Scaffolds and Formwork from Poles: Safe Loads, Bracing, and the Bay Size That Holds

Where steel tube scaffolding is unavailable or unaffordable, scaffolds and formwork are built from timber poles lashed or nailed together, and they are usually sized by copying whatever stood last time. This paper computes, from stated assumptions, the three quantities that decide whether such a structure holds: the buckling capacity of a pole upright as a function of its diameter and the height of one lift, the bending capacity of the horizontal poles that carry the platform, which fixes the greatest safe distance between uprights along the scaffold, and the load a pole can carry when it is used as a prop under wet concrete. The study is simulated, not measured, and not load-tested: it is a design calculation, and it does not replace scaffold inspection or local regulation, both of which remain necessary for any real structure. Under the stated values a braced upright of 90 mm diameter with a 2.0 m lift has a working buckling capacity of 23.84 kN, falling to 15.26 kN if the lift rises to 2.5 m and collapsing to 5.96 kN if the same lift is left unbraced so that it can sway. The platform, not the upright, fixes the bay: for a 90 mm ledger the greatest bay length is 1.52 m under a light load class of 1.5 kN/m2, 1.35 m under a medium class of 2.0 kN/m2 and 1.13 m under a heavy class of 3.0 kN/m2. Under a 0.20 m wet concrete slab imposing 6.8 kN/m2 in total, a 90 mm prop at a 2.0 m lift may carry a tributary area of at most 3.51 m2, a square grid spacing of 1.87 m, while at a 1.5 m lift the same pole may carry 6.23 m2, a spacing of 2.5 m. The design rule originated here combines these results into a small table of maximum bay length and lift height by pole diameter and load class, together with a bracing requirement stated as arithmetic rather than advice: because removing sway restraint quarters the buckling capacity, diagonal bracing on both faces of every lift is a structural necessity, and a pole scaffold with any unbraced lift should be treated as having the unbraced capacities, which are too small for ordinary platform loads at useful diameters.