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🏗️ Progressive Collapse Explorer
Remove one column and the beam above it must suddenly span two bays instead of one. Because moment goes with the square of span, the demand jumps by a factor of four before any dynamic effect is counted. Whether the building survives depends on whether it can bridge that gap in bending — or, when bending runs out, hang in catenary tension instead.
Loads
Structure
Live Result
—
APM load Ω(D + 0.25L)—
Normal single-bay moment—
Bridging moment over 2L—
Demand-capacity ratio—
Catenary tie force required—
Surviving mechanism—
Powered by the Alternate Path Method
(
engine/progressive-collapse.js). Verified before publishing that
the bridging amplification is EXACTLY 4.0 and stays exactly 4.0 across every
load and span tested — it is pure geometry, not a coincidence of one example.
The catenary tie force is cross-checked against
engine/funicular-arch.js and matches to machine precision: a
hanging cable and a compression arch are the same mathematics, so two separate
engines on this site must never disagree about it. When tie capacity is not
supplied the engine reports the outcome as unknown rather than guessing.Method
GSA (2016) · UFC 4-023-03 (archived 2026 → ASCE 76-23)Alternate Path Method — notional column removal, then check whether the remaining structure bridges. Load combination Ω(D + 0.25L).
Catenary actionT = wLᵇ²/(8·sag) — the second line of defence once flexure is exhausted, requiring both large deformation and real tie continuity.
Scope: single-bay notional removal, uniform load, flexural and catenary mechanisms only. Does NOT model 3D load redistribution, Vierendeel frame action, connection rotation capacity, dynamic removal transients, or slab membrane action. A teaching model of robustness concepts, not a substitute for full APM analysis.
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