Stiffness components keyed by their exact EXPRESS attribute name
(TranslationalStiffnessX, RotationalStiffnessZ, …).
The value is a number when the file gives a stiffness magnitude, and a
boolean when it uses the select's IfcBoolean branch — which is how a
fully fixed or fully free degree of freedom is written (.T. = fixed).
Collapsing the boolean branch to a number would report a rigid support as
stiffness 1.
OptionalnameCanonical EXPRESS type name, e.g. IfcBoundaryNodeCondition.
An
IfcBoundaryConditionleaf reduced to its named stiffness components.