Bolt Connector Pre-Load

You can apply an axial load or a combination of torque and a friction factor to simulate the prestress condition of a bolt connector. Enter the values under Pre-load in the Connector Bolt PropertyManager

For static studies, bolt connector definitions induce two successive analysis runs:
  1. For the first analysis run, the program applies the user-defined pre-load value to prestress the bolt connectors. All other types of loads are suppressed. After the analysis completes, the program calculates the tensile force of each bolt which it compares to the user-defined pre-load value.
  2. The program calculates a new adjusted pre-load value based on the difference between the tensile force and the user-defined pre-load value.
  3. The program runs the analysis once again with all the loads and the internally adjusted pre-load value for each bolt connector.

The first analysis run should typically include all loads that are present before the bolts are tightened. Such loads typically include gravity, but can also include all the loads representing the state of stress when the bolts are tightened (forces, thermal loads, spring pre-loads, etc.) It is not currently possible to select which loads to include in the first analysis where the program calculates the new adjusted pre-load value. For most cases, this limitation should have no major impact on the accuracy of the results.

If you use symmetrical bolts, enter the total pre-load value and ½ or 1/4 of the total mass of the bolt according to the selected symmetry type. Also, when you list bolt forces for symmetrical bolts after running a study, the results equal ½ or 1/4 of the total force.

Nonlinear Analysis

  • Keep Bolt Prestress is cleared

    The bolt's length at zero stress state L0 is determined based on the length of bolt at the start of analysis Lst, which corresponds to the un-deformed geometry state of the components attached through the bolt connector. The bolt's length at zero stress state is calculated from:

    L0 = Lst / (1+(P/A*E))

    As the nonlinear analysis progresses, the bolt's length Lstep at each analysis step adapts itself to the deformed geometry of the attached components as they deform due to the applied loads. The bolt's final stress at the end of the nonlinear analysis differs from the user defined pre-load stress. The bolt's axial load at each analysis step is calculated from:

    Pstep = A* E* (Lstep - L0) / L0

  • Keep Bolt Prestress is selected

    When this option is selected, the program first runs an analysis with the user defined pre-load P as initial condition without any external loads. The deformation of the parts connected through the bolt is calculated and is used to determine the bolt's length at zero stress state L0. Let’s define Lf as the deformed length of bolt which corresponds to the settlement of the connecting parts due to pre-stress. The bolt’s length at zero stress is then calculated from:

    L0= Lf / (1+(P/A*E))

    For the second step of the analysis, all aplied loads are included. The bolt's axial load at each analysis step is calculated from:

    Pstep = A* E* (Lstep - L0) / L0

    During the analysis, if (a) Lstep <= L0 then the bolt is loose, and if (b) Lstep > L0, the bolt is under tension and keeping parts together.

    Notation:
    • P: User defined axial pre-load
    • Pstep: Axial load of bolt at current analysis step
    • A: Bolt section area
    • E: Bolt material modulus of elasticity
    • L0: Original length of bolt at zero stress state (derivation differs depending on the status of Keep Bolt Prestress option)
    • Lst: Length of bolt at start of analysis (corresponds to the un-deformed geometry state of the components attached through the bolt)
    • Lf: Deformed length of bolt after settlement of connecting parts due to pre-stress (Keep bolt-prestress selected)
    • Lstep: Deformed length of bolt at current analysis step