How to Apply Distributed Loads Correctly in FEA
Distributed loads — pressure, line load, body force — must be applied with an understanding of how the software distributes them across elements. This guide explains the correct approach.
1. The Engineering Task
Apply a distributed load — pressure, line load, or inertia — to the FE model in a way that correctly represents the spatial variation and total resultant of the physical load.
2. When to Use This Method
Distributed loads are used for pressure on surfaces, aerodynamic loads on skins, hydrostatic pressure on tanks, and inertia loads on all structures with mass. They are the most common load type in structural FEA.
3. What You Need Before Starting
- The spatial distribution of the load: uniform, linearly varying, arbitrary field
- The total resultant force (for checking)
- The surface or region over which the load acts
- For inertia loads: the mass distribution and acceleration magnitude
4. Step-by-Step Method
- Determine the load distribution: is it uniform pressure, linearly varying (hydrostatic), or a mapped field (CFD pressure)?
- For uniform pressure: apply directly to the surface. The software distributes it across element faces automatically
- For linearly varying pressure (hydrostatic): define the pressure at two points and let the software interpolate, or use an analytical field
- For mapped pressure fields (CFD-to-FEA): map the pressure from the CFD mesh to the FE mesh using a surface mapping tool. Check that the total resultant matches
- For inertia loads: apply acceleration or gravity as a body force. The software multiplies by element density and volume to produce nodal forces
- Verify: extract the total reaction force and compare with the expected resultant. For pressure on a flat surface, Resultant = Pressure × Area
- For non-flat surfaces, the resultant depends on the projected area, not the surface area. Check the sign convention — pressure may be defined as positive in compression or tension depending on the software
5. What to Check
- Total resultant: does the reaction force equal the expected total load?
- Direction: is the pressure applied in the correct direction (into the surface vs away from it)?
- Distribution: for varying loads, does the contour plot show the expected spatial variation?
- Units: is the pressure in consistent units (Pa, MPa, psi)?
6. How to Interpret the Result
A correctly applied distributed load produces a reaction force that matches the expected resultant and a stress distribution that is smooth over the loaded surface. If the reaction is wrong, the load magnitude, direction or area is incorrect. If the stress is patchy, the mesh may be too coarse to represent the load distribution.
7. Common Mistakes
- Applying pressure to the surface area instead of the projected area when the total force is specified
- Using the wrong sign convention for pressure — positive may mean compression or tension depending on the software
- Not checking the total resultant — a factor of 1000 error in pressure units is common
- Forgetting that inertia loads require correct density — a density error directly scales the inertia force
- Mapping CFD pressure without checking that the total resultant matches between CFD and FEA
8. Further Reading
See the Loads, Load Cases & Structural Environments Knowledge category for distributed load theory. See How to Transfer CFD Pressure Loads into an FE Model for CFD-to-FEA mapping.