Rigid Connector & Restraint Classes Rigid Connector & Restraint Classes

Rigid connector and restraint classes are defined by using one of the following:


     ~CLASS, FIX, DF(1), ... DF(n),

     ~CLASS, SPR, DF(1), SPV(1), ... DF(n), SPV(n),

     ~CLASS, GAP, COEF

FIX and SPR classes define connections between specified degrees of freedom. For these connectors it is not permitted to define offsets. Instead, MOSES computes an offset for the second end of the connection so that the constraint is satisfied when the connection is defined. Thus, one should issue an &INSTATE command for each body prior to connection to establish the proper relative orientation. The values input when defining these classes are in the body system of the body to which the first node belongs. For both, DF(i) is the name of the degrees of freedom which will be restrained and must be chosen from the list: X, Y, Z, RX, RY, RZ. A FIX class will "fix" the specified degrees of freedom of the element nodes. If all degrees of freedom are to be restrained, then one can simply specify FIX with no other values. The SPR class is used to define linear springs. Here, SPV(i) is the value of the spring (bforce/blength for degrees of freedom X, Y, and Z, and bforce-blength/radian for degrees of freedom RX, RY, and RZ). These classes are used for defining both restraints and connectors. During a stress analysis, the behavior of a restraint and a connector with the same class is identical. During a simulation, however, a connector will behave differently than one may expect. During a simulation, rigid constraints are capable of restraining only translation. Thus, if one selects a rotational connector, it will only be applied during the stress analysis. Also, the connector applied during a simulation will be the same regardless of whether FIX or SPR was used to define the connection. The difference, however, will appear during the stress analysis where the specified flexibility will be applied.

A GAP connector is a rigid connector between two nodes. It will produce a force between the two nodes acting from the second node to the first to keep the distance between them greater than or equal to the distance between them when the gap was defined. Notice that for a GAP connector to be properly defined, the two nodes cannot be coincident. The vector from the second node to the first is called the gap direction and the length of this vector when the gap was defined is called the gap distance. The gap direction is considered to be a vector in the body to which the first node is attached. During a simulation, MOSES will compute the distance measured along the gap direction between the two nodes, and not allow this distance to be less than the gap distance. A gap cannot produce tension. If a gap has a specified friction coefficient, COEF, then it will behave differently during a simulation than during a stress analysis. For the stress analysis, the treatment is according to Coloumb's Law. For a simulation, a rigid connection is created perpendicular to the gap direction whenever the gap is active, i.e. when the gap is active, it prevents all relative motion between the two points.