For a piece under three metres, dead weight usually governs and the internal structure is a fabrication detail. Above six, wind governs absolutely: the overturning moment at the base grows with the projected area and with the height of its centroid, so a tall slender form can generate several hundred kilonewton-metres at the base while weighing very little.
That has three consequences a specifier should anticipate. The internal mast becomes a designed structural member with real dimensions, which constrains the form — you cannot taper a shell to nothing if there is a 450-millimetre section inside it. The base plate and holding-down arrangement become substantial, which affects the plinth detail. And the foundation becomes a groundworks contract designed by the project engineer to our issued loading plan.
The site parameters matter more than people expect. We work from the basic wind speed, terrain category, altitude and topography factors and the exposure of the specific position — not the town. A courtyard sheltered on three sides and an exposed waterfront promenade in the same city produce materially different answers.
Perforated, helical and highly three-dimensional forms are where published pressure coefficients run out. For those we use wind tunnel data for comparable geometries or, on the largest commissions, commission testing. A single-source digital result on a bespoke geometry is not something we are willing to sign, so every model is hand-checked at the critical section.
Serviceability matters as much as strength. Tip deflection on a cantilevered piece under the serviceability wind case is agreed with the facade or landscape consultant, typically at a limit of L/120. A piece that is perfectly safe but visibly moving is a client complaint waiting to happen.
