UFC 4-023-03
25 January 2005
3-2
ALTERNATE PATH METHOD.
The Alternate Path method is used in two situations: 1) when a vertical
structural element cannot provide the required tie strength, the designer may use the
AP method to determine if the structure can bridge over the deficient element after it
has been notionally removed, and 2) for structures that require Medium or High Levels
of Protection, the AP method must be applied for the removal of specific vertical load-
bearing elements which are prescribed in Section 3-2.3.
For MLOP and HLOP structures, perform and document a peer review
for all Alternate Path analyses. The peer reviewers must be independent and
qualified organizations who are approved beforehand by the building owner.
3-2.1
General.
This method follows the LRFD philosophy (ASCE 7-02) by employing load
factor combinations for extreme loading and resistance factors to define design
strengths.
It is recommended that 3-dimensional models be used to account for 3-
dimensional effects and to avoid overly conservative solutions. However, 2-dimensional
models may be used provided that the general response and 3-dimensional effects can
be adequately idealized.
There are three allowable analysis procedures: Linear Static, Nonlinear
Static, and Nonlinear Dynamic. These methods are summarized here and described in
detail in Sections 3-2.8 through 3-2.10.
Linear Static. The geometric formulation is based on small deformations
and the material is treated as linear elastic, with the exception of discrete
hinges that may be inserted, as described in Sections 3-2.7 and 3-2.8.
The full load is applied at one time to the structure, from which a vertical
load-bearing element has been removed.
Nonlinear Static: Both the material and geometry are treated as nonlinear.
A load history from zero load to the full factored load is applied to the
structure with a removed vertical load-bearing element.
Nonlinear Dynamic: The material and geometry are treated as nonlinear.
A dynamic analysis is performed by instantaneously removing a vertical
load-bearing element from the fully loaded structure and analyzing the
resulting motion.
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