Sometimes you may want to assign a prestress to a frame element. This is useful if you are modeling prestressing tendons in a section. To do this, you must apply a thermal load that forces the frame element to contract, which in turn induces a prestress.

First, a little mathematical theory:

Remember that

$\epsilon=\alpha \Delta T$

and that

$\sigma=E \epsilon$

So

$\sigma=E \alpha \Delta T$

If we know the prestress we want to apply, we must first compute the change in temperature, $\Delta T$, to apply. To do this, we rearrange the above equation:

$\Delta T=\frac{\sigma}{E \alpha}$

So, to apply a prestress to a frame element, we will first compute the change in temperature and then create a frame element thermal load using the following steps:

  1. Determine which material is being used and retrieve it using the Material from Name component. This will return an Elastic Material object.
  2. Deconstruct the Elastic Material using the Deconstruct Material component.
  3. Multiply the material’s E1 (elastic modulus in the local 1-direction) by the material’s A1 (temperature coefficient in the local 1-direction) using a Multiplication component.
  4. Create a Number Slider with the value of the prestressing, in consistent units with the elastic modulus. These units should correspond to the display unit system (or the model unit system if the display units are not overridden).
  5. Using a Division component, divide the prestress by the result of $E \alpha$. This is the required change in temperature, $\Delta T$.
  6. Reverse the sign of the temperature change using a Negative component so that the change is negative and will cause a contraction.
  7. Create a Design Frame Thermal Load load using the Create Design Frame Thermal Load component.
  8. Assign the thermal load to Design Frame Elements.