Point Loads

This section shows how to apply concentrated forces and concentrated moments at grid points.

• You can use the FORCE, FORCE1, and FORCE2 entries to apply concentrated forces directly to grid points.

• You can use the MOMENT, MOMENT1, and MOMENT2 entries to apply a apply

concentrated force in the translational degrees of freedom (T1, T2, and T3) at a grid point.

• Use FORCEAX and MOMAX to define forces or moments on conical shell elements.

• Use SLOAD to define a concentrated static load at a scalar point.

FORCE, FORCE1, FORCE2

You can apply concentrated forces directly to grid points with the FORCE, FORCE1, and FORCE2 entries.

• Use a FORCE entry if you want to define a static, concentrated force at a grid point by specifying a vector.

• Use a FORCE1 entry if the direction is determined by a vector connecting two grid points.

• Use a FORCE2 entry if the direction is specified by the cross product of two such vectors.

Using FORCE

The FORCE entry lets you specify the magnitude and direction of a force vector in any coordinate system as shown below. You can use the FORCE entry to define a static load applied to a

geometric 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 you used define the load.

The directions of the load components are the same as those indicated onFigure 5-2of“Defining Grid Points and Scalar Points”for displacement components.

Consider a force F acting on a cantilever beam:

A concentrated force F is applied to a CBAR element connecting grid points 1 and 2 using the FORCE entry. Its format is as follows:

1 2 3 4 5 6 7 8 9 10

FORCE SID G CID F N1 N2 N3

Field Contents

SID Load set identification number. (Integer > 0) G Grid point identification number. (Integer > 0)

CID Coordinate system identification number. (Integer ≥ 0; Default = 0)

F Scale factor. (Real)

Ni Components of a vector measured in coordinate system defined by CID.

(Real; at least one Ni ≠ 0.0)

In our example, the FORCE entry may be written as:

FORCE 2 2 10. 0. -1. 0.

or, in free field format,

FORCE,2,2,,10.,0.,-1.,0.

The load set identification number (SID in column 2) refers to a command defined in the Case Control Section (LOAD=2 in this example; the integer value 2 is arbitrarily chosen). Leaving column 4 blank means that the basic (default) coordinate system is used to specify the orientation of the load. The (0., -1., 0.) entries in columns 6, 7, and 8 refer to a vector in the -Y direction, defining the direction of application of the load. The force applied to the grid point is , given by

Equation 8-1.

where = (N1,N2,N3)

Thus, the value of F in column 5 is the full value of the applied load of 10 lb_fbecause vector (in this example) is of unit length.

The magnitude of the applied force is the scale factor (field 5) times the magnitude of the vector defined in fields 6 through 8. For example, the force applied with the following two FORCE entries is the same.

• “FORCE” in the NX Nastran Quick Reference Guide Using FORCE1 and FORCE2

• The FORCE1 entry lets you define a force by specifying a magnitude and two grid points (not necessarily the loaded grid point) to determine the direction.

• The FORCE2 entry lets you define a force by specifying a magnitude with the direction defined by the vector product of two other vectors.

The format of the FORCE1 and FORCE2 entries is as follows:

1 2 3 4 5 6 7 8 9 10

While the FORCE1 and FORCE2 forms are not used extensively, they are useful for some modeling situations as shown in the following example.

Suppose you wish to model a curved structure with edge loads as shown inFigure 8-2. The edge loads consist of 10 N per element (acting normal to the elements) and 8 N per element along the edge of each element (acting as a shear load). The FORCE1 and FORCE2 entries are the ideal choice to apply these loads. You could use the FORCE entry, but it would be more difficult because you would need to compute the normals for each of the loaded CQUAD4 elements.

Using FORCE1 and FORCE2 entries for the shear and normal loads, respectively, makes the task much easier.

The 10 N normal force for each element is divided equally between the two grid points along the edge. The directions of the resulting 5 N forces are applied using the FORCE2 entry by taking the cross product of the vectors defined by the edges to which the grid points are attached.

The result is a 5 N load acting normal to the plate. The 8 N shear force for each element is also divided up equally between the two grid points along the edge. The resulting 4 N loads are applied with the FORCE1 entry. The directions of these loads are determined by the line connecting the edge grid points.

The input file is shown inListing 8-4. Note that the normal forces and the shear forces are applied as one load condition. If you want the forces to act as separate loadings, you can use different IDs for the FORCE1 and FORCE2 entries and call them out using two separate subcases in your input file.

Figure 8-2. Edge Forces on a Plate Element

$ FILENAME - EDGE.DAT

GRID 2 60. -60. -2.

Note that the two loads are applied at the same grid points. This is perfectly acceptable. The total force acting at the grid points is the sum of all the applied loads. The OLOAD (applied load output request) output, which you request through the Case Control section, shows the applied force resultant acting at a set of user specified grid points. The OLOAD output is useful in verifying that the loads are applied correctly. A partial listing of the output file showing the OLOAD output is shown inFigure 8-3.

Figure 8-3. OLOAD Output See also

• “FORCE1” in the NX Nastran Quick Reference Guide

• “FORCE2” in the NX Nastran Quick Reference Guide Using MOMENT, MOMENT1, MOMENT2

MOMENT, MOMENT1, and MOMENT2 are similar to the three FORCE entries. The only difference is that you use force entries to apply concentrated force in the translational degrees of freedom (T1, T2, and T3), while you use moment entries apply concentrated moments in the rotational degrees of freedom (R1, R2, and R3).

• With MOMENT, you define a static, concentrated moment at a grid point by specifying a scale factor and a vector that determines the direction

• With MOMENT1, the software uses two grid points to determine direction ( , where is a unit vector parallel to the vector from grid 1 to grid 2).

• With MOMENT2, the software uses four grid points to determine direction ( , where is the unit vector parallel to the cross product of the vectors from G1 to G2, and G3 to G4).

The application of concentrated moments is analogous to forces. MOMENT has the following format:

1 2 3 4 5 6 7 8 9 10

MOMENT SID G CID M N1 N2 N3

Field Contents

SID Load set identification number. (Integer > 0)

G Grid point identification number at which the moment is applied.

(Integer > 0)

CID Coordinate system identification number. (Integer ≥ 0 or blank)

M Scale factor. (Real)

Ni Components of the vector measured in the coordinate system defined by CID. (Real; at least one Ni ≠ 0.0)

Consider moment M acting about the basic Z axis of the simply supported beam shown below:

In this case, the MOMENT entry could be written as

1 2 3 4 5 6 7 8 9 10

MOMENT 6 2 -18.6 0.0 0.0 1.

or, in free field format,

MOMENT,6,2,,-18.6,0.,0.,1.

The applied moment is given by

Equation 8-2.

where is the vector (N1, N2, N3). As was the case with FORCE entry, M is the full magnitude of the moment since is a vector of unit length. The direction of the applied moment is given by the sign of M according to the right-hand rule because = (0., 0., 1.) is a vector in the direction of the positive Z axis direction. Note that specifying M = 18.6 and = (0., 0., −1.) would produce an equivalent result.

See also

• “MOMENT” in the NX Nastran Quick Reference Guide

• “MOMENT1” in the NX Nastran Quick Reference Guide

• “MOMENT2” in the NX Nastran Quick Reference Guide Using FORCEAX, MOMAX

You can use the FORCEAX and MOMAX entries to define the loading of specified harmonics on rings of conical shell elements. You can use FORCE and MOMENT entries to apply concentrated loads or moments to conical shell elements if you defined such points with a POINTAX entry.

See also

• “FORCEAX” in the NX Nastran Quick Reference Guide

• “MOMAX” in the NX Nastran Quick Reference Guide Using SLOAD

You can use the SLOAD entry to define a concentrated static load at a scalar point or a grid point. With SLOAD, you can only specify the magnitude since only one component of motion exists at a scalar point.

See also

• “SLOAD” in the NX Nastran Quick Reference Guide