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Conductor Library
These properties are shown in the Conductor Library when the Cable Model Type is set to Linear.
Parameter | Description |
Modulus of Elasticity | This is the modulus of elasticity of the conductor which determines how much the conductor will stretch under load. It specifies the change in stress required for a unit change in the strain in the conductor. |
Coeff. of Linear Exp. | This is the coefficient of linear expansion which specifies how much the conductor will change length for a change in temperature. The higher this value is the more the conductor will sag when at maximum operating temperature. See article thermal effects for more details. |
Default Creep Temp. Offset | This is the temperature offset that will be applied to account for creep in the |
Theory
For a linear conductor, the stress-strain curve is defined by a straight line, and the relationship between stress (σ) and strain (ε) is expressed by Hooke's Law:
σ = Eε
Where:
σ represents the stress (force per unit area) applied to the material.
ε represents the strain (change in length divided by the original length) of the material.
E represents the modulus of elasticity also known as the material's Young's modulus.
Hence the slope of the stress-strain curve is linear and defines both the loading and unloading behavior of the conductor. The figure below illustrates the stress-strain curve for a conductor with the modulus of elasticity equal to 85GPa.
Note: As can be seen from the graph during loading and unloading, the change in stress-strain happens along the same curve, implying that there is no permanent deformations allowed. Given that a linear cable model does not allow for permanent deformations they cannot be used to account for preload situations such as a heavy snow/ice events (in this case a non-linear cable model would be required).
Worked Example
To illustrate how the modulus of elasticity is used to calculate the conductor length for a given tension for a linear conductor model consider the following example:
200m conductor length where the conductor modulus of elasticity is 200GPa and the axial force in the conductor is 20kN and the conductor cross section is 250mm^2. The stress in the conductor is calculated to be 20,000N / 0.00025m^2 = 80,000,000Pa. Using hookes law the strain = 80,000,000Pa / 200,000,000,000Pa = 0.0004%. Hence the length change due to elongation from strain for this 200m span would be 200m * 0.0004 = 0.08m. Therefore the conductor length when 20kN axial tension is applied to the span would be 200.08m.