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General Description
Section 1
1-80
The analysis sequence is as follows:
1.
Compute the unstressed length of the nonlinear members based
on joint coordinates, pretension, and temperature.
2.
Member/Element/Cable stiffness is formed. Cable stiffness is
from EA/L and the sag formula plus a geometric stiffness based
on current tension.
3.
Assemble and solve the global matrix with the percentage of the
total applied load used for this load step.
4.
Perform equilibrium iterations to adjust the change in directions
of the forces in the nonlinear cables, so that the structure is in
static equilibrium in the deformed position. If force changes are
too large or convergence criteria not met within default number
of iterations then stop the analysis.
5.
Go to step 2 and repeat with a greater percentage of the applied
load. The nonlinear members will have an updated orientation
with new tension and sag effects.
6.
After 100% of the applied load has converged, then proceed to
compute member forces, reactions, and static check. The static
check is not exactly in balance due to the displacements of the
applied static equivalent joint loads.
The load cases in a non linear cable analysis must be separated by
the CHANGE command and PERFORM CABLE ANALYSIS
command. The SET NL command must be provided to specify the
total number of primary load cases. There may not be any Multi-
linear springs, compression only, PDelta, NONLINEAR, or dynamic
cases.
Also for cables and preloaded trusses:
1.
Do not use Member Offsets.
2.
Do not include the end joints in Master/Slave command.
3.
Do not connect to inclined support joints.
4.
Y direction must be up.
5.
Do not impose displacements.
6.
Do not use Support springs in the model.
7.
Applied loads do not change global directions due to
displacements.