominantly as “one-way spanning”, such as precast slab units, profiled steel decking, timber decks on floor joists and cross-laminated timber (CLT) elements.
- For isotropic slabs, it also provides a way (as described below) to override the global stiffness modification factor with a modification factor specific to the selected slab only.
Define an orthotropic slab using relative stiffness factors
The stiffness modification X factor and Y factor can be set in the Analysis parameters property grid. They are visible only when Orthotropic and Calculate properties options are selected and can be set as follows:
Example: In a particular model in order to achieve highly orthotropic behavior you might enter an X factor of 0.001 while maintaining the Y factor as 1.0 as shown below.
Define an orthotropic slab by entering material properties manually
As an alternative to calculating the orthotropic material properties using X and Y factors, the properties can be also set manually if required. The properties take default values based on their state prior to deselecting the checkbox.
To increase the slab item’s orthotropicity, except adjusting Young’s moduli in the local X and Y directions, respectively, one might consider modifying the in-plane shear modulus, Gxy, as well.
To define properties manually in this way:
-
Unselect Calculate
properties
The properties take default values based on their state prior to deselecting the checkbox.
Example: In a particular
model in order to achieve highly orthotropic behavior you might reduce Ex while
maintaining the Ey unchanged. To increase the slab item’s orthotropicity, one might
consider modifying the in-plane shear modulus, Gxy, as well.
Override global stiffness modification factor with slab specific factors
By default the Apply modification factors option is selected, so that the stiffness modification factors defined in Analysis Settings > Modification Factors are applied.
If required, the above option can be cleared to enable the analysis parameters to be passed directly to the solver unmodified.
Furthermore, by selecting the Orthotropic and the Calculate properties options you are then provided access to X and Y factors which can be used in the place of the global stiffness modification factor. Although the implication is that you will apply these to make the slab orthotropic there is nothing to prevent you from specifying identical X and Y values. In this case, due to lack of differences between X and Y factors, the slab will be analyzed as an isotropic one. When used in this way, the Orthotropic property can be activated for isotropic slabs solely to override the global stiffness modification factor with a slab specific factor.
This is done as follows:
- Select any slab item within the slab of interest.
- In the Properties window, navigate to Slab parameters > Analysis parameters
- Select Orthotropic
- Select Calculate properties
- Clear Apply modification factors
- Enter the same value of X factor and Y factor.
Example: In a particular model the global factor might be 0.2 for typical thin flat slabs, but you could have a reason to override this with 0.3 for some much thicker transfer slab. This would be achieved by entering both the X factor and Y factor as 0.3 as shown below.
