Acceptance Criteria for the Nonlinear Alternative Load Path Analysis of Steel and Reinforced Concrete Frame Structures: Fid-Bau Portal

Acceptance Criteria for the Nonlinear Alternative Load Path Analysis of Steel and Reinforced Concrete Frame Structures

Weigand, J. M. / Bao, Y. / Main, J. A.

Alternative load path analysis is the primary approach for evaluating the potential for disproportionate collapse in structural design. In this approach, individual load-bearing elements are notionally removed from a structure, and the remaining structure is required to sustain the applicable gravity loads without collapse. Column loss in steel and reinforced concrete frame structures can result in large vertical deflections that subject the beams and their connections to significant axial deformations in addition to large rotations. Failure of members and connections in alternative load path analysis is evaluated by comparing the plastic rotations of these components to acceptance criteria, defined as rotation limits, based largely on data from seismic tests. Axial demands on members and connections, which are important in column loss scenarios, were not relevant in this seismic testing, and thus the corresponding rotation limits may not be appropriate for column loss. This paper compares current acceptance criteria for alternative load path analysis with experimental data reported in the literature under column loss scenarios for steel gravity frames with single-plate shear connections and for reinforced concrete moment frames. Significant variability is observed in the level of conservatism of these acceptance criteria, and factors contributing to this variability are discussed. A new approach for defining acceptance criteria is summarized, which provides improved risk-consistency by directly accounting for the interaction of axial and rotational demands on the connections under column loss scenarios.