Two Checks for Structures in a Moving Environment: Will the environment shake it apart, and will the environment outgrow it?

Two mandatory design checks for any engineered structure exposed to environmental loading — wind, waves, current, traffic, machinery or ice — stated so they can be applied by a designer and audited by a reviewer.

Check One asks whether the environment can shake the structure apart. It gives a forced-resonance criterion with explicit margins, a site-specific driving-frequency map, and — separately and mandatorily — a self-excited instability assessment covering flutter, galloping and vortex-induced lock-in. The two are independent, and the document explains why the field's most famous example, the 1940 Tacoma Narrows collapse, is an instance of the second and not the first. A designer who holds Tacoma Narrows as the canonical warning, runs the resonance check and finds a clean margin will believe the structure is protected against the very thing that destroyed that bridge. It is not.

Check One is also explicitly scoped: it governs structural members and does not govern power-take-off elements, which in an energy-absorbing machine are frequently designed to resonate. Resonance is the objective for the absorber and the failure mode for the structure, and a rule written without that scope will either forbid the machine from working or permit the structure to destroy itself. Where a resonant absorber is mounted to a structure that must not resonate, the mount becomes a compliance item in its own right.

Check Two asks whether the environment will outgrow the structure. It deliberately specifies no numerical climate multipliers. Instead it specifies a procedure that forces sourcing — name the design life and horizon year, select and justify the scenario, source each projected quantity from a citable record for the specific region, record the uncertainty range, and state the consequence of being wrong in each direction. It also requires a design-event register whose fourth row is mandatory: what the structure is explicitly not designed for. An inherited multiplier is the most dangerous kind of number in a design document, because it is load-bearing, looks authoritative, and is almost never checked.

The document contains no novel physics and claims none. It is a checklist, written so that the two commonest omissions become visible: treating resonance avoidance as though it covered self-excited instability, and inheriting numerical margins nobody ever traced to a source. Worked examples are drawn from marine and coastal engineering because that is where it was assembled; nothing in the method is marine-specific.

Companion to "What the Sea Pushes Against" (same author, same deposit date).