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📘 Stand at the outside edge and feel the lever arm

Stand on the outside edge: your weight acts through a lever arm, so the tread bends and the inner connection resists the moment.

5
lessons
~25 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. The step is a small cantileverExplain how a tread transfers a person's load through bending, shear, and torsion.A spiral tread behaves like a repeated offset support problem, with the outer edge creating a lever arm.
  2. The column and connections collect the forcesTrace reactions from the treads through the column, base, landing, and surrounding frame.The central pole is only one part of a load path whose connections and foundations must be designed together.
  3. Not every spiral stair uses one poleCompare central-column, helical-stringer, timber-frame, and masonry load paths.Different materials and geometries make the stair act as cantilevers, curved beams, or compression structures.
  4. Design loads are more than one personDistinguish distributed, concentrated, handrail, vibration, and serviceability demands.Safe design accounts for how people actually occupy and move on the stair, not just its self-weight.
  5. Read the stair as a force mapUse a simple load-path sketch to identify structural members and an honest inspection question.The visible helix is a clue; the structural story is the route from a footfall to the foundation.

Questions this course answers

Why does a person near the outside edge of a tread create a larger demand at the support?

An offset load has a larger moment arm, so the connection must resist more bending and twisting.

What usually happens to the central column's axial force as you move downward?

The lower column supports the reactions from all the treads and stair components above it.

How can a spiral stair without a central pole carry load?

The supporting members change, but the load still has to travel through them into the building or foundation.

Why does an engineer check a handrail as part of the stair structure?

Guard and handrail forces create their own load cases and can twist or pry at local connections.

What is the most useful first question when looking at an unfamiliar spiral stair?

Following the load path reveals which parts are structural and where the building must provide support.

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