Static Loads and Boundary Conditions

Any number of load sets can be defined in the Bulk Data section of the input file. However, only those sets selected in the Subcase Information section (as described in Linear Static Analysis, Inertia Relief and Nonlinear Static Analysis) will be used in the problem solution. The manner in which each type of load is selected is specified on the associated Bulk Data statement description.

The FORCE statement is used to define a static load applied to a grid point in terms of components defined by a local coordinate system. The orientation of the load components depends on the type of local coordinate system used to define the load.

The FORCE1 statement is used if the direction is determined by a vector connecting two grid points.

The MOMENT and MOMENT1 statements are used to define the application of a concentrated moment at a grid point.

The ACCEL, ACCEL1, and ACCEL2 entries are used to define the application of static acceleration loads at grid points in the model.

Pressure loads on triangular and quadrilateral elements are defined with a PLOAD2 card. The positive direction of the loading is determined by the order of the grid points on the element connection card (using the right-hand rule). The magnitude and direction of the load is automatically computed from the value of the pressure and the coordinates of the connected grid points. The load is applied to the connected grid points.

PLOAD pressure loads are used in a similar fashion to define the loading of any three or four grid points, regardless of whether or not they are connected with two-dimensional elements.

Pressure loads on the HEXA, PENTA and TETRA solid elements are defined with the PLOAD4 card. The pressure is defined positively outward from the element. The magnitude and direction of the equivalent grid point forces are automatically computed using the iso-parametric shape functions of the element to which the load has been applied. Pressure loads on the QUAD4 and TRIA3 elements can also be applied using the PLOAD4 card.

The PLOAD1 card is used to describe concentrated, uniformly distributed or linearly distributed loads on the CBAR or CBEAM elements.

The GRAV Bulk Data Entry is used to specify a gravity load by providing the components of the gravity vector in any defined coordinate system. The gravity load is obtained from the gravity vector and the mass matrix of the structural model. Because the gravitational acceleration is not calculated at scalar points, you are required to introduce gravity loads at scalar points directly.

The RFORCE statement is used to define a static loading condition due to a centrifugal force field. A centrifugal force load is specified by the designation of a grid point that lies on the axis of rotation and by the components of rotational velocity and acceleration in any defined coordinate system. In the calculation of the centrifugal force, the mass matrix pertains to a set of distinct rigid bodies connected to grid points. Deviations from this viewpoint, such as the use of scalar points or the use of mass coupling between grid points, can result in errors.

Temperature loads can only be defined at grid points. The temperatures of the connected grid points are given on the TEMP and TEMPD Bulk Data Entries. The thermal expansion coefficients are defined on the material definition cards. The mere presence of a thermal field does not imply the application of a thermal load. A thermal load will not be applied unless you make a specific request in the Subcase Information section.

The LOAD Bulk Data card defines a static loading condition that is a linear combination of load sets consisting of loads applied directly to grid points, pressure loads, gravity loads, and centrifugal forces. This card must be used if gravity loads are to be used in combination with loads applied directly to grid points, pressure loads, or centrifugal forces. The application of the combined loading condition is requested in the Subcase Information section by selecting the set number of the LOAD combination.

It should be noted that the equivalent loads (thermal and enforced displacement) must have unique set identification numbers and be selected separately in the Subcase Information section. For any particular solution, the total load will be the sum of the applied loads (grid point loading, pressure loading, gravity loading, and centrifugal forces) and the equivalent loads.

Zero enforced displacements may be specified on SPC or SPC1 cards. Zero displacements result in non-zero forces on the grid point constrained (SPC forces).

The SPCADD statement allows the combination of different SPC sets.

For inertia relief, the reaction degrees of freedom for the computation of the acceleration load are defined through SUPORT or SUPORT1 statements. Up to six degrees of freedom can be defined per subcase.

Non-zero enforced displacements may be specified on SPC or SPCD cards. The SPC card specifies both the component to be constrained and the magnitude of the enforced displacement. The SPCD card only specifies the magnitude of the enforced displacement. When an SPCD card is used, the component to be constrained must be specified on either an SPC or an SPC1 card. The use of the SPCD card avoids the decomposition of the stiffness matrix when changes are only made in the magnitudes of the enforced displacements.

The equivalent loads resulting from enforced displacements of grid points are calculated by the program and added to the other applied loads.

If the magnitudes of the enforced displacements are specified on SPC cards, the application of the load is automatic when you select the associated SPC set in the Subcase Information section.

If the magnitude of the displacement is defined on an SPCD card, the load is applied if you select the associated LOAD set in the Subcase Information section.