Primary / Secondary forces

Primary and secondary forces are special type of 1D results which are available only for " Defining the load case parameters" with type Prestress connected to special 1D members in model - " Parameters of the post-tensioned internal tendon".

Theory

The calculation of primary and secondary forces is different in case of statically determinate or indeterminate structure. This can be explained on the two cases of simply supported and continuous beam.

- simply supported beam (determinate structure) - deformation from prestressing is free and not restrained by boundary conditions

- continuous beam (indeterminate structure) - deformation from prestressing is prevented by middle support which cause additional reaction in the middle support caused by this system

Primary forces

These forces are obtained from the distribution of horizontal and vertical component of the prestressing force.

The prestressing represents a sef-equlibrating system and it means there are no other reactions.

Mpp-primary bending moment from prestressing

Vpp-primary shear force from prestressing in this case 0

Secondary forces

Forces are caused by the reactions of statically indeterminate structures.

Mps-secondary bending moment due to prestressing

Vps-primary shear force from prestressing in this case 0

Total resultants

It is the summation of primary and secondary forces.

Mp =Mpp+Mps

Vp =Vpp+Vps

 

Usage

  1. Calculate a FEM analysis of the model
  2. Go to Tree > Results > Beams > 1D internal forces
  3. Set properties of the command to specify mainly:
    • selection of 1D members where the results are evaluated
    • load type must be set on Load case with type Prestress
    • in subgroup Prestressingselect one of possibilities Primary forces / Secondary forces / Total resultants
    • specific result value from the available list to be visualized
  4. Click on [Refresh] action button to display the results

Values

Value Description Notes
Primary forces

These forces are obtained from the distribution of horizontal and vertical component of the prestressing force

1) As default, the internal forces refer to the local axes of the individual 1D member

2) The local axes can be displayed by selecting the corresponding option in the View parameters settings.
Secondary forces Forces are caused by the reactions of statically indeterminate structures
Total resultants It is the summation of primary and secondary forces