Why a Tall Tower Moves With the Sun

By Michael Stern, JDS Development Group

Designing for movement rather than against it is an old admission in building practice. It came from watching real structures in the weather and noticing that the face of a building the sun is on runs warmer than the face in shade, so the two sides of one frame want to be different lengths at the same moment. This is for developers, facade consultants and structural engineers deciding how much tolerance a tall building needs and where to spend it.

On a low building the difference is small enough to absorb without much thought, and the convention that grew up around it looks like housekeeping. On a tower the same effect accumulates over height, and the housekeeping becomes the design.

Why does a tall building move when the sun hits one side?

Because the sunlit face gets warmer than the shaded face, and warm material is longer than cool material. One side of the frame grows while the other does not, so the tower curves gently toward the cooler side until the two even out again after dark.

The underlying behaviour is not special to buildings. Materials change dimension when their temperature changes, and concrete, steel and aluminium each do it at their own rate. A single member sitting in uniform air simply gets longer and then gets shorter again, and nobody notices.

What makes a tower different is that the heating is uneven by definition. A building has a sunny side and a shaded side at the same instant, so the expansion is not uniform across the plan, and the frame has nowhere to put the difference except into curvature. The taller and narrower the frame, the further the top travels for the same difference between the two faces.

A sunlit surface also runs hotter than the air around it, which is the part that gets underestimated. Dark cladding, still air and a clear sky push the temperature of a wall well above what a thermometer on the sidewalk reports, and the design temperature range for a facade is therefore wider than the weather record for the city.

The movement is also cyclical rather than cumulative, which is both the relief and the problem. The building comes back every night, so nothing drifts permanently out of position. It also means every detail that accommodates the movement is worked through that cycle every clear day for the life of the building.

What the daily movement asks of the facade

The facade is where it shows up first, because the cladding is attached to a frame that is quietly changing shape underneath it. Each panel is hung so that it can move relative to its neighbours, and the gap between panels is sized for the range the frame will impose rather than for how the elevation looks in a rendering.

The anchors do the real work. A panel is held at one point that carries its weight and restrained at others in a way that permits sliding, so the panel follows the frame without being stretched by it. Get that backwards and the cladding becomes a structural element nobody designed.

Those gaps are not left open. They are bridged by a sealant chosen for how far it can stretch and compress without tearing or losing adhesion, which is a performance question rather than a cosmetic one. The width of the joint and the capability of the sealant are a single decision made twice, once by the engineer and once by the specifier, and they have to arrive at the same number.

At 111 West 57th Street the proportions make all of this less forgiving. The tallest and most slender residential building in the Western Hemisphere has very little plan dimension across which to absorb a difference between two faces, so the tolerance has to be found in the detail rather than in the geometry. SHoP Architects were working a facade across a form where the structure and the skin are never far apart.

How is a tower detailed to accommodate thermal movement?

In order, and early. The structural engineer establishes how much the frame will move, the facade consultant turns that figure into joint widths and anchor types, and every trade that crosses the facade line sizes its own tolerances against the same number.

The sequence matters because each step spends part of the budget the step before it set. If the frame movement is established late, the facade has already been detailed against a guess, and the guess is generous in the wrong direction more often than not. Reordering that on site costs more than getting it in sequence ever would.

Shafts are the quiet case. Elevator guide rails run the full height of a structure that is moving and have to stay aligned closely enough for the car to ride them, so the rail brackets are detailed to slip rather than to clamp. An elevator that rumbles on hot afternoons and rides clean at night is usually telling you something about the frame, not about the elevator.

Interior finishes come last and suffer most. A partition built tight to the slab above will crack at its head long before anything structural complains, which is why the head detail in a tall building is a slip track rather than a solid connection. The same logic governs stone, plaster and anything else rigid that spans between two things moving independently.

Can a tower be built stiff enough to stop it?

No, and trying is the expensive mistake. Stiffness resists load, and a temperature change is not a load, so a stiffer frame warms and lengthens exactly as the lighter one does while costing more to put up.

What stiffness actually buys is restraint, and restraint turns free movement into stress. A frame tied down hard against its own expansion carries that expansion as force in the members and at the connections, and the detail that was meant to prevent movement becomes the detail that concentrates it.

Here a competent engineer may well disagree with me. On a heavy concrete tower with a thermally broken facade, the daily swing from the sun is modest next to the drift the same building sees in a windstorm, and the argument is that tolerance spent on thermal movement would do more good spent on wind. That reading is right about magnitude and wrong about frequency.

The honest way to settle the question is to measure rather than argue. I have made the same case about measuring a finished tower for wind, and the instruments do not care which effect they are recording, because they report the motion the building actually has rather than the motion it was predicted to have.

The compromise I accept is a maintenance obligation. A joint that accommodates movement has a consumable in it, and sealant has a service life considerably shorter than the structure around it, so a facade detailed this way commits an owner to going back up and doing it again.

A building that is allowed to move is a building that has to be looked after, and that trade is made once, on paper, years before anyone is up there making good on it.

Retain the structural engineer to establish the movement, a facade consultant to turn it into joint widths and anchor types, and an elevator consultant to confirm the shaft tolerances still work once the frame is moving through its daily cycle. None of the three can do another's part, and a project that skips one of them finds out which one on site, at the worst possible moment in the schedule.

A tall building is never standing still, and the good ones are detailed by people who assumed that from the first drawing.

Michael Stern is the founder and chief executive officer of JDS Development Group.

Photos by Malena Gonzalez Serena and Christian Ladewig on Unsplash

Sources

CBD 47. Extreme Temperatures at the Outer Surfaces of Buildings
Thermal Expansion and Contraction
CBD 155. Joint Movement and Sealant Selection