UFC 4-023-03
25 January 2005
3-2.2
Load and Resistance Factor Design for Alternate Path Method.
Following the LRFD approach, the Design Strength provided by a member
and its connections to other members in terms of flexure, axial load, shear and torsion is
taken as the product of the strength reduction factor Φ and the nominal strength Rn
calculated in accordance with the requirements and assumptions of applicable material
specific codes, including the application of a material over-strength factor Ω as
appropriate. Note that for wood construction, a time effect factor λ is also included; see
Section 7.1.
Design Strength = Φ Rn ≥ Required Strength
Equation (3-2)
where
Φ
= Strength reduction factor
Rn
= Nominal strength, calculated with the appropriate
material specific code, including over-strength
factors Ω where applicable.
Per the LRFD approach, the Design Strength must be greater than the Required
Strength which is the internal force created by the factored loads.
For the purposes of this UFC, all strength reduction factors Φ are taken
from the material specific design code, as defined in Chapters 4 through 8.
3-2.3
Removal of Load-Bearing Elements for the Alternate Path Method.
Load-bearing elements are removed from the AP model in two cases: 1) in
structures with elements that cannot provide the vertical required tie strength, the
deficient element must be removed, and 2) for MLOP and HLOP structures, the location
and size of the removed element are specified to verify that the structure has adequate
flexural resistance to bridge over the missing element. The details of the type, location,
and size of the removed load-bearing elements are described in the following sub-
paragraphs.
3-2.3.1
Structures With Deficient Vertical Tie Force Capacity.
The definition of the size and type of load-bearing element that must be
removed is dependent upon the construction material and is presented in each of the
material-specific chapters (Chapters 4 to 8).
3-2.3.2
MLOP and HLOP Framed and Flat Plate Structures.
For structures with Medium and High Levels of Protection, multiple AP
analyses are performed, with the load-bearing elements removed from the plan
locations specified in the following sub-sections.
3-5
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