Boundary Load
Use a Boundary Load to apply tractions or pressure on boundaries.
Shell Properties

Force
Select the Load typeForce per unit area, Pressure, Total force, Resultant, or for 2D components, Force per unit length. Then enter values or expressions based on the selection and the space dimension.
For Force per unit area, the traction components are given explicitly.
For Total force, COMSOL Multiphysics divides the total force by the area of the boundaries where the load is active. Then the force is applied in the same way as for a Force per unit area. When working with curved boundaries or local coordinate systems, use this option carefully, as the result is not always intuitive.
For Pressure, a scalar input is given, and the orientation of the load is given by the normal to the boundary. The pressure is positive when directed toward the solid. In a geometrically nonlinear analysis, the current surface normal and area are used.
For Resultant, enter the Force and Moment with respect to a point. Select the Application point defined usingCentroid, Point, or Coordinates. For Centroid, the coordinates of the application point is the centroid of the selected boundaries. For Point, select a geometrical point in the section Application Point. For Coordinates, enter the global coordinates of the Application point, xa.
When using the Resultant option, it is formally possible to select a nonplanar boundary, but the traction distribution may then be unpredictable.
After selecting a Load type, the Load list normally only contains User defined. When combining with another physics interface that can provide this type of load, it is also possible to choose a predefined load from this list.
FA
N/m2
FL
Ftot
F, M
Traction Field
This section is only shown when Resultant has been selected as Load type. The distribution of the tractions over the boundaries is controlled by the settings here.
Select a Traction distributionBeam or User defined.
When Beam is selected, the traction field approximately matches the stress distribution over a beam cross section.
For User Defined, you can write expressions for the traction distributions. Enter expressions for the six dimensionless vector-valued traction distribution functions, q1, q2, q3, q4, q5, and q6. Usually, the local coordinates of the loaded region would be used for this purpose, but there is no limitation on the form of the functions. The built-in variables for the local coordinates are named <physics_tag>.<load_tag>.x2 and <phys_tag>.<load_tag>.x3, for example solid.bndl1.x2. The default value is the distribution that is used when the option Beam is selected.
The local coordinates of the boundary, x2 and x3, are defined in the following way:
x2 is the axis around which the principal area moment of inertia is larger.
When User Defined is selected, you can also select a Weight functionNone, Circular, or User Defined. This function acts as a multiplier to the traction distribution functions. When None is selected, the weight function w = 1. For Circular, enter a radius rw of a circle, outside of which no load is applied (w = 0). Inside the circle, w = 1. For User Defined, enter a weighting expression. Usually, the built-in local coordinates of the loaded region would be used for this purpose, but there is no limitation on the form of the function. A time- or parameter-dependent function can, for example, be used to model a moving load.
The traction distribution functions have the property that the total traction field is
where the coefficients ci are chosen so that the given force and moment resultants are obtained.
Symmetry
This section is only shown when Resultant has been selected as Load type. If the resultant force is applied in a symmetry or antisymmetry plane, this fact needs to be taken into account when creating the corresponding traction field.
Select a symmetry type — None, Symmetry, or Antisymmetry. When one of the types of symmetry is chosen, specify the symmetry plane by entering a Normal Vector, nsym, and a Point, xsym, located in the plane.
When specifying a resultant load in a symmetry plane, you still provide the full value of force and moment, as if there were no symmetry in the model. Any components of the given load that do not fulfill the selected type of symmetry will be discarded.
Linear Buckling
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.
If you are performing a linear buckling analysis with a combination of live and dead loads, select the Treat as dead load check box to indicate that the load contributions from this node are constant. The default is that a load is proportional to the load factor.
 
You can add the Phase subnode to specify the phase of this load in a frequency domain analysis.
You can assign this load to a load group. See Load Cases in the Structural Mechanics Modeling chapter.
Location in User Interface
Context Menus
Ribbon
Physics tab with Solid Mechanics or Multibody Dynamics selected:
Physics tab with Layered Shell selected:
Physics tab with Thin Layer selected: