You must use LOAD if you want to combine gravity loads with loads applied directly to grid points, pressure loads, or centrifugal forces, even if there is only one loading condition.
You can define up to 300 (Si, Li) pairs with a single LOAD entry.
For any particular solution, the total static load will be the sum of the applied loads (grid point loading, pressure loading, gravity loading, and centrifugal forces) and the equivalent loads.
The resulting combined load is determined by:
Equation 8-20.
Where:
• (S) is the overall scale factor you define on the LOAD entry
• (Si) is the scale factor you define on the LOAD entry for each individual load set
• {Li} is the applied load vector corresponding to load set ID Li
You request subcases, which may or may not include different loading conditions, in the Case Control Section. Three Case Control commands (LOAD, TEMP(LOAD), and DEFORM) are used in selecting loading conditions for subcases. Each of these commands identifies a unique set whose SID is the same as the SID of one or more Bulk Data entries. Figure 8-19shows the Bulk Data entries you can select with each of the three Case Control commands.
Figure 8-19. Load Combinations LOAD Example 1
As an example of the LOAD entry combining two loads, consider the umbrella tent shown in Figure 8-20.
Figure 8-20. Umbrella Tent Subjected to Gravity and Wind Loads
The input file containing the combined loading is shown inListing 8-13. To apply the wind pressure in the correct direction, a local coordinate system is defined so that the local Y axis makes an angle of 5 degrees with the ground. This coordinate system is referenced on the PLOAD4 continuation entry. Since all of the elements in the model are exposed to the wind load, the simplest method is to use the thru option on the PLOAD4 entry (fields 8 and 9).
The gravity load of 386.4 entered on the GRAV entry corresponds to a 1 G load in the English system. Whenever you are working with an acceleration vector in terms of Gs, make sure that you convert the load such that its units are consistent with your structure.
$
$ GRID, ELEMENT, AND CONSTRAINTS ARE NOT SHOWN
$ ENDDATA
Listing 8-13. Umbrella Tent with Wind and Gravity Loads
The gravity load is defined by the GRAV entry with a loading set ID of 1. The wind loading is defined using the PLOAD4 entry with a loading set ID of 2. The LOAD entry (ID of 5) is used to combine the gravity and wind loadings. The LOAD entry is selected by the LOAD = 5 command in the Case Control Section. If the GRAV and the PLOAD4 entries were assigned the same ID, a fatal error would have resulted.
LOAD Example 2
Assume that your model has one concentrated force of 15.2 lbs in the y direction applied to grid point 12, and one concentrated moment of 6.4 inch-lbs about the x-axis applied to grid point 127.
It is required to double the value of force and triple the value of moment for your next analysis.
The LOAD Bulk Data entry may be written with an overall scale factor (S) of 1.0 and loadset scale factors (Si) of 2.0 for force and 3.0 for moment. Thus,
In Case Control:
LOAD = 22
In Bulk Data:
1 2 3 4 5 6 7 8 9 10
LOAD 22 1.0 2.0 30 3.0 40
FORCE 30 12 15.2 0. 1. 0.
MOMENT 40 127 6.4 1. 0. 0.
9 Constraints
• Introduction to Constraints
• Single-point Constraints
• Automatically Applying Single-point Constraints
• Enforced Displacements at Grid Points (SPCD, SPC)
• Multipoint Constraints
• Rigid Body Supports
• Surface-to-Surface Gluing
• Linear Contact Using Constraints
9.1 Introduction to Constraints
A constraint is the enforcement of a prescribed displacement (i.e., component of translation or rotation) on a grid point or points. There are two basic types of constraints in NX Nastran: single point constraints (SPCs) and multipoint constraints (MPCs).
• A single point constraint is a constraint applied to an individual grid point. Single point constraints can enforce either zero displacement or nonzero displacement.
• A multipoint constraint is a mathematical constraint relationship between one grid point and another grid point (or set of grid points).
The boundary conditions of a static structure (fixed, hinged, roller support, etc.) typically require that various degrees of freedom be constrained to zero displacement. For example, consider a grid point fixed in a rigid wall. All six displacement degrees of freedom—three translational directions and three rotational directions—must be constrained to zero to mathematically describe the fixed boundary condition.
Real world structures often don’t have simple or ideal boundary conditions. Because a model’s constraints greatly influences its response to loading, you must try to constrain your model as accurately as possible.
This chapter describes how you apply constraints. To understand how constraints are processed, you need to be familiar with the NX Nastran set notation and matrix operations.
9.2 Single-point Constraints
A single point constraint (SPC) applies a fixed value to a translational or rotational component at an individual grid point or to a scalar point. Common uses of single point constraints are to specify the boundary conditions of a structural model by fixing the appropriate
degrees-of-freedom and to eliminate unwanted degrees-of-freedom with zero stiffness. Multiple sets of single point constraints can be provided in the Bulk Data Section, with selections made at execution time by using the subcase structure in the Case Control Section. This procedure is particularly useful in the solution of problems having one or more planes of symmetry.
You can use single-point constraints to:
• To constrain a structure to ground.
• To apply symmetric or antisymmetric boundary conditions by restraining the degrees of freedom that must have a zero value to satisfy symmetry or antisymmetry.
• To remove degrees of freedom that are not used in the structural analysis (i.e., are not connected to any structural elements or otherwise joined to the structure).
• To remove degrees of freedom that are very weakly coupled to the structure. This condition can occur, for example, to the rotations about the normal of a slightly curved shell. In this case, a judgment must be made whether to remove the degree of freedom using an SPC (in which case the structure may be over-constrained), or to leave it in the problem (in which case the stiffness matrix is nearly singular). A reasonable rule is to constrain the degree of freedom if its stiffness is less than 10-8as large as the stiffness in another direction at the same grid point.
The elements connected to a grid point may not provide resistance to motion in certain directions, causing the stiffness matrix to be singular. Single point constraints are used to remove these degrees-of-freedom from the stiffness matrix.
For example, consider a planar structure composed of membrane and extensional elements. The translations normal to the plane and all three rotational degrees-of-freedom must be constrained since the corresponding stiffness matrix terms are all zero. If a grid point has a direction of zero stiffness, the single point constraint doesn’t need to be exactly in that direction. It only needs to have a component in that direction. This allows the use of single point constraints for the removal of such singularities regardless of the orientation of the global coordinate system.
Although the displacements will depend on the direction of the constraint, the internal forces will be unaffected.