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General Nonlinear Parameters for Analysis

Total Load Cycles

It describes the number of total load cycles for the nonlinear analysis. The number is equal to the total number of load cycles required in the Analysis.

Maximum Iterations for each Load Cycle

It describes the maximum number of iteration for each load cycle in the analysis. If the equilibrium condition is satisfied before this number is reached or the iteration number is equal to this assigned iteration number, another load step will be imposed until the permitted number of load steps is reached. The tolerance for equilibrium check is 0.1 % by default. That is, when the Euclidean norms of the unbalanced displacements and the unbalanced forces are less than respectively 0.1 % of the total applied forces and the total accumulated displacements, the equilibrium condition is assumed to have been satisfied.

Number of Iterations for Tangent Stiffness Matrix

It describes the number of iterations for the tangent stiffness matrix to reform during the iterative process. When this number is specified to be very large or simply equal to the “maximum number of iterations for each load cycle” above, the iterative scheme will then become the modified Newton Raphson method. If the number here is specified as "1”, it becomes the Newton Raphson method. If the number is between these two extremes, the method is a mixed Newton-Raphson method. When compared to modified Newton Raphson method, the Newton Raphson method generally requires less number of iterations for convergence, but longer time for each iteration. It is recommended to use the Newton Raphson method.

Incremental Load Factor

This factor will be used as the first load factor used for the analysis and the load factor increment in subsequent analysis. It is different from the design load factor behind “header load” which is multiplied to the input load to obtain the design load vector and will not appear in the plotting of equilibrium or load-deflection curve with its value generally taken as, for example, 1.6 for wind, 1.4 for self-weight etc. The load factor described here is used as the ratio of the current applied load to the input design load. For example, if a structure yields at a load factor of 2.6, it means when the applied load is 2.6 of the design load, the structure yields.

Imperfection Method & Direction

It describes the direction of initial imperfection with different methods. It can be no initial imperfection, initial imperfection in one principal plane causing less severe effect than initial imperfection in both the principal planes. The minimum magnitude of initial imperfection is taken as 1/1000 of the member length if the initial imperfections are allowed. For some sections such as cold-formed sections, this value may not be adequate. For global imperfections of a structure, the notional force can be used in place of member imperfections.

Magnitude of Imperfection for Global Eigenvalue Mode

This value is the magnitude of imperfection when eigenvalue buckling mode is adopted. After the eigenvalue analysis, the eigen-mode is determined and a set of initial imperfection is determined for the structure with this mode shape. This number is for the magnitude (maximum) of the initial deflection for the eigen-mode which is then added to the initial geometry of the structure.

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