the building would collapse under impact. But a detailed finite element model is
necessary to determine if localized collapse needs to be evaluated.
If the model is also used to compute instructure shock spectra to evaluate the
functionality of SSCs supported by the target structure, the structural components that
support these SSCs should be explicitly represented by separate mass and stiffness
elements. Also, the discretization of mass and stiffness should be refined enough that
the high frequency content of the impact force is appropriately reflected in the generated
C.220.127.116.11 Structural Evaluation Criteria.
Even if one or more structural components fail (i.e., the computed ductility ratio is more
than the permissible value), the building or the structure as a whole may not collapse
unless a mechanism (i.e., no resistance to additional load) is formed due to the failure of
these components. To determine if collapse would occur, a nonlinear analysis (having
both geometric and material nonlinearity) can be performed in which the stiffness of the
structural components shall be set approximately to zero as soon as the deformation
reaches the ductility limit. If the structure can withstand the impact force without forming
a mechanism, no structural collapse will occur.
For certain target structures, their deformation, displacement, or strain may be required
to remain within some specified limits well below those associated with the permissible
ductility ratios given in Section 18.104.22.168. Examples of such structures are (1) concrete
structures or vaults that must perform a confinement function and therefore are allowed
only infinitesimal cracks, and (2) structures that must not deform excessively to avoid
pounding on adjacent safety-related systems or structures. Permissible deformation or
strain limits for these structures shall be based on their performance requirements.