Loads & Pressure Faces
How forces, pressures, and gravity are applied in FEM — and where mistakes happen.
Loads are the external forces acting on the structure. The solver distributes them across the mesh and computes the resulting stress and deformation field.
Types of loads in Dr.Q
Force (nodal or surface)
A concentrated force applied to a surface, edge, or point. Dr.Q distributes the total force evenly across all nodes in the selection.
- Surface force: most common — models a clamping jaw, a contact patch, a hand load
- Point force: applied to a single node — creates a stress singularity (artificial spike), avoid where possible
Pressure
A distributed load per unit area, always acting normal to the surface. Positive pressure pushes inward; negative pressure (suction) pulls outward.
Pressure is the right choice when the intensity is known (e.g., 10 bar internal pressure in a vessel). Force is the right choice when the total load is known (e.g., a 500 N bolt preload).
Pressure automatically scales with the element area. If you remesh with finer elements, the total force stays the same. With nodal forces, you'd have to reapply the load manually.
Gravity / self-weight
Applies a body force to the entire model based on material density and gravitational acceleration (9.81 m/s²). Requires that density is defined in the material properties.
Don't forget self-weight for heavy structures or when static equilibrium checks matter.
Bearing load
A sinusoidal pressure distribution over a cylindrical surface — physically correct for pin or shaft loads where contact only occurs on the loaded side (not the full circumference).
Which face does pressure act on?
Pressure always acts on the surface normal direction. In Dr.Q, you select the face and the software infers the outward normal automatically. Check the preview arrow to confirm the load direction is correct before solving.
Load combinations
Real structures experience multiple load cases. Run separate analyses for each combination — bending alone, torsion alone, combined — and superpose results if you are in the linear regime.
Superposition is only valid for linear analysis. For nonlinear problems (large deformation, contact, plasticity), you must apply all loads simultaneously in a single analysis.
Common mistakes
- Wrong units: Dr.Q uses N and mm by default. A load entered as 1000 N when you meant 1000 kN is a factor-of-1000 error.
- Force on a point: Causes a singularity at the node. Distribute the force over a face instead.
- Forgetting self-weight: Especially critical for large castings and weldments where the structure's own weight is a significant fraction of total load.
- Wrong pressure direction: A suction load modelled as compression, or vice versa, gives completely wrong results. Always check the load preview arrow.