UFC 4-023-03
25 January 2005
2-1.3
Medium and High Level of Protection Design Requirement.
For the purposes of this UFC, the Medium and High Levels of Protection are
combined. Three requirements must be satisfied: tie forces, alternate path, and
additional ductility requirements. The requirements are illustrated in the flowchart in
Figure 2-3.
2-1.3.1
Tie Force Requirements for MLOP and HLOP.
For MLOP and HLOP structures, the designer must provide adequate
horizontal and vertical tie force capacities. However, if a structural member cannot
provide the required vertical tie force capacity, the designer must either re-design the
member or use the Alternate Path method to prove that the structure can bridge over
the element when it is removed. For elements with inadequate horizontal tie force
capacity, the Alternate Path method cannot be used. In this case, the designer must re-
design the element in the case of new construction or retrofit the element for existing
construction.
2-1.3.2
Alternate Path Requirements for MLOP and HLOP.
The structure must be able to bridge over specific vertical load-bearing
elements that are notionally removed from the structure. The plan locations of the
removed vertical load-bearing elements include, as a minimum, the center of the short
side, the center of the long side, and the building corner, as discussed in Section 3-2.3.
In addition, vertical load-bearing elements are removed wherever there is a significant
variation or discontinuity in the structural geometry, such as re-entrant corners and
abrupt changes in bay sizes.
For each plan location of a removed element, an Alternate Path analysis is
performed for every floor, one at a time; thus, if there are three plan locations and eight
stories, twenty four AP analyses must be performed. If bridging cannot be
demonstrated for one of the removed load-bearing elements, the structure must be re-
designed or retrofitted to increase the bridging capacity. Note that the structural re-
design or retrofit is not applied to just the deficient element, i.e., if a structure cannot be
shown to bridge over a removed typical column at the center of the long side, the
engineer must develop suitable or similar re-designs or retrofits for that column and
other similar columns. For instance, a re-design might consist of additional positive
moment rebar at a reinforced concrete beam-column joint; this new design must be
applied to other columns on that external column line.
For MLOP and HLOP structures, the designer must perform and
document a peer review for all Alternate Path analyses. The reviewer must be an
independent organization with demonstrated experience performing progressive
collapse design.
2-1.3.3
Additional Ductility Requirements for MLOP and HLOP.
2-4
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