Use the Load Application Control for Nonlinear Static Analysis form to select a load-controlled (i.e., Full Load), displacement-controlled, or quasi-static nonlinear static analysis. For all three options, the pattern of loads acting on the structure is determined by the combination of loads specified in the applied loads table on the Load Case Data form. If you don't know what to do, select Full Load control. It is the most common, physical situation.
Load Application Control. Select the appropriate option:
Full Load. Select full load when the magnitude of load that will be applied is known and the structure is expected to be able to support that load. An example would be when applying gravity load because it is governed by nature. Select a displacement component to monitor in the Monitored Displacement area of the form.
Displacement Control. Select displacement control when the distance the structure is to be moved is known, but the amount of load is unknown. This is most useful for structures that become unstable and may lose load-carrying capacity during the course of the analysis. Typical applications include static pushover analysis and snap-through buckling analysis.
Control Displacement. Choose the appropriate option.
Use Conjugate Displacement option. Choose the Use Conjugate Displacement option and then select a displacement component to be monitored in the Monitored Displacement area of the form. Be sure to choose a displacement component that monotonically increases during loading. Use the Use Conjugate Displacement option if the analysis is having trouble converging. The conjugate displacement is a weighted average of all displacements in the structure where each displacement degree of freedom is weighted by the load acting on that degree of freedom. In other words, it is a measure of the work done by the applied load.
Use Monitored Displacement option. Choose the Use Monitored Displacement option and select a displacement component to be monitored in the Monitored Displacement area of the form.
Load to a Monitored Displacement Magnitude of edit box. Use this edit box to specify the magnitude of the displacement that is the target for the analysis. The program will attempt to apply the load to reach the specified displacement.
Monitored Displacement. Use this area of the form to specify the displacement component to be monitored. This may be a single degree of freedom at a joint, or a previously defined generalized displacement (select the displacement from the drop-down list).
Important: Using displacement control is NOT the same as applying displacement loading (see Joint Loads - Ground Displacements) on the structure! Displacement control is simply used to MEASURE the displacement that results from the applied loads, and to adjust the magnitude of the loading in an attempt to reach a certain measured displacement value.
Quasi-Static (run as time-history) This is similar to displacement control in that the distance the structure is to be moved is known. However, the solution is found by applying a specified amount of load incrementally, and solving using dynamic analysis rather than static analysis. This is most useful for structures that become unstable and may lose load-carrying capacity during the course of the analysis, and for which a solution could not be found using displacement control. Typical applications include static pushover analysis and snap-through buckling analysis.
Although a target displacement is sought, the amount of load applied must be specified in advance. This may require some trial and error to determine a load large enough to reach the target displacement or failure of the structure, but not so large as to lose detail in the pushover curve. In many cases, a load on the same order of magnitude as the weight of the structure is a good starting point.
This load will be ramped up slowly in a number of time steps equal to the Minimum Number of Saved States specified on the Results Save for Nonlinear Static Case form. The analysis will continue after ramping up the load until the target displacement is reached or the Maximum Number of Saved States is reached.
Control Displacement. Choose the appropriate option.
Use Conjugate Displacement option. Choose the Use Conjugate Displacement option and then select a displacement component to be monitored in the Monitored Displacement area of the form. Be sure to choose a displacement component that monotonically increases during loading. Use the Use Conjugate Displacement option if the analysis is having trouble converging. The conjugate displacement is a weighted average of all displacements in the structure where each displacement degree of freedom is weighted by the load acting on that degree of freedom. In other words, it is a measure of the work done by the applied load.
Use Monitored Displacement option. Choose the Use Monitored Displacement option and select a displacement component to be monitored in the Monitored Displacement area of the form.
Load to a Monitored Displacement Magnitude of edit box. Use this edit box to specify the magnitude of the displacement that is the target for the analysis. The program will attempt to apply the load to reach the specified displacement.
Monitored Displacement. Use this area of the form to specify the displacement component to be monitored. This may be a single degree of freedom at a joint, or a previously defined generalized displacement (select the displacement from the drop-down list).
Quasi-static Parameters
Time History Type option. Currently only Nonlinear Direct Integration History is available
Output Time Step Size edit box. Select a time step to control the speed of the analysis. The idea is to load slowly enough to minimize inertial and damping effects during load application, except in local regions during strength loss.
Mass Proportional Damping edit box. Choose a value large enough to minimize spurious vibration, but not so large as to significantly delay load application.
Hilber Hughes Taylor Time Integration Parameter, Alpha edit box. This parameter controls the stability of the time integration during nonlinear behavior. The range is from 0 to -1/3. The default value of -1/3 provides the most stability, whereas the value of 0 provides the highest accuracy. The effect of this parameter upon accuracy is not generally significant for quasi-static loading.
Comments: For well-behaved problems, displacement control is usually most efficient. When displacement control is unable to find the desired solution, try quasi-static control Start with a load magnitude on the order of the weight of the structure, use default parameters, and adjust as necessary. If the structure does not reach the desired displacement and is still stable, try increasing the load. If the structure has lost load but not reached the desired displacement, increasing the load is not required. Try increasing the time step size, damping value, or maximum number of saved states. In the case of progressive collapse, reaching the target displacement may not be possible or even necessary, since the structure has no further useful capacity.
Quasi-static analysis uses an artificial mass to provide an inertial reaction against strength loss that creates a stable path for load redistribution. The base reaction plotted for the pushover curve subtracts the inertial and damping forces from the applied load so that it primarily represents stiffness forces. In the case of rapid strength loss, significant vibration may be seen in the base reaction force. This is not generally important with respect to the main goal of determining the deformation (ductility) demand upon the components of the structure.
Access the Load Application Control for Nonlinear Static Analysis form as follows:
Click the Define menu > Load Cases command to display the Load Cases form.
Click the Add New Case button or highlight a previously defined load case and click the Add Copy of Case or the Modify/Show Case button to display the Load Case Data form.
Select Nonlinear Static for the Load Case Type.
Click the Load Application Modify/Show button.