Adjustable props and shoring

Frequently asked questions

What actually decides how many props are needed?

The load coming down and what each prop is rated to take AT ITS EXTENDED LENGTH — and that second part is what catches people out. A prop's capacity falls sharply as it is extended, because a longer slender column buckles at a lower load: the same prop rated at several tonnes closed can be rated at a fraction of that near its full extension. So a calculation done on the closed rating and executed at full extension can be out by a factor of several. The load itself is the tributary area of whatever is being supported, including the floors above it, and the wall's self-weight — which for masonry is substantial and routinely underestimated. And each prop needs something to stand on: a prop on a suspended floor is transferring the load to that floor, which then needs propping in turn, all the way to the ground.

What is a strongboy, and why does it exist?

It is a bracket that fits the head of a prop with a flat blade projecting sideways, and the blade slides into a bed joint that has been raked out above the working area — so the masonry is supported from a course of brickwork rather than by a plate across its face. It exists because the alternative, a needle, means cutting a hole right through the wall and propping both sides, which is far more work. The limit is that the load path is now a few courses of brickwork above the blade, so the arrangement depends on the masonry above being sound and bonded, and it is not suitable where the wall is in poor condition, where the load is high, or where there is an opening immediately above. That judgement is the reason temporary works are designed rather than arranged.

Why is temporary works a designed activity rather than a site decision?

Because the building is at its least stable exactly while it is being propped, and the failures are sudden. A wall with an opening cut in it and no lintel is relying entirely on the props; if one is on soft ground, at too great an extension, not plumb, or missing because the space was awkward, the load redistributes to the others and the sequence runs away. British practice formalises this with a temporary works coordinator and a design for anything non-trivial, and American practice with shoring and reshoring requirements in the formwork standards. The recurring domestic failure is simpler than any of that: propping installed correctly and then removed too early, before the beam has bearing or the concrete has strength.