25 January 2005
approaches employ the AP method, but with specific modifications that are tailored for
the typical threats and structures considered by each organization. This UFC replaces
the previous DoD guidance (ITG 2001).
SUMMARY OF PROGRESSIVE COLLAPSE DESIGN PROCEDURE.
The design requirements presented in this UFC were developed such that
two structural response modes are available to provide different levels of resistance to
progressive collapse. The first level of progressive collapse design employs Tie Forces,
which are based on a "catenary" response of the structure. The second level employs
the Alternate Path method, in which the structural mode is "flexural", as the building
must bridge across a removed element. A significant portion of the design guidelines
and criteria in this UFC are based on the British Standards approach, as discussed in
more detail in Appendix B.
For existing and new construction, the level of progressive collapse design
for a structure is correlated to the Level of Protection (LOP) that the Project Planning
Team develops and provides to the designer. At the lower LOPs [Very Low Level of
Protection (VLLOP) and Low Level of Protection (LLOP)], only Indirect Design is
employed, by specifying the required levels of Tie Forces. However, in the case that an
adequate Tie Force cannot be developed in a vertical structural element, then the
Alternate Path method is applied to verify that the structure can bridge over the deficient
element. For Medium Level of Protection (MLOP) and High Level of Protection (HLOP),
the Alternate Path method is also applied to verify satisfactory flexural resistance in
addition to the catenary resistance provided by the Tie Forces. Finally, for MLOP and
HLOP, additional ductility requirements are specified for ground floor perimeter vertical
load-bearing elements, to improve the resistance to progressive collapse
It is expected that the majority of new and existing DoD facilities will be
assigned VLLOP or LLOP ratings and the design to resist progressive collapse will
require the application of only the Tie Force criteria. In general, these requirements will
be met without much difficulty and can usually be satisfied by application of good
connection detailing practice.
Inspection requirements to verify conformance with this UFC are provided in
Appendix G. These inspection requirements are modifications to the provisions of the
2003 International Building Code (2003 IBC), which cover construction documents,
structural tests and special inspections for buildings that have been designed to resist
SECURITY ENGINEERING UFC SERIES.
This UFC is one of a series of security engineering Unified Facilities Criteria
that cover minimum standards, planning, preliminary design, and detailed design for
security and antiterrorism. The manuals in this series are designed to be used