An MPC (multi-point constraint) is a constraint that defines the response of one or more nodal degrees-of-freedom (called dependent degrees-of-freedom) to be a function of the response of one or more nodal degrees-of-freedom (called independent degrees-of-freedom). The general form of the MPC, which most of the major structural analysis codes support (referred to as the explicit MPC type in Patran) is as follows:

Where U0 is the dependent degree-of-freedom, Ui the independent degrees-of-freedom, and Ci the constants. The term to the left of the equal sign is called the dependent term and the terms to the right of the equal sign are called the independent terms. C0 is a special independent term called the constant term.

An example of an explicit MPC is:

which specifies that the x displacement of node 4 is equal to half the x displacement of node 5 minus half they displacement of node 10 plus 1.0. There are four terms in this example, one dependent term, two independent terms, and a constant term.

MPCs can be used to model certain physical phenomena that cannot be easily modeled using finite elements, such as rigid links, joints (revolutes, universal, etc.), and sliders, to name a few. MPCs can also be used to allow load transfer between incompatible meshes. However, it is not always easy to determine the explicit MPC equation that correctly represents the phenomena you are trying to model.

To help with this problem, many analysis codes provide special types of MPCs (sometimes called “implicit” MPCs) which simulate a specific phenomena with minimum user input. For example, most analysis codes support an implicit MPC type which models a rigid link, in which an independent node is rigidly tied to one or more dependent nodes. All the user is required to input are the node IDs. The analysis code internally generates the “explicit” MPCs necessary to cause the nodes to act as if they are rigidly attached.

In addition to the implicit MPC types supported by the analysis code, there are implicit MPC types supported by the analysis code translator. These are converted into “explicit” form during the translation process. This allows Patran to support more MPC types than the analysis code supports itself.

Patran supports the creation of all MPC types through the use of a single form, called Create MPC Form (for all MPC Types Except Cyclic Symmetry and Sliding Surface), with two exceptions: the Cyclic Symmetry and Sliding Surface MPC types. These two MPC types have special capabilities which require special create forms. See Create MPC Cyclic Symmetry Form and Create MPC Sliding Surface Form.

Before creating an MPC, first select the type of MPC you wish to create. Once the type has been identified, Patran displays the proper form(s) to create the MPC.

A list of the MPC types which are supported by the MSC analysis codes can be found in the application module User’s Guide or application Preference Guide for the respective analysis code. You will only be able to create MPCs which are valid for the current settings of the Analysis Code and Analysis Type preferences. If the Analysis Code or Analysis Type preference is changed, all existing MPCs, which are no longer valid, are flagged as such and will not be translated. Invalid MPCs are still stored in the database and are displayed, but they cannot be modified or shown. However, they can be deleted. An invalid MPC can be made valid again by setting the Analysis Code and Analysis Type preferences back to the settings under which the MPC was originally created.

The principal difference between one MPC type and the next is the number and makeup of the dependent and independent terms. A term is composed of up to four pieces of information:

For example, a dependent term of the explicit MPC type consists of a single node and a single degree-of-freedom, while an independent term of the explicit MPC type consists of a coefficient, a single node, and a single degree-of-freedom. As another example, the dependent and independent terms of the Rigid (fixed) MPC type consist of a single node.

The number of dependent and independent terms required or allowed varies from one MPC type to the next. For example, the Explicit MPC type allows only one dependent term while allowing an unlimited number of independent terms. Conversely, the Rigid (fixed) MPC type allows one independent term while allowing an unlimited number of dependent terms. Other MPC types allow only one dependent and one independent term, or one dependent and two independent terms.

Whenever one or more degrees-of-freedom are expected for an MPC term, a listbox containing the valid degrees-of-freedom is displayed on the form. A degree-of-freedom is valid if:

In most cases, all degrees-of-freedom which are valid for the current Analysis Code and Analysis Type preferences are valid for the MPC type. There are some cases, however, when only a subset of the valid degrees-of-freedom are allowed for an MPC. For example, an MPC may allow the user to select only translational degrees-of-freedom even though rotational degrees are valid for the Analysis Code and Analysis Type preference.

The degrees-of-freedom which are valid for each Analysis Code and Analysis Type Preference are listed in the analysis code or analysis code translator User’s Guide.

MPCs are displayed as a set of lines which connect each dependent node (node appearing as part of a dependent term) to each independent node (node appearing as part of an independent term). The dependent nodes are circled to distinguish them from the independent nodes (see Figure 3‑1). MPCs are treated like elements in Patran because they:

In certain cases, Patran allows you to create several multi-point constraints (called Sub-MPCs) at one time which are stored as a single MPC entity with a single ID. The following rules apply:

When multiple Sub-MPCs are created, they are displayed as shown in Figure 3‑2, with the ID of the MPC displayed at the centroid of each Sub-MPC. The translators will treat each Sub-MPC as a separate MPC, but Patran treats the collection of Sub-MPCs as a single entity.

When Create is the selected Action and MPC is the selected Object, the Create MPC form is displayed. Several MPC types are valid under the Type option menu.

The Define Terms form appears when the Define Terms button is selected on the Create MPC form. Use this form to define the dependent and independent terms of an MPC.

Use this form to Create an MPC which defines a set of cyclic symmetry boundary conditions between the nodes in two regions.

Use this form to create an MPC which defines a sliding surface between the nodes in two coincident regions. The translational degree-of-freedom (normal to the surface) of coincident nodes in the two regions are tied while all others remain free.