The thing that decides the structural design of a very slender tower is not a safety requirement. It is a threshold at which people begin to notice motion and object to it. That sentence surprises most people who have not worked on one, and it reorders everything downstream of it.

Safety is comfortably satisfied long before comfort is. A structure can be entirely sound while swaying enough to make occupants uneasy, and uneasiness is not a marginal concern: it is what determines whether the building can be occupied as intended. So the criterion that governs is subjective, it concerns perception rather than capacity, and it has to be turned into a number the engineering can be checked against.

That conversion is worth understanding because it is where the softness enters. Perception of motion varies by person, by what they are doing, by how long it lasts and by whether they can see something stationary. There is no single threshold that is true for everybody. What the profession uses instead are criteria derived from studies of how people respond, expressed as accelerations over a defined period, and the choice of which criterion applies is itself a judgment made early and rarely revisited.

Then the mechanism that produces the motion, which is less intuitive than it sounds. Air moving past a tall body does not flow smoothly around it. It separates at the corners and sheds in an alternating pattern, and that alternation applies a rhythmic force perpendicular to the wind direction. If the rhythm of the shedding approaches the rhythm at which the building naturally wants to sway, the two reinforce and the response grows out of proportion to the wind that caused it.

This is why a slender tower can be least comfortable in a moderate wind rather than a severe one. The alternation has a frequency that depends on wind speed, so there is a speed at which it lines up with the building. Faster is not automatically worse. That is a genuinely counter intuitive property and it is why a purely load based intuition misleads here.

How the criterion actually gets established is by testing a specific shape in a specific surrounding context, because the surroundings change the flow. A model goes into a wind tunnel with its neighbours reproduced around it, and the response is measured rather than calculated. The result belongs to that configuration and no other.

Which produces the limitation I find most interesting about the whole discipline. The surroundings are not fixed. Somebody can build next door afterward, changing the flow the tower was tested in, and nobody is obliged to ask. So a building is designed against a condition that is accurate at the moment of testing and is not guaranteed to persist, and the response is to carry margin rather than to predict.

The responses available fall into three families. Change the shape so the shedding cannot organize into a clean rhythm, which is why the good answers at this proportion tend to be visually irregular in some way. Add damping so energy is absorbed. Or stiffen the structure so there is less response to absorb. In practice serious buildings use some of each, because each one is expensive past a certain point and the combination is cheaper than any single answer taken far.

Damping deserves a plain description because it is written about as though it cancels movement. It does not. It removes energy from a structure already in motion, so the motion is smaller and decays sooner. The building still moves. The target is to get below perception, not to zero, and anyone describing it otherwise has misunderstood it or is selling something.

Here is where a competent engineer could take the other side of my view. I think the profession is too comfortable with how soft the comfort criterion is. An enormous amount of expensive structure is sized by a threshold derived from studies of how people report feeling, applied to a population of occupants nobody has met, in a building that does not exist yet. That is not a criticism of the engineering, which is rigorous given the input. It is a question about the input, and the standard answer, that the criteria are conservative and have served well, is reasonable and may simply be right.

The honest limit is that none of this resolves until the building is occupied and people either complain or do not. There is no intermediate confirmation.

For a project at this proportion, a wind consultant belongs in the work before the shape is settled, alongside a structural engineer with supertall experience. The tower I have worked on that shows all of this most clearly is 111 West 57th Street.