LUSAS 활용
활용사례 및 기타 자료
Step-by-step nonlinear analysis setup
LUSAS 활용
How to solve the problem of convergence failure in construction stage-by-stage analysis
Analysis by construction stage is a nonlinear analysis that performs analysis in the form of adding a structure or adding a load in the next stage, using the deflection and stress state of the previous stage as the initial conditions.
In reality, after Stage 1 is completed, Stage 2 is constructed, and in the LUSAS model, each stage is modeled as Deactivate / Activate after expressing the entire structure. In reality, when Stage 2 is constructed, the boundaries between Stages 1 and 2 disappear and the top surface of Stage 2 will be constructed without sagging, but in modeling, if sagging occurs in Stage 1 due to its own weight or external force, it will also affect the shape of Stage 2, which has not yet been constructed.
To overcome these differences and to more strictly express the actual construction process, LUSAS applies Displacement Reset (Attribute menu) when creating (Activating) a new structure to compensate. However, if the change in the shape of the structure or the change in the load is too large during this process, the nonlinear analysis solution cannot be obtained.
In a step-by-step analysis model of an LNG tank using a 3D shell model, if the analysis is not completed until the final stage, we will summarize the method of completing the analysis by correcting the model.
(1) Check the interpretation progress
For construction stages (load cases) where the analysis has been completed, a rainbow-shaped icon is displayed as shown in the figure below, but it is not displayed if the nonlinear analysis solution is not found.
(2) Copy load case
Right-click on the step (load case) where the analysis failed to open the pop-up menu and execute Copy & Paste. This will create a new load case by duplicating all conditions.
(3) Check the changed contents
Identify any structures or loads added to the load cases where the analysis failed.
In the example below, no load conditions have been added, only the structure has been added and activated. You can select the added part using the Select Assignment menu.
(4) Delete properties inherited from the previous step
In order to converge the nonlinear analysis, the added part from the analysis result of the previous step is adjusted so that it is added gradually rather than all at once. To this end, all loads from the previous step are removed from the load case for which the solution is to be found.
One thing to note when a new structure is activated is that although the gravity applied to the entire structure is deleted, the gravity of the newly created structure in the current load case is also deleted. In this case, you need to define the load acting in the direction of gravity as a Body Force and re-apply it to the newly created structure.
(5) Automatic Control Settings
Specify Automatic Control for load cases where the analysis failed as follows.
By doing this, a nonlinear solution is obtained by gradually increasing the load added in the previous step from 10% specified in Starting load factor to 100% specified in Max total load factor.
This load case is only for conducting nonlinear analysis, and there is no need to view the results for each load size, so you can uncheck Same as previous loadcase and set plot file = 0. (This is to reduce the time to create the result file and the size of the result file.)
(6) Restore Manual Control
If you set Nonlinear Control in one load case, it will be inherited in subsequent load cases. In order for the analysis to proceed according to the conditions specified for each load case, the load case that was initially created by duplication is re-designated as Manual Control.
If you specified plot file = 0 in the previous step , you must change it back to 1 to save the result file.
In the duplicated load case, the Activate item is deleted because it has already been reflected in the load case to which Automatic Control was applied in the previous step.
(7) Perform interpretation
Analysis by construction stage is a nonlinear analysis that performs analysis in the form of adding a structure or adding a load in the next stage, using the deflection and stress state of the previous stage as the initial conditions.
Among the pop-up menus linked to material data, Temperature Dependence… Select to specify the temperature and the material properties at that temperature.
Axisymmetric elements are used to easily proceed with analysis by modeling only one section instead of modeling the entire structure with axisymmetric behavior. We summarize contents related to loads, boundary conditions, and result extraction in structural analysis using axisymmetric elements.