What Is a Progressive Collapse Mitigation Strategy?

The design approaches

Tie forces (the indirect — the prescriptive integrity: the horizontal and the vertical ties binding the structure; the catenary action — the slabs and the beams spanning the lost column), the alternate path (the direct — the notional removal analyzed; the structure bridging the gap; the acceptance — the deformation limits), and the enhanced local resistance (the key elements — the columns designed for the specific threat; the hardening). The threat-independent (the notional removal — the design not tied to the specific blast or impact).

When mitigation is required

The occupancy categories (the UFC's — the military's required; the GSA's — the federal's; the IBC's — the high-occupancy's structural integrity), the risk assessment (the threat and the consequence — the owner's decision), and the consequence class (the Eurocode's — the CC3's systematic risk assessment). The new vs. the existing (the retrofit — the ties added, the hardening; the existing's constraints).

Structural detailing

The continuity (the reinforcement continuous — the beams' top and bottom steel; the slabs' integrity steel), the ductility (the rotation capacity — the connections developing the catenary; the detailing for the large deformations), and the redundancy (the multiple load paths — the structure's robustness). The analysis (the nonlinear static — the pushdown; the dynamic — the sudden-removal; the acceptance per the UFC/GSA). The mitigation's cost (the 1-5% premium — the integrity steel and the detailing) is the resilience bargain.

The bottom line

Progressive collapse mitigation (UFC 4-023-03, GSA) uses tie forces, alternate-path analysis, or enhanced local resistance — detail continuity and ductility for the notional element removal at a 1-5% premium.

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