| Progressive (or disproportionate) collapse is an effect of localized failure propagation. During a catastrophic event, a local member or section exhibits plastic behavioral characteristics due to disproportionate stresses and displacements beyond its strength and serviceability. The energy applied to the system is beyond that of the member's maximum design allowable, and therefore failure is imminent. Upon failure, the loads are transferred to adjacent members and sections. If these excessive loads are beyond the strength of the adjacent members, then the collapse propagates further.;Security/blast engineering to mitigate progressive collapse emphasizes prescriptive design rules and deterrence. Direct design methods must be utilized to mitigate progressive collapse, if all else fails. Since blast durations are comparatively minute and transient compared to conventional loading, direct design methods utilize multiple path redundancies and dynamic load factoring to quarantine and arrest propagation.;Once deterrence fails, the structural design is the last line of defense when the event is imminent. Select direct design methods are discussed for the purposes of security engineering (See Section E.0 Design Code). These design methods assume complete failure of local members and are designed to arrest progressive collapse. Assaults against a structure, in this matter are cataclysmic—this is the game changer. However, the design codes are not exclusive to blast mitigation; the methods can be used for natural disaster resilience and select robustness. |
