Civil Engineering Reference
In-Depth Information
perturbation steps, the solution for a single set of applied loads, can be pre-
dicted. However, for static analyses covered in this topic, it is possible to find
solutions for multiple load cases. In this case, the overall analysis procedure
can be changed from step to step. This allows the state of the model (stresses,
strains, displacements, deformed shapes, etc.) to be updated throughout all
general analysis steps. The effects of previous history can be included in the
response in each new analysis step by calling the results of a previous history.
As an example, after conducting an initial condition analysis step to include
residual stresses in cross sections, the initial stresses in the whole cross section
will be updated from zero to new applied stresses that accounted for the
residual stress effect in metal structures.
It should be noted that linear perturbation steps have no effect on sub-
sequent general analysis steps and can be conducted separately as a whole
(overall) analysis procedure. In this case, the data obtained from the linear
perturbation steps can be saved in files that can be called into the subsequent
general analysis steps. For example, linear eigenvalue buckling analyses,
needed for modeling of initial overall and local geometric imperfections,
can be conducted initially as a separate overall analysis procedure, and buck-
ling modes can be extracted from the analyses and saved in files. The saved
files can be called into subsequent static general analyses and factored to
model initial geometric imperfections. The most obvious reason for using
several steps in an analysis is to change the analysis procedure type. However,
several steps can also be used to change output requests, such as the boundary
conditions or loading (any information specified as history or step-
dependent data). Sometimes, an analysis may be progressed to a point where
the present step definition needs to be modified. ABAQUS [1.29] provides
the ability to restart the analysis, whereby a step can be terminated prema-
turely and a new step can be defined for the problem continuation. History
data prescribing the loading, boundary conditions, output, etc., will remain
in effect for all subsequent general analysis steps until they are modified or
reset. ABAQUS [1.29] will compare all loads and boundary conditions spec-
ified in a step with the loads and boundary conditions in effect during the
previous step to ensure consistency and continuity. This comparison is
expensive if the number of individually specified loads and boundary con-
ditions is very large. Hence, the number of individually specified loads and
boundary conditions should be minimized, which can usually be done by
using element and node sets instead of individual elements and nodes.
Most current general-purpose finite element computer program divides
each step of analysis into multiple increments. In most cases, one can choose
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