{"id":791,"date":"2026-07-21T06:31:37","date_gmt":"2026-07-21T06:31:37","guid":{"rendered":"https:\/\/simutecra.com\/blog\/?p=791"},"modified":"2026-07-23T05:52:23","modified_gmt":"2026-07-23T05:52:23","slug":"how-to-select-boundary-conditions-fea","status":"publish","type":"post","link":"https:\/\/simutecra.com\/blog\/how-to-select-boundary-conditions-fea\/","title":{"rendered":"How to Select Boundary Conditions in FEA (With Examples)"},"content":{"rendered":"\n

The model looked correct. The geometry was accurate, the material properties were verified against the datasheet, and the mesh had passed a convergence study. But the maximum stress result was three times higher than the hand calculation predicted, and the deformation pattern made no physical sense, the beam appeared to stretch in a direction with no applied load. After an hour of troubleshooting, the engineer found the problem: one fixed support had been applied to the wrong face, locking a translational degree of freedom that should have been free. The stress result was an artifact of the boundary condition, not a property of the structure.<\/p>\n\n\n\n

Boundary conditions are the most consequential modeling decision in any FEA setup<\/strong>. Errors in material properties typically introduce errors of 10 to 30 percent. Errors in mesh density affect stress gradients locally. But wrong boundary conditions can change results by a factor of 2 to 10 or make them completely meaningless, overstiffening a structure, creating artificial reactions, locking stress states that do not exist in the physical system, or producing rigid-body motion that prevents the solver from converging at all.<\/p>\n\n\n\n

This article covers the complete framework for boundary condition selection: the physics of how constraints represent physical supports, the reference table of 11 BC types with their correct applications and failure modes, the symmetry and antisymmetry decision tree, the checklist for eliminating rigid-body motion without over-constraining, worked examples for four common structural scenarios, and the diagnostic workflow for identifying BC errors when results look wrong. The goal is a boundary condition strategy that represents what the structure actually experiences in service, not what is convenient to model.<\/p>\n\n\n\n

What Boundary Conditions Actually Represent<\/strong><\/h2>\n\n\n\n

A boundary condition in FEA is a mathematical statement that removes degrees of freedom from nodes<\/strong> at the model boundary. In a 3D solid model<\/a>, each node has six potential degrees of freedom: three translational (UX, UY, UZ) and three rotational (RX, RY, RZ). A support in the physical world constrains some subset of these DOFs. The job of the engineer is to identify which DOFs the physical support actually removes, and then constrain exactly those DOFs and no others.<\/p>\n\n\n\n

\"The<\/figure>\n\n\n\n

The fundamental principle is kinematic equivalence<\/strong>: the boundary conditions applied to the FEA model should produce the same kinematic behavior, the same pattern of allowed and prevented movement, as the physical support or interface they represent. A pinned joint allows rotation but prevents translation. A roller allows translation in one direction and rotation but prevents translation in the perpendicular direction. A welded connection prevents all relative movement between the connected members. Each of these has a direct FEA equivalent, and the mapping must be made deliberately.<\/p>\n\n\n\n

Two failure modes define the extremes of incorrect BC selection. Under-constraining<\/strong> leaves one or more rigid-body modes active: the structure can translate or rotate as a rigid body under the applied loads, which means the stiffness matrix is singular, the solver either fails to converge or produces arbitrarily large displacements, and no meaningful stress result is possible. Over-constraining<\/strong> removes DOFs that the physical1<\/a><\/sup> support does not actually prevent: the model is artificially stiffer than the real structure, reactions appear at nodes that carry no real load, and stresses are elevated or depressed by the artificial stiffness added by the phantom constraints.<\/p>\n\n\n\n

The Six Rigid Body Modes and Why Each Must Be Removed<\/strong><\/h3>\n\n\n\n

A free 3D solid body in space has six rigid-body modes<\/strong>: three translations (UX, UY, UZ) and three rotations (RX, RY, RZ). The FEA solver can only produce a unique displacement solution if all six are constrained by either physical supports, symmetry conditions, or inertia relief. Missing even one rigid-body mode makes the global stiffness matrix singular. In practice, the solver either returns an error (singular matrix, zero pivot detected) or, in poorly implemented codes, returns a displacement field that has no physical meaning.<\/p>\n\n\n\n

The challenge is that the six modes are not always obvious from the problem setup<\/strong>. A pressure vessel supported by two fixed rings appears fully constrained, but if the rings allow free rotation about the vessel axis, the rotation RZ about the axis of symmetry is unconstrained and a rigid-body mode remains. A bracket bolted to a wall is overconstrained if the bolt holes are modeled as fully fixed: in reality the bolts provide a stiff but not rigid connection, and modeling them as fixed introduces artificial bending resistance at the attachment plane that does not exist in the physical system.<\/p>\n\n\n\n

The 11 Boundary Condition Types: Reference Table<\/strong><\/h2>\n\n\n\n

The following table covers the 11 boundary condition types used in structural FEA<\/a>, with their degrees of freedom constrained, correct applications, common errors, and conditions where they should not be used.<\/p>\n\n\n\n

BC Type<\/strong><\/td>DOFs Constrained<\/strong><\/td>Typical Use Case<\/strong><\/td>Common Errors<\/strong><\/td>When to Avoid<\/strong><\/td><\/tr>
Fixed support (encastre)<\/td>All translational + rotational (UX, UY, UZ, RX, RY, RZ)<\/td>True built-in wall, welded base plate, press-fit mandrel<\/td>Over-constraining: real joints have compliance; fixed BC makes structure too stiff<\/td>Any joint with known rotation or compliance, use spring or partial constraint instead<\/td><\/tr>
Pinned \/ hinge support<\/td>All translational (UX, UY, UZ); rotations free<\/td>Pin joints, roller bearings in one plane, bolted shear plates<\/td>Forgetting to check if the model is under-constrained (missing rotation restraint about the pin axis)<\/td>Thin-shell models where moment transfer must be captured<\/td><\/tr>
Roller \/ sliding support<\/td>One translational DOF (normal to surface)<\/td>Symmetry planes, sliding guides, frictionless contact surfaces<\/td>Using roller on the wrong axis; creates rigid-body motion if applied to all supports of a body<\/td>Any joint with friction, use contact with friction coefficient instead<\/td><\/tr>
Symmetry BC<\/td>Normal displacement = 0; in-plane rotations = 0<\/td>Structures and loads that are geometrically and load-symmetric<\/td>Applying symmetry to a model with antisymmetric loads (buckling, out-of-plane loads)<\/td>Any load or geometry that breaks the symmetry plane assumed<\/td><\/tr>
Antisymmetry BC<\/td>Tangential displacements = 0; normal rotation = 0<\/td>Structures with antisymmetric loading (e.g., torsion on a symmetric shaft)<\/td>Confusing symmetry and antisymmetry conditions, they constrain opposite DOFs<\/td>Problems with mixed symmetric and antisymmetric load components (use superposition)<\/td><\/tr>
Remote displacement \/ coupling<\/td>Translates far-field displacement to a surface or set of nodes<\/td>Applying a prescribed displacement over a complex surface; gear load application<\/td>Over-stiff coupling: rigid coupling distributes load uniformly; may not represent actual load distribution<\/td>Flexible structures where load distribution depends on local stiffness, use pressure load instead<\/td><\/tr>
Displacement-controlled load<\/td>Prescribes UX\/UY\/UZ at a node or surface<\/td>Fatigue testing simulation, indentation, press-fit analysis<\/td>Forgetting that reaction force, not displacement, is the output of interest; confusing load-controlled and displacement-controlled results<\/td>When the applied force (not displacement) is the known boundary condition in service<\/td><\/tr>
Spring support (elastic foundation)<\/td>Connects node(s) to ground via spring stiffness<\/td>Soil foundation, vibration isolators, elastomeric mounts<\/td>Using wrong spring stiffness units (force\/length vs. force\/length\/area); single spring vs. distributed spring<\/td>Any case where a proper contact or material model can represent the support more accurately<\/td><\/tr>
Pressure load<\/td>Applies distributed force per unit area normal to a surface<\/td>Fluid pressure, contact load distribution, bearing pressure<\/td>Applying pressure in global coordinates instead of surface-normal; sign convention errors (positive = outward or inward)<\/td>Point or line loads, use force BCs or distributed edge loads instead<\/td><\/tr>
Inertia relief<\/td>Applies self-equilibrating body force to remove rigid-body modes without artificial supports<\/td>Free-flying structures (aircraft in flight, satellites, vehicles under acceleration)<\/td>Model is not in static equilibrium: inertia relief won’t converge if applied loads are not balanced by inertia<\/td>Any structure that has physical ground supports, use physical support BCs instead<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Fixed vs Pinned vs Roller: The Most Common Selection Error<\/strong><\/h2>\n\n\n\n

The most frequent BC selection mistake in structural FEA is the inappropriate use of the fixed support<\/strong>. A fixed support in FEA constrains all six DOFs at the applied nodes: three translations and three rotations. This represents a perfectly rigid, zero-compliance joint. The physical structures that actually behave this way are rare: a precision ground surface welded to a massive, infinitely stiff base plate, or a specimen gripped in a testing machine chuck with no slop. Nearly every other real support has some compliance, some rotation capacity, or both.<\/p>\n\n\n\n

The consequence of using a fixed support where a pinned or spring support is appropriate is artificial moment resistance<\/strong>. The fixed BC introduces a bending moment reaction that does not exist in the physical system. For beams in bending, this can change the moment diagram completely: a simply supported beam (pin-roller) has a parabolic moment distribution with maximum at midspan and zero at the supports. <\/p>\n\n\n\n

A fixed-fixed beam has the same midspan moment but also has end moments that reduce the peak by a factor of two. If the real beam is simply supported and the model applies fixed BCs at both ends, the predicted peak stress at midspan is half the actual value, a non-conservative error of 50 percent that a safety factor of 1.5 would not catch.<\/p>\n\n\n\n

When a Fixed Support is Actually Correct<\/strong><\/h3>\n\n\n\n

A fixed support is the correct choice when the physical joint satisfies two conditions: (1) the joint provides negligible rotation relative to the connected structure under the applied loads<\/strong>, and (2) the analyst has verified that any error from the fixed assumption is smaller than the required accuracy of the analysis<\/strong><\/p>\n\n\n\n

The most common cases where fixed supports are defensible: a shaft end pressed into a heavy housing where the housing compliance is small compared to the shaft compliance; a welded bracket on a thick plate where the plate bending stiffness is an order of magnitude greater than the bracket stiffness; and a specimen in a fatigue testing fixture where the grip compliance has been measured and confirmed to be below one percent of the specimen deformation. In all other cases, a pinned, spring, or contact BC more accurately represents the physical behavior<\/strong><\/p>\n\n\n\n

How to Verify Your Support Representation<\/strong><\/h3>\n\n\n\n

A practical method for checking whether the support type choice is materially affecting results is the support sensitivity study<\/strong>. Run the analysis with three support representations at the critical location: (1) fully fixed, (2) pinned (translations fixed, rotations free), and (3) spring support with an estimated spring stiffness from the supporting structure. If the stress result at the critical location differs by less than 5 percent between these three cases, the support representation is not governing the result and either choice is acceptable. If the difference exceeds 5 percent, the support stiffness must be accurately characterized, either by measurement, by modeling the supporting structure explicitly, or by analyzing both bounding cases and bracketing the result.<\/p>\n\n\n\n

Symmetry and Antisymmetry Boundary Conditions<\/strong><\/h2>\n\n\n\n

Symmetry and antisymmetry boundary conditions are among the most powerful tools in structural FEA because they allow an analyst to model only a fraction of the full structure<\/strong> while recovering the complete solution. A half-model with correct symmetry BCs gives the same stress and displacement results as the full model at the symmetric locations, at half the computational cost. A quarter-model gives the same results at one-quarter the cost. In complex 3D analyses where the full model would require millions of elements, symmetry reduction is not a convenience, it is what makes the analysis tractable.<\/p>\n\n\n\n

\"Symmetry<\/figure>\n\n\n\n

The condition for applying symmetry BCs is strict: both the geometry and the loading must be symmetric about the cut plane<\/strong>. If the geometry is symmetric but the load is not, symmetry BCs cannot be applied directly, the load must be decomposed into symmetric and antisymmetric components using superposition, each component analyzed on the half-model with its appropriate BC, and the results superposed to recover the full solution.<\/p>\n\n\n\n

Symmetry vs Antisymmetry: The DOF Rule<\/strong><\/h3>\n\n\n\n

The DOF assignment for symmetry and antisymmetry conditions is the source of most symmetry BC errors. The correct assignment follows directly from the physics of each case:<\/p>\n\n\n\n

Symmetry BC on the XY plane (cut plane normal to Z): <\/strong>UZ = 0 (no displacement normal to the cut plane), RX = 0, RY = 0 (no rotation about axes in the cut plane). The structure is free to move in X and Y and to rotate about Z.<\/p>\n\n\n\n

Antisymmetry BC on the XY plane: <\/strong>UX = 0, UY = 0 (no displacement in the cut plane), RZ = 0 (no rotation about the normal to the cut plane). The structure is free to move in Z and to rotate about X and Y.<\/p>\n\n\n\n

A common mnemonic: symmetry constrains the normal displacement and in-plane rotations; antisymmetry constrains the in-plane displacements and the normal rotation<\/strong>. These are exactly opposite. Applying symmetry BCs to an antisymmetric load case gives a physically impossible result, the deformation is forced to be symmetric about a plane where the physics demands antisymmetry, and the stress field is completely wrong.<\/p>\n\n\n\n

Symmetry BC Decision Table<\/strong><\/h3>\n\n\n\n
Condition<\/strong><\/td>Geometry Symmetric?<\/strong><\/td>Load Symmetric?<\/strong><\/td>BC to Apply<\/strong><\/td>Half-Model Valid?<\/strong><\/td><\/tr>
Full symmetry<\/td>Yes<\/td>Yes (same magnitude, direction, and distribution on both sides)<\/td>Symmetry BC on cut plane: normal disp = 0, in-plane rotations = 0<\/td>Yes, use half model, multiply reactions by 2<\/td><\/tr>
Antisymmetry<\/td>Yes<\/td>Antisymmetric (equal magnitude, opposite direction about the plane)<\/td>Antisymmetry BC: tangential displacements = 0, normal rotation = 0<\/td>Yes, use half model for each antisymmetric load case<\/td><\/tr>
Partial symmetry (load breaks symmetry)<\/td>Yes<\/td>No, load is not symmetric or antisymmetric about the plane<\/td>No symmetry BC applicable. Use superposition: decompose load into symmetric + antisymmetric components, solve each on half-model, superpose<\/td>Only with load decomposition, not directly<\/td><\/tr>
Cyclic symmetry<\/td>Yes (periodic geometry)<\/td>Yes (same load repeats every sector)<\/td>Cyclic symmetry BC: matching DOFs on sector boundaries with phase angle<\/td>Yes, one sector only, with cyclic BC on cut faces<\/td><\/tr>
No symmetry<\/td>No<\/td>Any<\/td>No symmetry BC, full model required<\/td>No<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Eliminating Rigid-Body Motion: The Constraint Checklist<\/strong><\/h2>\n\n\n\n

The systematic approach to eliminating rigid-body motion without over-constraining the model uses a degree-of-freedom audit<\/strong>: before running the analysis, count how many rigid-body modes remain active given the applied BCs, and verify that exactly six are removed in 3D (or three in 2D). This can be done by hand for simple geometries and by using the solver’s modal analysis capability (free-vibration analysis with zero stiffness boundary) for complex geometries.<\/p>\n\n\n\n

The 6-DOF Removal Checklist for 3D Models<\/strong><\/h3>\n\n\n\n
    \n
  1. List all supports and their constrained DOFs: <\/strong>For each support applied to the model, write down which of the six DOFs (UX, UY, UZ, RX, RY, RZ) it constrains. Do this before running the analysis.<\/li>\n\n\n\n
  2. Count total constrained DOFs: <\/strong>Sum the constrained DOFs across all supports. For a statically determinate support, this should equal exactly 6. For a statically indeterminate support (more than 6 constraints), verify that the redundant constraints represent real physical rigidity, not modeling convenience.<\/li>\n\n\n\n
  3. Identify which global directions are unconstrained: <\/strong>Check each of UX, UY, UZ, RX, RY, RZ independently. Any that are unconstrained by at least one support represent active rigid-body modes that will cause solver failure or meaningless results.<\/li>\n\n\n\n
  4. Check for degenerate constraints: <\/strong>Two supports that both constrain UX at the same end of a beam remove only one rigid-body mode (translation in X), not two. Spatially separated supports constraining the same DOF each contribute to moment equilibrium but remove only one translational rigid-body mode. Count modes removed, not constraints applied.<\/li>\n\n\n\n
  5. Verify rotational modes specifically: <\/strong>Rotational rigid-body modes are the most commonly missed. A long shaft constrained at both ends in UX, UY, UZ may still be free to rotate about its own axis (RZ if Z is the shaft axis) if no torque reaction is provided. Bearings that allow free rotation in one direction are modeled as constraining only the two transverse translations, not the axial rotation.<\/li>\n\n\n\n
  6. Run a modal analysis pre-check if in doubt: <\/strong>Set all material properties and BCs, then run a free-vibration (modal) analysis requesting the first 12 modes. Any modes with eigenfrequency near zero (less than 0.001 Hz in a structural model) are rigid-body modes. A correctly constrained 3D model has exactly zero near-zero modes. This check takes seconds and definitively identifies under-constrained or over-constrained models before the stress analysis is run.<\/li>\n<\/ol>\n\n\n\n
    <\/td>CRITICAL: The Zero-Frequency Modal Check<\/strong>
    <\/mark>Running a quick modal analysis before any static stress run is the fastest way to confirm the model is correctly constrained. In any major FEA solver (Abaqus, ANSYS, Nastran, Calculix), request 6 to 12 modes with no pre-load. A correctly constrained 3D model returns zero near-zero-frequency modes. If any modes appear at essentially zero frequency, they are rigid-body modes, the model is underconstrained and the stress analysis will give meaningless results. Fix the missing constraints, re-run the modal check, then proceed to the stress analysis. This takes two minutes and eliminates the most common class of FEA error<\/a>.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Worked Examples: BC Selection for Four Common Structural Problems<\/strong><\/h2>\n\n\n\n

    Example 1: Cantilever Bracket, Fixed vs Spring Support<\/strong><\/h3>\n\n\n\n

    A steel angle bracket is bolted to a 10mm thick steel plate with four M8 bolts. The bracket carries a vertical downward point load of 2000 N at its free end. The plate is itself bolted to a concrete wall. Determine the correct FEA boundary conditions.<\/p>\n\n\n\n

    Physical analysis: <\/strong>The bracket is bolted to the plate: the bolts provide both shear resistance and some moment resistance through bolt-head bearing, but not a rigid fixed connection. The plate is bolted to the wall: similar semi-rigid connection. A fully fixed BC at the bracket base overestimates the rotational stiffness of the bolt connection. A pinned BC underestimates it.<\/p>\n\n\n\n

    Recommended BC approach: <\/strong>Model the bracket with a spring support at the base plate interface. Estimate the rotational stiffness of the bolt group using beam-on-elastic-foundation theory: k_rot = n_bolts x F_bolt_tension x lever_arm^2, where F_bolt_tension is the bolt pretension and lever_arm is the distance from the bolt group centroid to the outermost bolt. Run the analysis with fixed and pinned BCs as bounding cases. If the peak stress at the bracket root differs by less than 15 percent between pinned and fixed, the exact support stiffness is not critical and either bound is acceptable.<\/p>\n\n\n\n

    Example 1: BC Sensitivity Check (Pseudo-code \/ Calculation Record)<\/strong>
    PROBLEM: Cantilever bracket, 2000 N vertical load at tip
    Support: 4x M8 bolts, bolt circle radius 30mm, plate thickness 10mm

    CASE A: Fully fixed support (RX=RY=RZ=UX=UY=UZ=0 at base face)
      Peak stress at bracket root: 185 MPa
      Reaction moment at base: 2000 N x 120mm arm = 240,000 N-mm

    CASE B: Pinned support (UX=UY=UZ=0, rotations free at base face)
      Peak stress at bracket root (from bending): 248 MPa
      Reaction moment at base: 0 (moment-free support)

    CASE C: Spring support (k_rot estimated from bolt group)
      k_rot = 4 bolts x 15,000 N pretension x (30mm)^2 = 54,000,000 N-mm\/rad
      Peak stress at bracket root: 212 MPa

    DIFFERENCE (A vs B): (248-185)\/185 = 34% -> support type is significant
    DECISION: Model bolt group explicitly or use spring BC with k_rot = 54 MN-mm\/rad
              Do NOT use simple fixed or pinned BC for this problem
    <\/code><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Example 2: Pressure Vessel, Symmetry BC Setup<\/strong><\/h3>\n\n\n\n

    A cylindrical pressure vessel with a hemispherical end cap is subjected to internal pressure of 5 MPa. The vessel has a 200mm internal diameter and a 10mm wall thickness. The geometry is axisymmetric and the load (internal pressure) is also axisymmetric. Set up the FEA BCs.<\/p>\n\n\n\n

    Physical analysis: <\/strong>Both geometry and loading are axisymmetric. The correct approach is either a full 3D model with no symmetry (expensive) or an axisymmetric 2D model exploiting the full rotational symmetry. Since the pressure is uniform around the circumference and along the axis, there is no preferred direction that breaks symmetry.<\/p>\n\n\n\n

    Recommended BC approach: <\/strong>Use a 2D axisymmetric element model. Apply symmetry BC at the vessel midplane: UZ = 0 (no axial displacement at the mid-length cut plane). Apply internal pressure as a distributed load on all internal faces. The axisymmetric element formulation inherently constrains the circumferential rigid-body rotation. The result is a quarter-model (one quarter of the meridional cross-section) that fully captures the stress field in both the cylindrical wall and the end cap junction, the critical location for hoop-to-meridional stress transition.<\/p>\n\n\n\n

    Verification: <\/strong>Thin-wall pressure vessel theory gives hoop stress = pR\/t = 5 x 100 \/ 10 = 50 MPa and meridional stress = pR\/2t = 25 MPa. The FEA result should match these values in the cylindrical section away from the end cap, and show the stress concentration at the cylinder-to-hemisphere junction. If the FEA hoop stress in the cylindrical section deviates from 50 MPa by more than 2 percent, the axisymmetric BC or the mesh is incorrect.<\/p>\n\n\n\n

    Example 3: Shaft in Bending, Avoiding Over-Constraint at Bearings<\/strong><\/h3>\n\n\n\n

    A steel shaft of 40mm diameter and 300mm length is supported by two deep-groove ball bearings, one at each end. A central gear load applies a 5000 N radial force at midspan. Set up the FEA BCs.<\/p>\n\n\n\n

    Physical analysis: <\/strong>Deep-groove ball bearings constrain radial displacement in both transverse directions (UX and UY if Z is the shaft axis) but allow free rotation about the shaft axis (free RZ) and allow small axial float (UZ is constrained by one bearing and free at the other in a typical two-bearing arrangement). They also allow free rotation about the transverse axes (RX and RY) to accommodate shaft bending.<\/p>\n\n\n\n

    Recommended BC approach: <\/strong>At bearing 1 (fixed bearing): constrain UX, UY, UZ. Leave RX, RY, RZ free. At bearing 2 (floating bearing): constrain UX, UY only. Leave UZ, RX, RY, RZ free. Apply the 5000 N radial load at the midspan node. This represents the actual kinematic behavior of the bearing arrangement and eliminates the artificial bending stiffness that a fixed BC at the bearing seats would introduce.<\/p>\n\n\n\n

    Common error: <\/strong>Applying fully fixed BCs at both bearing seats. This constrains RX and RY at both ends, making the shaft appear as a fixed-fixed beam in bending. The actual behavior is a simply supported beam (pin-roller in the radial planes). The fixed-fixed model predicts midspan bending stress that is half the correct value, a 50 percent non-conservative error.<\/p>\n\n\n\n

    Example 4: Free-Flying Structure, Inertia Relief<\/strong><\/h3>\n\n\n\n

    A satellite solar panel is analyzed for structural loads during launch. The panel is attached to the satellite body through a hinge mechanism, and the load is a distributed body force (launch acceleration of 8g applied to all elements). There are no external ground reactions in the launch configuration, the panel is in free flight relative to any ground reference. Set up the FEA BCs.<\/p>\n\n\n\n

    Physical analysis: <\/strong>There is no physical ground support to apply as a BC. The panel is accelerating with the entire satellite, and the structural loads arise from the difference in acceleration response between different parts of the panel. Applying an artificial fixed BC to any node would introduce a spurious reaction force that does not exist in the real loading condition and would completely invalidate the stress result.<\/p>\n\n\n\n

    Recommended BC approach: <\/strong>Apply inertia relief<\/strong>: the body force (8g acceleration) is applied to all elements as a body load, and the solver automatically computes a self-equilibrating set of reaction forces distributed across the structure such that the sum of external forces is zero. The structural deformation and stress arise from the differential loading that the inertia relief cannot balance, the true elastic loads. In Nastran this is activated with PARAM, INREL, -1 or -2; in Abaqus with *INERTIA RELIEF.<\/p>\n\n\n\n

    Verification check: <\/strong>After running inertia relief, verify that the sum of all reaction forces and moments reported by the solver is zero (within numerical tolerance). Any non-zero net reaction indicates that the applied load is not in equilibrium with the body forces, which means the inertia relief assumption is violated, the model has a net unbalanced force that requires a physical support, and the BC strategy must be revised.<\/p>\n\n\n\n

    Diagnosing Boundary Condition Errors in Results<\/strong><\/h2>\n\n\n\n

    When FEA results look wrong, stress concentrations at support nodes, implausible deformation patterns, reaction forces that do not balance the applied loads, or convergence failures, the boundary conditions are the first place to investigate. The following diagnostic workflow identifies the most common BC errors systematically.<\/p>\n\n\n\n

    \"Diagnosing<\/figure>\n\n\n\n

    Diagnostic Step 1: Check Reaction Force Equilibrium<\/strong><\/h3>\n\n\n\n

    After any static analysis<\/a>, the sum of all reaction forces and moments must equal the sum of all applied loads<\/strong>, within the numerical tolerance of the solver (typically less than 0.1 percent for well-conditioned problems). If they do not balance, a BC has been applied incorrectly: either the wrong nodes are constrained (causing a reaction at an unloaded location) or the constraint direction is wrong (creating a reaction in a direction that has no applied load component).<\/p>\n\n\n\n

    Request a complete reaction force summary from the solver at every support node. Verify each reaction against a hand calculation of expected magnitude and direction. An unexpected reaction at a node that should be free (for example, a moment reaction at a pin joint where no moment can be transferred) immediately identifies the incorrect BC.<\/p>\n\n\n\n

    Diagnostic Step 2: Inspect Deformation Shape<\/strong><\/h3>\n\n\n\n

    Exaggerated deformation plots (typically shown at 100x to 1000x actual displacement) reveal BC errors that stress contours obscure. The deformation pattern must be physically plausible given the applied loads and supports<\/strong><\/p>\n\n\n\n