Keywords: progressive collapse, steel structures, alternate path, moment connections.

This paper demonstrates the analytical modelling procedures and design requirements for steel framed regular building structures. The structural demand is computed using the Alternate Path (AP) method, for loads prescribed in the Unified Facilities Criteria - Design of Buildings to Resist Progressive Collapse, developed and updated by the U.S. Department of Defense. The AP method is one of the required approaches for progressive collapse resistant design of building structures that fall under Occupancy Category III or IV. The AP method is used to assess the ability of the structural frame to transfer loads carried by a notionally removed column to the rest of the structure, and ultimately to the foundation. This paper demonstrates the process of estimating the required external load on various portions of the structure and the computation of structural demand after the notional removal of a primary load-carrying element. The procedures demonstrated in this paper are limited to structures where linear elastic analysis is permitted. Importance of moment resisting connections at appropriate locations is discussed. It is concluded that moment resisting frames in building perimeter will not be sufficient to transfer loads in the event of loss of an internal column. Additional moment resisting frames are needed internally, in addition to the perimeter moment resisting frames.

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