Why a Prop's Safe Working Load Drops as It Extends

Adjustable steel props all share one behaviour that catches people out: the safe working load is not a single number. It depends on how far the prop is extended — and the difference is large.

Extension is everything

A BS prop that safely carries around 35 kN near its closed length may be down to roughly a third of that at full extension. The load-capacity curve falls steadily as the inner tube extends, which is why prop schedules always state the working height, not just the prop size. (Our BS props conform to BS 4074:1982 and are tested in accordance with BS 5507 Part 3:1982, with a safety factor of 2 on the published curves.)

The plumb rules that come with the numbers

• Props must be plumb and loaded concentrically — the safe working loads allow for only 1.5° out of plumb.
• As a practical site check, that means about 25 mm of lean per metre of height: 50 mm at 2 m, 75 mm at 3 m, 125 mm at 5 m — no more.
• If anything is bolted to the prop, include the attachment height in the extension when reading the load table.

Choosing the right prop range

BS props cover working heights from 1.07 m up to 4.87 m across five sizes. Light and medium-duty props cover 1.5–5.0 m where loads are more modest, and trench struts — the short, claw-ended members used against timber walings — hold a constant safe working load of 35 kN across their range, which keeps spacing consistent when sizes are mixed.

Not sure which prop, at which spacing, for your load case? See the prop range — or send us the details and we'll size it for you.