UFC 4-023-03
25 January 2005
B-3.5
Cold-formed Steel Tie Forces.
As cold-formed steel construction is similar to wood frame, identical tie force
requirements are used. These values might be adjusted in the future, with additional
research and analysis,
B-4
ALTERNATE PATH METHOD.
The Alternate Path methodology provided in this UFC is a combination of two
existing approaches, presented in ITG 2001 and GSA 2003. There are some
modifications, most notably in the Deformation Limits for some of the materials, as
discussed later.
As the description of the AP method in the UFC is straightforward, only
significant topics or changes from existing guidance are discussed in the following
sections.
B-4.1
Removal of Load-Bearing Elements.
As discussed in the UFC, the AP method for MLOP and HLOP requires that
load-bearing elements be removed from every floor, after their plan location is identified.
The main motivation for this requirement is that DoD facilities could be attacked with
artillery, rockets, mortars, or rocket propelled grenades, all of which could damage a
structure at upper floors. Many buildings are more susceptible to progressive collapse if
the damage initiates at higher elevations (due to the reduced reserve capacity from the
fewer number of floors above) and this requirement will motivate the designer to
distribute additional strength and ductility to the upper levels.
For some multi-story structures, the results of the AP analyses will indicate
that similar responses are expected for element removal at a range of floors. If the
designer performs AP analyses for a lower floor and a higher floor and can show that
the intermediate floors will have similar results, the intermediate floors do not have to be
analyzed. However, this conclusion must be documented and provided as part of the
engineering analysis.
Finally, as shown in Figure 3-4, the column or wall is removed from the
structural model without degrading the capabilities of the joint at the upper end of the
member. Physically, this is unlikely to happen in an accidental or man-made event and
critics of this approach usually refer to the column deletion as the "immaculate removal."
However, it should be emphasized that the AP method is not intended to replicate an
actual event; the goal is to verify that the structure has satisfactory flexural resistance to
allow bridging across an area with localized damage.
B-13
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