UFC 4-023-03
25 January 2005
of the structure and not to direct blast effects. The recent escalation of the domestic
and international terrorist threat has increased the probability that other US government
structures will be attacked with explosives or other violent means.
1-3.2
Hardening of Structures to Resist Initial Damage
As the initiating event is unknown, the requirements in this UFC are not
intended to directly limit or eliminate the initial damage. This is consistent with UFC 4-
010-01, which applies where there is a known risk of terrorist attack, but no specific
terrorist threat is defined; in this case, the goal is to reduce the risk of mass casualties
in the event of an attack. For cases where specific explosive threats against a building
have been identified, design guidelines for specific blast hardening can be found in 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. Even if a
structure is designed to resist an identified or assumed threat, the progressive
collapse requirements of this UFC will still apply.
1-3.3
Design Approaches.
ASCE 7-02 defines two general approaches for reducing the possibility of
progressive collapse: Direct Design and Indirect Design.
1-3.3.1
Direct Design Approaches.
Direct Design approaches include "explicit consideration of resistance to
progressive collapse during the design process..." These include: 1) the Alternate
Path (AP) method, which requires that the structure be capable of bridging over a
missing structural element, with the resulting extent of damage being localized, and 2)
the Specific Local Resistance (SLR) method, which requires that the building, or parts of
the building, provide sufficient strength to resist a specific load or threat.
1-3.3.2
Indirect Design Approaches.
With Indirect Design, resistance to progressive collapse is considered
implicitly "through the provision of minimum levels of strength, continuity and ductility".
The commentary in ASCE 7-02 goes on to present general design guidelines and
suggestions for improving structural integrity. These include: 1) good plan layout, 2)
integrated system of ties, 3) returns on walls, 4) changing span directions of floor slabs,
5) load-bearing interior partitions, 6) catenary action of the floor slab, 7) beam action of
the walls, 8) redundant structural systems, 9) ductile detailing, 10) additional
reinforcement for blast and load reversal, if the designer must consider explosive loads,
and 11) compartmentalized construction. However, no quantitative requirements for
either direct or indirect design to resist progressive collapse are provided in ASCE 7-02.
1-2
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