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 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 moments can
cause a plastic hinge to form in flexural 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.
Structure specific considerations for hinge locations for concentrated loads should be
See Figure C-5 and Table C-3 for plastic hinge properties. These properties
are adapted from the reinforced concrete member rotation requirements of the UFC. It
should be noted that reinforced concrete member allowable nonlinear capacity is based
on absolute rotation, independent of member section properties. Nonlinear hinge
definition should account for this.