At street level, wind is weather. At 150 metres, wind is a load case that shapes the plan, sizes the core, decides whether the top-floor tenant's coffee shows rings on a Tuesday. Rotterdam sits in the windiest corner of a windy country; every tower we draw is negotiated with the west wind before it is negotiated with anyone else.
The two problems, kept apart
High-rise wind design is really two problems that punish you for confusing them. The first is strength: the tower must stand up in the storm that arrives once in fifty years. That one is soluble with enough concrete, and it is rarely what drives the design. The second is comfort — accelerations near the crown in the wind that arrives every fortnight — and it is merciless, because people feel two milli-g before any engineer can measure damage.
Comfort is governed less by how hard the wind pushes than by how rhythmically it lets go. Air peels off a tower's corners in alternating whirls — vortex shedding — and if the shedding rhythm approaches the tower's own natural frequency, the building starts a slow lateral dance. Slender towers with clean rectangular plans are the best dancers, which is unfortunate, because slender rectangles are what the pro-forma wants.
You cannot argue with a frequency. You can only change the building until the wind loses the beat.
our façade engineer, at the first Maas Vertex tunnel session
The toolbox, in the order we reach for it
- Shape first: chamfered or softened corners, a tapering shaft, a setback that breaks the shedding rhythm. Free once drawn, and it never needs maintenance.
- Mass and stiffness second: an outrigger storey at the datum can halve crown acceleration — and become a winter garden while it's there.
- Damping last: a tuned mass damper is admitting the shape lost an argument. Sometimes correct, never celebrated.
On Maas Vertex the crown chamfer went through two tunnel campaigns in Delft. The final geometry cut peak accelerations by a third and street-level gusts at the quay by nearly as much. The drawing that changed — one corner detail, repeated forty-one times — is not the drawing anyone photographs. It is the reason the terrace on the crown is usable in April.
The discipline we hold ourselves to: the wind tunnel gets its review before the design gets its applause. A silhouette that cannot stand still in a February south-wester is not a silhouette; it is a proposal.