UFC 4-023-03
25 January 2005
APPENDIX B
COMMENTARY
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INTRODUCTION.
The goal of these design requirements is to provide a rational and uniform
level of resistance to progressive or disproportionate collapse in new and existing DoD
structures. These requirements are threat-independent and are not intended to provide
resistance to the local damage that may initiate the progressive collapse. Structural
hardening of conventional structures to resist specific threats is covered in UFC UFC 4-
013-01 Structural Design to Resist Explosives Effects for New Buildings and UFC 4-
013-02 Structural Design to Resist Explosives Effects for Existing Buildings.
As suggested by Ellingwood and Leyendecker 1977, this UFC employs a
"combined approach", in which Indirect Design is used for "normal" buildings by
specifying minimum levels of strength, ductility, redundancy, and continuity. If the
building is "unusual" or the consequences of a progressive collapse event are severe,
then explicit consideration of the resistance to progressive collapse must be considered
through a Direct Design approach. This combined approach is thought to add minimal
expense while significantly improving the ability of structures to resist progressive
collapse. The British employ a similar combined approach in their Building Standards.
In this UFC, different levels of design requirements are specified, depending
upon the facility's required Level of Protection (LOP). Following the procedure laid out
in UFC 4-020-01 Security Engineering Facility Planning Manual, the Project Planning
team determines the LOP, which is a measure of the asset value of the facility and
reflects the number of occupants, mission criticality, as well as other factors. It is noted
that the LOP is a threat-independent quantity. A Very Low LOP (VLLOP) or Low LOP
(LLOP) corresponds to buildings that are "normal"; Medium LOP (MLOP) and High LOP
(HLOP) correspond to "unusual" structures. Likewise, the British have recently
proposed the addition of a risk/consequence approach to their Building Standards, to
specify a level of progressive collapse design that reflects the value and vulnerability of
each structure.
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DESIGN REQUIREMENTS.
For Indirect Design in VLLOP and LLOP structures, "Tie Force" requirements
are specified in this UFC. Tie Forces are the tensile capacities provided by the
structural elements and connections and are intended to keep the columns vertical and
to transfer loads from damaged portions of the structure to un-damaged sections. For
MLOP and HLOP structures, both Indirect Design (Tie Forces) and Direct Design
(Alternate Path method) are required. In the Alternate Path method, the designer must
verify that the structure can bridge over a removed structural element. Additional details
and background on Tie Forces and the Alternate Path method are provided later.
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