Using SUBCOM to Combine Subcases
8.5 Distributed Loads
There are three Bulk Data entries available to apply distributed loads to element surfaces.
• Use PLOAD define a uniform static pressure load on a triangular or quadrilateral surface.
• Use PLOAD1 and PLOAD2 to apply a distributed load to the CQUAD4, CTRIA3, and CSHEAR elements only.
• Use PLOAD4 to apply distributed loads on any of the two-dimensional elements and on the surfaces of three-dimensional elements.
PLOAD
You can use the PLOAD entry to define a uniform normal static pressure load on triangular or quadrilateral surfaces by using grid points. You can apply PLOAD to 2-D (surface) or 3-D (solid) elements.
The PLOAD entry is different from the other PLOADi entries because it references three or four grid points rather than the element itself. When you use PLOAD to load a CTRIA3 element, the total force acting on the element is divided evenly among the three corner points. When you use PLOAD to load a CQUAD4 element, the total force acting on the element is distributed to corner grid points using the geometric shape of the element. The resultant of the applied corner loads acts through the centroid of the element. The direction of the pressure load is determined using the right-hand rule by the numbering sequence of the grid points on the PLOAD entry.
The PLOAD entry has the following form:
1 2 3 4 5 6 7 8 9 10
PLOAD SID P G1 G2 G3 G4
Field Contents
SID Load set identification number. (Integer > 0)
P Pressure. (Real)
Gi Grid point identification numbers. (Integer > 0; G4 may be zero or blank) Grid Points G1, G2, G3, and G4 define either a triangular or quadrilateral surface; if G4 is zero or blank, the software assumes that the surface is triangular. The direction of the pressure load is determined by applying the right-hand rule to the grid point ordering sequence of the surface.
Pressure is applied in the opposite direction by making the value of P negative.
See also
• “PLOAD” in the NX Nastran Quick Reference Guide
PLOAD1
You can use the PLOAD1 entry to apply a distributed load to a CBAR, CBEAM, or CBEND element. You can use the PLOAD1 entry for both concentrated and linearly distributed forces.
• For the CBAR and CBEAM elements, you can apply the linearly distributed force between any two locations on the element (or off the element if you wish.)
• For the CBEND element, you can only apply the linearly distributed loads to linearly varying forces and moments between the end points.
You can use PLOAD1 entry to define either distributed or concentrated loads at intermediate points on CBAR and CBEAM elements. You define applied loads at the end points of CBAR and CBEAM elements with either FORCE or MOMENT entries. You can also use PLOAD1 to define only distributed loads for the CBEND element. For the CBEND element, you can only use PLOAD1 to define
You can use the CBARAO entry is used to define intermediate points on CBAR elements where you want to obtain stress and/or force output. You can also use the PLOAD1 to define only distributed loads for the CBEND element. The distributed load is always applied along the entire length of the element and does not define loads at intermediate points on the CBEND.
The PLOAD1 entry cannot be used to define concentrated loads at intermediate stations on the CBEND element.
You can apply the load can be applied along the entire element length, a segment of the length, or at a point along the length. The form of the PLOAD1 entry is shown below. The meanings of X1, X2, P1, and P2 are shown inFigure 8-4.
Format
1 2 3 4 5 6 7 8 9 10
PLOAD SID EID TYPE SCALE X1 P1 X2 P2
Field Contents
SID Load set identification number. (Integer > 0)
EID CBAR, CBEAM, or CBEND element identification number.(Integer > 0) TYPE Load type. (Character: “FX”, “FY”, “FZ”, “FXE”, “FYE”, “FZE”, “MX”,
“MY”, “MZ”, “MXE”, “MYE”, “MZE”)
SCALE Determines scale factor for X1, X2. (Character: “LE”, “FR”, “LEPR”,
“FRPR”)
X1, X2 Distances along the CBAR, CBEAM, or CBEND element axis from end A. (Real; X2 may be blank, 0 ≤ X1 ≤ X2)
P1, P2 Load factors at positions X1, X2. (Real or blank)
Figure 8-4. PLOAD1 Convention in CBAR or CBEAM Elements
The TYPE option in field 4 lets you specify the type of the load. The load can be a concentrated force or moment in the basic or element coordinate system. If the applied load is to be a concentrated load, leave fields 8 and 9 blank and the concentrated load will be applied at the X1 location. If both X1 and X2 are input, the load will be taken as a linearly varying load between X1 and X2.
The SCALE option in field 5 lets you specify the location of the load on the CBAR and CBEAM elements using field 5 of the PLOAD1 entry.
• With the “LE” or “LEPR” methods, you specify the actual start and end positions of the load as measured from end A of the element. When using this method, the distances X1 and X2, as shown inFigure 8-5, are in the same units as the dimensions used for the model.
• With the “FR” or “FRPR” methods, you specify the percent (using “X/XB”) along the element where the load starts and ends. A value of 0.0 defines end A, while a value of 1.0 defines end B. Again, the start of the load is measured from end A.
For both methods of describing the location of the loads, you can also specify whether the applied load is to be a direct load (scale “LE” and “FR”) or a projected load (scale “LEPR” or “FRPR”). For the projected loads, distributed loads are entered in terms of the projected length of the element as shown inFigure 8-5. Remember that if you input the loads in terms of the basic coordinate system (“FX”, “FY”, “FZ”, “MX”, “MY”, or “MZ”), then the projected angle a is with respect to the basic coordinate system, not the element coordinate system.
PLOAD1Examples
This section contains a series of examples that illustrate different uses of the PLOAD1 entry.
PLOAD1 Example 1
In this example, we used PLOAD1 to apply a uniformly distributed load over the full length of a CBAR element using fractional (normalized) scaling.
Note that P1 = P2 = 12.6 lbf/inch
X1 = 0.0 fractional scaling
X2 = 0.0
1 2 3 4 5 6 7 8 9 10
PLOAD1 SID EID TYPE SCALE X1 P1 X2 P2
PLOAD1 36 52 FY FR 0.0 -12.6 1.0 -12.6
Note: You can also use PLOAD1 to apply a uniformly distributed load over the full length of a CBAR element using length scaling.
1 2 3 4 5 6 7 8 9 10
PLOAD1 SID EID TYPE SCALE X1 P1 X2 P2
PLOAD1 36 52 FY LE 0.0 -12.6 2.0 -12.6
PLOAD1 Example 2
In this example, we used PLOAD1 to apply a linearly varying distributed load to the interior of a CBAR element using length scaling:
1 2 3 4 5 6 7 8 9 10
PLOAD1 SID EID TYPE SCALE X1 P1 X2 P2
PLOAD1 36 52 FY LE 1.0 -3.1 2.0 -8.4
PLOAD Example 3
The example demonstrating the use of the PLOAD1 entry consists of applying a direct linearly varying load to the three-bar structure shown inFigure 8-5.
Figure 8-5. Distributed Load on CBAR Elements The input file for this example is shown inListing 8-5.
$
GRID 4 3000. 0.0 0.0 123456
$
PBAR 1 1 100.0 100.0 100.0 100.0
$
MAT1 1 4.E4 .3
$
PLOAD1 1 11 FY LE 300. 23. 800. 45.
$ ENDDATA
Listing 8-5. PLOAD1 Loading
Note the that the starting location for the linearly varying load starts and ends at 0.3 m (300 mm) and 0.8 m (800 mm), respectively. This is the distance measured from end A. One of the most common mistakes is to make these distances 1.3 m and 1.8 m, respectively, while thinking that it is the actual distance measured in the basic coordinate system. It is not; it is the distance measured from end A of the loaded element-in this case, grid point 2.
When you use the PLOAD1 entry to define a distributed load, the CBARAO (CBAR Additional Output) entry is helpful. You use the CBARAO entry to request stress and force output at intermediate locations along the CBAR element. The CBARAO output for element 2 is shown in Figure 8-6.
Figure 8-6. CBARAO Output for the Center Element
The CBARAO entry is one of the best tools available for model checkout when you’re using linearly varying loads on the CBAR elements. The CBARO output lets you easily generate a shear and moment diagram as shown inFigure 8-7 below.
Figure 8-7. Shear and Moment Diagram Generated from CBARAO Output For CBEAM elements, you can use the PBEAM entry to directly request intermediate output along the element.
PLOAD1 Example 4
You can also use PLOAD1 to apply a concentrated load at an interior point of a CBAR element using fractional scaling.
1 2 3 4 5 6 7 8 9 10
PLOAD1 SID EID TYPE SCALE X1 P1 X2 P2
PLOAD1 36 52 FY FR 0.5 -1000.
PLOAD1 Example 5
Another feature of the PLOAD1 entry is the ability to apply projected loads. To apply a load as a projected load, you choose a scale (field 5) of “LEPR” for the actual length from end A or “FRPR”
for the fractional distance from end A. This example shows illustrates how you can use PLOAD1 to define a projected load using a snow load on a truss structure as shown inFigure 8-8.
Figure 8-8. Planar Truss Structure with a Snow Loading
The snow load on the truss is 500 N/m acting in the basic Y-direction. The input file containing the grid points and PLOAD1 entries is shown inListing 8-6.
$
GRID 8 3250. 1250. 0.0 345
GRID 9 6500. 2500. 0.0 345
GRID 11 9750. 1250. 0.0 345
$
Listing 8-6. Truss with Snow Loading
To see the effect of using projected forces, the resulting SPC forces are shown inFigure 8-9. The total reaction of the loads obtained by adding the SPC forces is 6500 N in the Y-direction. The total reaction load is equal to the projected length of 13 meters times the distributed load of 500 N/m. If the projected option had not been used, the applied load would have been 6964.2 N, which is the total length of the top members times the distributed load.
Figure 8-9. SPC Forces Due to the Snow Loading
Note: With PLOAD1, if you’re using a concentrated load and don’t need the additional features that the CBEAM element offers, you should use the CBAR element because of the way the equivalent end loads are generated.
• For the CBAR element, the equivalent end loads are generated explicitly.
• For the CBEAM element, the end loads are generated by numerical integration along the length of the element, which may not be as accurate as the method used for the CBAR element.
A linearly varying load applied with a PLOAD1 entry is accurate when used with either element.
PLOAD2
You can use the PLOAD2 entry to apply a normal uniform pressure load to CQUAD4, CSHEAR, or CTRIA3 2-D (surface) elements using element IDs. The positive direction of the loading is determined by the order of the grid points on the element connection entry, using the right hand rule. The software computes the magnitude and direction of the load from the value of the pressure and the coordinates of the connected grid points. NX Nastran applies the load to the connected grid points.
The PLOAD2 entry has two forms:
1 2 3 4 5 6 7 8 9 10
PLOAD2 SID P EID1 EID2 EID3 EID4 EID5 EID6
Alternate form:
PLOAD2 SID P EID1 “THRU” EID2
Field Contents
SID Load set identification number. (Integer > 0)
P Pressure value. (Real)
EIDi Element identification number. (Integer ≥ 0 or blank; for the “THRU”
option, EID1 < EID2)
The direction of the pressure is determined using the connected GRID points in the same right-hand rule sense as the PLOAD entry (i.e., with respect to the positive element z axis). In addition, you use the “THRU” option, all referenced elements must actually exist.
The PLOAD2 entry is similar to the PLOAD entry except that the PLOAD2 references the element ID instead of the grid points where the element is attached. The PLOAD2 is usually preferred over the PLOAD entry because it is easier to use. The PLOAD2 entry, similar to the PLOAD entry, is limited to pressure acting normal to the element surface. The direction of the pressure load is determined by the numbering sequence of the grid points on the connectivity entry (CQUAD4, CTRIA3, etc.).
See also
• “PLOAD2” in the NX Nastran Quick Reference Guide
PLOAD4
The PLOAD4 entry lets you create the most general pressure definition. You can use PLOAD4 to apply pressures and/or tractions to any of the two-dimensional elements and the surfaces of the three-dimensional elements. With PLOAD4, you can create a pressure load that’s either normal to the surface or that contains a traction (not normal to the surface) component. In addition, you can define a different value of pressure at each corner.
You can use PLOAD4 with CQUAD4, CQUAD8, CQUADR, CTRIA3, CTRIA6, and CTRIAR, CHEXA, CPENTA, and CTETRA elements. Since the surface hyperelastic elements CQUAD4, CQUAD8, CQUAD, CTRIA3, and CTRIA6 are plane strain elements, you can’t apply pressure loads on them.
The software automatically computes the magnitude and direction of the equivalent grid point forces using the isoparametric shape functions of the element to which the load has been applied.
Load intensities P1, P2, P3, (and P4) act at corner points G1, G2, G3, (and G4) for triangular (and quadrilateral) elements. The default direction of positive pressure for faces of solid elements is inward.
You can set the direction of the pressure can be set by one of two methods.
• By default, the software uses the element normal to determine the direction of the pressure.
For two-dimensional elements, the direction of positive pressure is in the direction of the outward normal as is determined by applying the right-hand rule to the ordering sequence of the grid points on the connectivity entry. For surfaces of solid elements, the direction of positive pressure is inward toward the center of the element. The face of the solid to which you are applying the pressure is determined by specifying the appropriate corner grid points.
• Alternatively, you can input the direction of the pressure defined by the PLOAD4 entry using an optional coordinate system and a vector entered on the continuation line. Using a local coordinate system, you can define a pressure acting at any angle to the surface.You can also apply loads acting parallel to the surface (tractions).
Another feature unique to the PLOAD4 entry is the ability to apply a nonuniform pressure. You can enter the pressure at each of the corner grid points to create a linearly varying pressure load.
If you enter the pressure for the first grid point only, the software assumes that the pressure is constant over the element.
The format of the PLOAD4 entry is:
1 2 3 4 5 6 7 8 9 10
PLOAD4 SID EID P1 P2 P3 P4 G1 G3 or G4
CID N1 N2 N3
The alternate format is
1 2 3 4 5 6 7 8 9 10
PLOAD4 SID EID1 P1 P2 P3 P4 “THRU” EID2
CID N1 N2 N3
P1, P2, P3, P4 Load per unit surface area (pressure) at the corners of the face of the element.
G1 Identification number of a grid point connected to a corner of the face. Required data for solid elements only.
G3
Identification number of a grid point connected to a corner diagonally opposite to G1 on the same face of a CHEXA or CPENTA element.
Required data for the quadrilateral faces of CHEXA and CPENTA elements only. G3 must be omitted for a triangular surface on a CPENTA element.
G4
Identification number of the CTETRA grid point located at the corner; this grid point may not reside on the face being loaded. This is required data and is used for CTETRA elements only.
Field Contents
CID Coordinate system identification number.
N1, N2, N3
Components of the vector measured in coordinate system defined by CID. Used to define the direction (but not the magnitude) of the load intensity.
If P2, P3, and P4 are blank fields, the load intensity is uniform and equal to P1; P4 is left blank for a triangular face. In addition, for pressure that acts normal to the face, the continuation entry is not used.
See also
• “PLOAD4” in the NX Nastran Quick Reference Guide PLOAD4 Example 1
Consider the curved plate shown inFigure 8-10. PLOAD4 entries are to be used to apply a normal pressure to each of the six CQUAD4 elements.
Figure 8-10. Pressure Loads on the CQUAD4s
This input file is given inListing 8-7. When using the PLOAD4 entry, you may specify a PLOAD4 entry for every element or use the alternate method of specifying several elements that have the same pressure. The alternate method is used for this example.
$ FILENAME - PRESS.DAT
Listing 8-7. CQUAD4 Elements with Pressure Loads
The resulting deflection is shown inFigure 8-11. The deflected shape is not what you would expect. Element 2 appears to bend up. The problem occurs because the normal for element 2 is reversed (see the ordering of the grid point IDs on the CQUAD4s). The pressure on this element is acting upward. This type of mistake is quite common.
Figure 8-11. Displacements Due to Pressure Load PLOAD4 Example 2
Specify the PLOAD4 entry for a uniform normal pressure load applied to the CHEXA solid element shown in the following figure:
1 2 3 4 5 6 7 8 9 10
PLOAD4 SID EID P1 P2 P3 P4 G1 G3 or G4
CID N1 N2 N3
PLOAD4 12 100 26.4 5 7
This load is selected in the Case Control Section with the command LOAD = 12. Leaving P2, P3, and P4 blank assigns a uniform pressure value of 26.4 lb/in2.