Analysis Overrides [FEA]
An Analysis Override changes a property of an object for one analysis case only, without editing the object's base definition. The same model can therefore be analysed with different properties in different cases — cracked sections for seismic, uncracked for service; a bearing fixed in one case and free in another; a support removed to check a jacking condition.
Overrides change the stiffness matrix (or whatever property they target) for their case. They generate no load and add no mass.
Analysis Case: The case the override applies to. This is the parameter that scopes everything else; an override with no case assigned applies nowhere.
Engineering influence. Overrides are how one model serves several code checks with different stiffness assumptions. Assigning an override to the wrong case silently applies a cracked-section or released-bearing assumption to an analysis that should have used the base properties, and nothing in the results announces it.
Nodes
Object: The node whose property is overridden.
Property: The node property being changed — typically one of the six DOF restraint or spring values.
Value: The value used in this case.
Engineering influence. This is how a support condition is changed per case: releasing a restraint to model a jacking operation or a bearing replacement, fixing a normally-free direction to represent a temporary condition, or substituting a different spring stiffness for a case where the soil behaves differently. Because it changes restraint, it changes the whole load path: releasing a support redistributes its reaction to the others and can, if over-used, leave the structure a mechanism for that case.
The value follows the node's own sign convention — 0 free, negative fixed, positive a spring stiffness — so an override entered as 1 to "fix" a direction produces an almost-free spring instead.
Springs
Object: The spring whose property is overridden.
Property: The spring property being changed — usually a directional stiffness.
Value: The value used in this case.
Engineering influence. The standard use is substituting an isolator's or bearing's stiffness per case: the small-amplitude stiffness for a service or wind case, and the effective secant stiffness for a seismic one. Because the isolated period depends directly on this, an override here can change the dynamic response of the whole structure. Setting a stiffness to zero removes the connection for that case, which may leave the model unstable.
Lines
Object: The line element whose property is overridden.
Property: The property being changed — section area, inertia, torsion constant, modulus, or one of the corresponding factors.
Value: The value used in this case.
Engineering influence. This is the most common override in practice: applying cracked-section properties to concrete members for a seismic or long-term case while keeping gross properties for service checks. Reducing a member's inertia softens it, lengthens periods, and sheds force into stiffer adjacent members — so softening a pier reduces its own moment while increasing demand on its neighbours. That redistribution means an override is not conservative in a single direction, and the whole structure has to be reviewed after applying one.
Overriding torsion constant is the other frequent case, since uncracked J grossly overstates the torsional stiffness a cracked concrete member delivers.
Surfaces
Object: The shell whose property is overridden.
Property: The property being changed — thickness, modulus, or one of the membrane/bending modifiers.
Value: The value used in this case.
Engineering influence. Used to apply cracked-section stiffness to a deck or wall for a specific case. Because the shell modifiers act in local axes, an override applied across a mesh with inconsistent local axes softens different physical directions on different elements.
Volumes
Object: The solid whose property is overridden.
Property: The property being changed.
Value: The value used in this case.
Custom
Object: Any object in the model, for overriding a property not covered by the typed tabs above.
Engineering influence. The general-purpose escape hatch. It is powerful and correspondingly easy to misuse: there is no type checking that the property named is meaningful for the object selected, and a property name that does not resolve produces an override that silently does nothing.
Several columns on this object read blank in the sheet because their values are held per-tab; the Object/Property/Value trio you edit is the one belonging to the active tab. Check you are on the right tab for the object type you intend to override.
Verification
Run the case with and without the override and compare. If nothing changes, the override is not being applied — the usual causes are an unassigned case, a property name that does not resolve, or the wrong tab.
Check periods and mode shapes after a stiffness override; a cracked-section override should measurably lengthen periods.
Check reactions after a restraint override; releasing a support must redistribute its reaction to the others while the total stays the same.
Confirm the base model is unchanged by reviewing the object's own spreadsheet row.
List all overrides before signing off a case, and confirm each one is intended for that case.
Where an override softens a member, check the members around it for the increased demand.
Common mistakes
Forgetting an override exists. The object sheets show base values, so a case can behave differently from what the model appears to say, indefinitely.
Assigning the override to the wrong analysis case, applying cracked properties to a service check or gross properties to a seismic one.
Using the node sign convention wrongly — entering 1 to fix a direction, which creates an almost-free spring.
A property name that does not resolve, producing an override that does nothing without warning.
Editing on the wrong tab for the object type being overridden.
Softening a member and reviewing only that member, missing the increased demand redistributed into its neighbours.
Overriding a stiffness to zero and destabilizing the model for that case.
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