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Chapter 2 Using Multiframe
Tension and Compression only members
Tension only and compression only members can be considered by a nonlinear
analyses. These types of members are removed or reinstated to the structure at the
end of each iteration based upon the axial deformation of the member.
Analysis Parameters
Nonlinear analysis in Multiframe is performed using a Newton-Raphson solution
scheme. The user may control the solution process by adjusting the number of
load increments, the maximum and minimum number of iterations, the
convergence tolerance and the type of convergence norm.
No. of Load Increments
The number of steps in which the loads are applied to the structure. For most
analyses it is sufficient to use a single load increment.
Maximum iterations per increment
The maximum number of iterations to be performed in a single load increment.
This value is set to avoid analyses that may not converge.
Minimum iterations per increment
The minimum number of iterations to be performed in a single load increment.
Unless analysis time is an issue we suggest using a minimum of 3 iterations to
ensure that the solution has converged. This is particularly important in tension
only/compression only analyses where several iteration are often necessary to
obtain a reliable solution.
Convergence Tolerance
The solver will continue to iterate until the selected converge norm is less than the
Convergence Tolerance. A smaller tolerance produces a more accurate solution
but will take longer to complete the analysis.
And more, with the new dialog, users are allowed to set different convergence
tolerances for residual displacement and unbalanced forces. A tight tolerance for
displacement and a slightly loose tolerance for force will normally get a satisfied
solution as users require.
The analysis reports show the max residual displacement & force at each iteration.
This could help users to determine proper convergence tolerances for their design
model or even for each load case when the nonlinear analysis doesn’t converge.
Type of Convergence Norm
Convergence of the solution within each increment is tested using a Residual
Force or Displacement. The Residual Force norm is computed by dividing the
maximum unbalanced force at the end of an iteration by the maximum force
applied in the current increment. The Displacement norm is determined as the
maximum change in displacement in an iteration which is normalised by the
maximum total displacement at any joint in the structure. The user may choose
whether to use a either or both of these norms to test convergence. Unless analysis
time is an issue we suggest that both norms be used to test convergence as it is
desirable for the value of both norms to be small at the end of an analysis.
Also see: Patch Internal Forces
To display the patch internal forces

Page 188
Choose Patch Internal Forces from the Results sub-menu under the
Display menu