25 January 2005
Progressive Collapse Load Combination.
Per the requirements of the UFC, the following load combination is required
for alternate path analysis:
(1.2D + 0.5*L + 0.2*W) where D = DL + SDL + CL
This load combination is doubled per the UFC over the bays adjacent to the removed
column on all floors for the AP analysis.
For the nonlinear alternate load path method, plastic hinges are allowed to
form along the members. These hinges are based on maximum moment values
calculated using phi factors and over-strength factors per the UFC. However, only
flexural moments can cause a plastic hinge to form in members, and only the axial-
moment interaction (PMM) can cause a plastic hinge to form in a column. Any shear or
torsion values that would cause a hinge to form would result in an immediate failure.
Theoretically hinges can occur anywhere along the beam. However, hinges
are allowed to occur at the ends of each member and at the midspan of the flexural
members. This simplifies the model by placing hinges in the most probable locations.
See Figure D-6 and Table D-3 for plastic hinge properties. These properties
are adapted from the steel member ductility requirements of the UFC.
Figure D-6 Nonlinear Hinge Definition
Μrel = hinge capacity (defined relative to member's yield moment)