Materials are required to define cross-sections. The material properties affect the stiffnesses of members.
Name
You can define any name for the material. If the description matches an entry in the library, RSTAB imports the stored material properties. To select a material in the library, click the
button at the end of the input line. The import of materials is described in Chapter Material Library.
For materials from the library, the 'Basic Material Properties' are set by default and cannot be changed. If you want to use user-defined properties of the material, tick the User-defined material check box in the 'Options' dialog section (see User-defined material paragraph).
Main
The Main tab manages the basic material parameters.
Material type
The material type is used to define the category of the material. It controls the parameters and factors relevant for the design. The material type also defines the partial safety factors of the material, which are taken into account for the design depending on the standard.
For a material taken from the library, one of the following material types is preset.
Material model
The list offers the 'Isotropic | Linear Elastic' and 'Isotropic | Timber | Linear Elastic (Members)' (for wood materials) material models for selection.
Isotropic Linear Elastic
The linear-elastic stiffness properties of the material do not depend on directions. They can be described as follows:
| E | modulus of elasticity |
| G | shear modulus |
| ν | Poisson's ratio |
The following conditions apply:
- E > 0
- G > 0
- ν > -1
Isotropic Timber Linear Elastic (Members)
This material model can be selected for materials of the 'Timber' type. It allows you to represent, for example, the properties of an oriented strand board (OSB) in a member model, including different stiffnesses depending on the installation position. You can define the position of the board in the 'Isotropic Timber | Linear Elastic (Members)' tab using the two lists.
Modulus of elasticity
The modulus of elasticity E describes the ratio between normal stress and strain.
Shear modulus
The shear modulus G is the second parameter for describing the elastic behavior of a linear, isotropic, and homogenous material. In this case, the deformation is based on shear stress.
Poisson's ratio
Poisson's ratio ν is required to determine the transverse strain. Generally, Poisson's ratio for isotropic materials is between 0.0 and 0.5. Therefore, for a value of 0.5 or higher (for example, rubber), it is assumed that the material is not isotropic.
The relation between the modulus of elasticity, shear modulus, and Poisson's ratio for an isotropic material is described in the Poisson's ratio equation.
When you enter a User-defined material with its isotropic properties, RSTAB determines the Poisson's ratio based on the values of the modulus of elasticity and the shear modulus. If necessary, you can change this presetting in the 'Definition type' list.
Definition type for material properties
| E | G | (ν) | Poisson's ratio determined from modulus of elasticity and shear modulus |
| E | (G) | ν | Shear modulus determined from modulus of elasticity and Poisson's ratio |
| E | G | ν | The modulus of elasticity, shear modulus, and Poisson's ratio are independent of each other. |
Specific weight / Mass density
The specific weight γ describes the weight of the material per volume unit. The specification is especially important for the 'Self-weight' load type. The automatic self-weight of the model is determined from the specific weight and the cross-sectional areas of the used members.
The density ρ describes the mass of the material per volume unit. This information is required for dynamic analyses.
Coefficient of Thermal Expansion
The coefficient of thermal expansion α describes the linear correlation between changes in temperature and length (elongation of material due to heating, shortening due to cooling).
The coefficient of thermal expansion is relevant for the 'Temperature' and 'Temperature change' load types.
User-defined material
The material properties for materials from the library are preset. Therefore, they cannot be changed directly in the input fields.
To adjust the properties of a material, tick the User-defined material check box in the 'Options' dialog section. Then, the input fields in the 'Basic Material Properties' dialog section of the 'Main' tab become accessible. Use the 'Material Values' tab also to change design-specific properties.
Stiffness Modification
In the case of a user-defined material, you can adjust the stiffness, for example, to consider safety factors or reduced material properties. Two options are available for selection in the 'Modification Type' list:
- Division factor for E and G modules
- Multiplier factor for E and G modules
In the 'Parameters' dialog section, enter the factor by which the material stiffness is to be adjusted.
Temperature-dependent
To define a linear elastic material with temperature-dependent stress-strain properties, select the User-defined and Temperature-dependent check boxes in the 'Options' dialog section. Then, you can define the temperature-dependent material properties in the 'Temperature-Dependent' tab. These material properties are taken into account for objects that are thermally stressed by temperature or temperature change.
In the 'Temperature-Dependent Property' list, select a material property; for example, the modulus of elasticity. Then, use the
button to create the required table rows so that you can enter the temperatures with the corresponding values row by row. You can also use the
button to import data from an Excel spreadsheet.
The 'Reference Temperature' defines the stiffnesses for the objects that have no temperature loads. When a reference value, for example, of 300 °C is set, the reduced modulus of elasticity of that point of the temperature curve is applied for all members.