UFC 4-023-03
25 January 2005
B-4.2
Factored Loads for Alternate Path Method.
B-4.2.1
Dynamic Load Case.
The load case in Section 3-2.4.1 is taken from ASCE 7-02, Section C2.5,
"Load Combinations for Extraordinary Events." ASCE 7-02 states that "For checking a
structure to determine its residual load carrying capacity following occurrence of a
damaging extraordinary event, selected load-bearing elements should be notionally
removed and the capacity of the remaining structure evaluated" using the load
combination in Section 3-2.4.1.
B-4.2.2
Static Load Case.
The factor of 2.0 acting on the Dead, Live and Snow Loads in Section 3-2.4.2
is used to account for the localized inertial effects due to the loss of vertical support over
a short, finite period of time. The factor 2.0 is used in GSA 2003 and has been
validated as conservative through a number of numerical simulations of progressive
collapse.
The increased loads are only applied to the bays adjacent to and above the
removed load-bearing element, as detailed in Section 3-2.4.2. The increased loads are
limited to these areas as they are most likely to experience significant inertial loading,
whereas the rest of the structural will experience much smaller motion.
B-4.2.3
Loads Associated with Failed Elements.
When an element fails, the element's load must be transferred to the rest of
the structural model. For Nonlinear and Linear Static analysis, the loads applied above
the removed column or wall are doubled to account for inertial effects that can't be
represented in a static solution. As these loads are already increased by a factor of 2.0,
they are redistributed, without increase, to the structure below, over an area that is
equal to or smaller than the loaded area that the failed element was supporting.
For Nonlinear Dynamic analysis, the entire structure is loaded as detailed in
Section 3-2.4.1, without the factor of 2 that is used in Static analysis for the structural
areas above the removed element. However, for Nonlinear Dynamic analysis, the loads
from a failed element are doubled before being applied to the area below, to grossly
account for the effect of structural elements falling upon other elements. The load is
applied instantaneously, as is the removal of the structural element, which may induce
significant dynamic response in the structure. The choice of 2.0 is based on
engineering judgment. While the peak loads in a perfect impact will be much higher
than 2.0, it is unlikely that elements will fail completely and fall intact upon the lower
level. It is more likely that the element will be partially restrained, e.g. with rebar that is
still embedded in the concrete, shear connectors between floor systems and beams,
non-load-bearing walls, and other non-structural elements.
B-14
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