You can apply an axial load or a combination of torque and a friction
factor to simulate the prestress condition of a bolt connector. You can enter the values
under Pre-load in the Connector Bolt PropertyManager
For static studies, bolt connector definitions induce two successive
analysis runs:
- For the first analysis run, the program applies the
user-defined preload 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 preload value.
- The program calculates a new adjusted preload value based on
the difference between the tensile force and the user-defined preload
value.
- The program runs the analysis once again with all the loads
and the internally adjusted preload 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 preloads, etc.) It is not possible to
select which loads to include in the first analysis where the program calculates the
new adjusted preload 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 preload 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 the bolt at the start of analysis Lst, which
corresponds to the undeformed 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 preload 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 preload 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 us define Lf as the deformed
length of bolt, which corresponds to the settlement of the connecting
parts due to prestress. 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 applied 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 preload
- 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 undeformed geometry state of the
components attached through the bolt)
- Lf: Deformed length of bolt after
settlement of connecting parts due to prestress (Keep
bolt-prestress selected)
- Lstep: Deformed length of bolt at
current analysis step
For a bolt connector to maintain its prestress load and reach solution convergence,
it is recommended to maintain the ratio of the modulus of elasticity of the attached
components, Ec, over the modulus of elasticity of the bolt, Eb, as Ec / Eb >
0.5.