How to Extract Bolt Loads from an FE Model

1. The Engineering Task

Extract the axial force, shear force and bending moment in each bolt from the FE model results, such that the loads can be used for bolt strength and joint separation checks.

2. When to Use This Method

Every bolted joint modelled in FEA where the bolt loads are needed for assessment. The extraction method depends on the bolt modelling approach — beam, solid or connector element.

3. What You Need Before Starting

• The bolt modelling approach used: beam element, solid bolt, connector element or spring
• The coordinate system for reporting forces (bolt axial direction vs global)
• The analysis steps: preload step and external load step

4. Step-by-Step Method

1. For beam bolt models: extract the beam axial force (S11 or N1), shear forces (S12, S13) and bending moments (M1, M2) at the beam midpoint or at the cross-section of interest
2. For solid bolt models: create a cross-section through the bolt shank and integrate the stress to obtain the axial force, shear force and bending moment. Most post-processors provide a "free-body cut" or "section force" tool for this
3. For connector elements (CBUSH, CFAST): extract the connector forces directly from the element output
4. Report the bolt loads in the bolt local coordinate system: axial (along the bolt axis), shear (perpendicular to the bolt axis)
5. Extract the loads at the critical load step — typically the maximum external load step, not the preload step
6. For the preload check: extract the bolt axial force after the external load is applied and confirm it has not dropped below the minimum required preload

5. What to Check

• Are the forces reported in the bolt local coordinate system, not the global system?
• For solid bolts: has the section force been integrated correctly? Check by comparing the sum of nodal forces with the expected reaction
• Has the preload been retained under the external load? If the bolt axial force has dropped significantly, the joint may have separated
• For multi-bolt joints: have all bolts been extracted, or just the one expected to be critical?

For beam bolt models, the beam axial force after preload application should equal the specified preload. If it does not, the preload was not applied correctly.

6. How to Interpret the Result

The extracted bolt loads are used as input to the bolt strength checks (shear, tension, interaction) and the joint separation check. The bolt with the highest combined load is the critical bolt. For a well-designed joint, the bolt loads should be consistent with the expected load distribution — end bolts carry more, inner bolts carry less.

7. Common Mistakes

• Extracting forces in the global coordinate system instead of the bolt local system
• Not checking that the preload is retained after the external load is applied
• Extracting only the preload step and not the external load step
• For solid bolts, using the peak stress instead of the integrated section force
• Only extracting the critical bolt and not all bolts — a different bolt may be critical under a different load case

8. Further Reading

See the Joints & Interfaces Knowledge category for fastener load distribution theory. See How to Model Bolts and Fasteners in FEA for the modelling approaches that produce these loads.