6.1.3 Multiple load case analysis

**Products: **ABAQUS/Standard ABAQUS/CAE

A multiple load case analysis:

is used to study the linear responses of a structure subjected to distinct sets of loads and boundary conditions defined within a step (each set is referred to as a load case);

can be much more efficient than an equivalent multiple perturbation step analysis;

allows for the changing of mechanical loads and boundary conditions from load case to load case;

includes the effects of the base state; and

can be performed with a static perturbation or steady-state dynamics, direct procedure.

Load cases

A load case refers to a set of loads and boundary conditions comprising a particular loading condition. For example, in a simplified model the operational environment of an airplane might be broken into five load cases: (1) take-off, (2) climb, (3) cruise, (4) descent, and (5) landing. Often a load case is defined in terms of unit loads or prescribed boundary conditions, and a multiple load case analysis refers to the simultaneous solution for the responses of each load case in a set of such load cases. These responses can then be scaled and linearly combined during postprocessing to represent the actual loading environment. Other postprocessing manipulations on load cases are also common, such as finding the maximum Mises stress among all load cases. These types of load case manipulations can be requested in the Visualization module of ABAQUS/CAE (see the ABAQUS/CAE User's Manual).

Using multiple load cases

A multiple load case analysis is conceptually equivalent to a multiple step analysis in which the load case definitions are mapped to consecutive perturbation steps. However, a multiple load case analysis is generally much more efficient than the equivalent multiple step analysis. The exception occurs when a large number of boundary conditions exist that are not common to all load cases (i.e., degrees of freedom are constrained in one load case but not others). It is difficult to define what “large” is since it is model dependent. The relative performance of the two analysis methods can be assessed by performing a data check analysis for both the multiple load case analysis and the equivalent multiple step analysis. The data check analysis writes resource information for each step to the data file, including the maximum wavefront, number of floating point operations, and minimum memory required. If these numbers are noticeably larger for the multiple load case step compared to those across all steps of the equivalent multiple step analysis (the number of floating point operations should be summed over all steps before comparing), the multiple step analysis will be more efficient.

Although generally more efficient, the multiple load case analysis may consume more memory and disk space than an equivalent multiple step analysis. Thus, for large problems or problems with many load cases it is again advisable, as described above, to compare resource usage between the multiple load case analysis and the equivalent multiple step analysis. If resource requirements for the multiple load case analysis are deemed too large, consider dividing the load cases among a few steps. The resulting analysis (a hybrid of multiple load cases and multiple steps) will require fewer resources while retaining an efficiency advantage over an equivalent pure multiple step analysis.

Defining load cases

You define a load case within a static perturbation or direct-solution steady-state dynamic analysis step. Load case definitions do not propagate to subsequent steps. Only the following types of prescribed conditions can be specified within a load case definition:

Boundary conditions

Concentrated loads

Distributed loads

Distributed surface loads

Inertia-based loads

Each load case definition is assigned a name for postprocessing purposes.

Input File Usage: | Use the first option to begin a load case and the second option to end a load case: |

*LOAD CASE, NAME= Prescribed conditions specified within a load case definition apply only to that load case. Prescribed conditions can be specified outside the load case definitions, in which case they apply to all load cases in the step. |

ABAQUS/CAE Usage: | Load module: |

In ABAQUS/CAE if a step contains load cases, all prescribed conditions in the step must be included in one or more load cases. |

Load cases can be defined only in perturbation steps with the following procedures:

Static

Direct-solution, steady-state dynamic

Restart from a particular load case

Submodeling using results from other than the first load case in the global analysis

Importing and transferring results

Cyclic symmetry analysis

Contour integrals

Design sensitivity analysis

Boundary conditions can be specified both outside and inside load case definitions in the same step. Specifying a boundary condition outside the load case definitions in a step is equivalent to including it in all load case definitions in the step (i.e., the boundary condition will be applied to all load cases). Unless any boundary conditions are removed in the perturbation step, the boundary conditions that are active in the base state will propagate to all load cases in the perturbation step. If any boundary condition is removed in a step with load cases (either outside or inside load case definitions), the base state boundary conditions will not be propagated to any load case in the step. See “Boundary conditions,” Section 27.3.1, for more information.

**Note:
**In ABAQUS/CAE if a step contains load cases, all boundary conditions in the step must be included in one or more load cases.

Concentrated, distributed, and distributed surface loads can be specified both outside and inside load case definitions in the same step. Inertia relief loads can be specified either outside load case definitions or inside load case definitions in the same step but not both simultaneously.

Specifying one of these load types outside the load case definitions in a step is equivalent to including it in all load case definitions in the step (i.e., the loading will be applied to all load cases). As with any perturbation step, perturbation loads must be defined completely within the perturbation step (see “Applying loads: overview,” Section 27.4.1).

**Note:
**In ABAQUS/CAE if a step contains load cases, all loads in the step must be included in one or more load cases.

In a step containing one or more load cases, only field output requests to the output database and output requests to the data file are supported. Output requests to the results file and history output requests to the output database are not supported. Output requests can be specified only outside load case definitions, and they apply to all load cases in a step. The step propagation rules for output requests are the same as for other perturbation steps (see “Output,” Section 4.1.1).

All of the field output variables normally available within a particular procedure are also available during a multiple load case analysis (see “ABAQUS/Standard output variable identifiers,” Section 4.2.1). The output corresponding to each load case is stored in a separate frame on the output database with the load case name included as a frame attribute. The Visualization module of ABAQUS/CAE and the ABAQUS Scripting Interface (see Chapter 8, “Using the ABAQUS Scripting Interface to access an output database,” of the ABAQUS Scripting User's Manual) can be used to access and manipulate load case output. ABAQUS/Standard does not perform consistency checks on the physical validity of the load case manipulations. For example, the linear superposition of two load cases, each with different boundary conditions, is allowed even though the combined results may not be physically meaningful.

*HEADING … *STEP, PERTURBATION *STATICor*STEADY STATE DYNAMICS, DIRECT … *OUTPUT, FIELD … *BOUNDARYData lines to specify boundary conditions for all load cases.*DLOADData lines to specify distributed loads for all load cases.*CLOADData lines to specify point loads for all load cases.*DSLOADData lines to specify distributed surface loads for all load cases.*INERTIA RELIEFData lines to specify inertia relief loading directions. (This option cannot be used inside load cases if it is used here.)… *LOAD CASE, NAME=name1*BOUNDARYData lines to specify boundary conditions for first load case.*DLOADData lines to specify distributed loads for first load case.*CLOADData lines to specify point loads for first load case.*DSLOADData lines to specify distributed surface loads for first load case.*INERTIA RELIEFData lines to specify inertia relief loading directions. (This option cannot be used outside load cases if it is used here.)*END LOAD CASE *LOAD CASE, NAME=name2Load and boundary condition options for second load case*END LOAD CASE …Subsequent load case definitions… *END STEP