9.3.1. Generating and Controlling Mechanical Loads
The total perturbed loads are calculated as follows:
= +
where:
{Fend} = total loads at the end of the load step of the current restart point (load applications
are read from the .LDHI file). By default, all loads of {Fend} are deleted except for displacement
boundary conditions, inertia loads, and non-mechanical loads.
{Fadd} = additional (new) loads prescribed by the user in the second phase of the linear perturb-
ation analysis (after the first SOLVE,ELFORM command is invoked). This additional loading is
optional.In the first phase of a linear perturbation analysis, the ANTYPE,,RESTART command resumes the Job-
name.RDB database and read in the Jobname.LDHI file to establish the {Fend} load.
New load application (adding to {Fadd}) or load removal (changing {Fend}) can occur only in the second
phase of the linear perturbation analysis (after the first SOLVE,ELFORM command), allowing flexibility
in controlling the final {Fperturbed} that will be used.
{Fperturbed} is used differently in each linear perturbation analysis type:
- In a linear perturbation static analysis, {Fperturbed} is used to compute the static analysis solution.
- In a linear perturbation modal analysis, {Fperturbed} is calculated and stored in the Jobname.FULL
and Jobname.MODE files for a subsequent mode superposition, PSD, or other type of modal-based
linear dynamic analysis.- In a linear perturbation eigenvalue buckling analysis, {Fperturbed} is used to calculate the linearly per-
turbed displacements; these displacements are used for generation of the linearly perturbed stress
stiffening matrix and thus the load factor for eigenvalue buckling analysis. Note that this load can
be totally independent of the load used in the base analysis.- In a linear perturbation full harmonic analysis, {Fperturbed} is used in the frequency steps for the har-
monic solution. {Fperturbed} can be frequency dependent and can use complex input.9.3.2. Generating and Controlling Non-mechanical Loads
Non-mechanical loads (including thermal loads) must remain unmodified in the first phase of the linear
perturbation analysis so that a detailed nonlinear snapshot for various solution matrices and element
history variables can be regenerated; thus, total load contributions to {Fperturbed} include non-mechan-
ical loads in the first phase of a linear perturbation analysis.
In the second phase of a linear perturbation analysis, you cannot change, add, or remove non-mechan-
ical loads with these exceptions: thermal loads can be defined in the second phase of a linear perturb-
ation static analysis or a linear perturbation eigenvalue buckling analysis by specifying a new temperature.
In a linear perturbation static or buckling analysis, assuming that the base analysis is nonlinear, the
reference temperature is the temperature from which the linear perturbation analysis is restarted (and
not the reference temperature [TREF] from the base analysis). If the base analysis is linear, then the
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