Why Window Washing Is Designed Into a Slender Tower

By Michael Stern, JDS Development Group

Two window cleaners hang on ropes against the glass facade of a tall office building.

On the facade drawings of a tower there is a note, small and usually pushed to the corner of an elevation sheet, that says every exterior surface must be reachable by the building maintenance unit. Nobody argues with it in a design meeting because it reads like housekeeping. It governs the shape of the building more than most of the lines drawn beside it.

I am writing this for developers, architects and facade consultants who are about to draw a setback or a slender floor plate and have not yet asked who will clean it. The note is the first place that question gets answered, and on a tall narrow building the answer is a good deal more physical than the note suggests.

A window gets cleaned and a panel gets inspected by someone, standing somewhere, held by something. That sentence is the whole subject of this piece, and each of its three parts takes up room in the building.

The facade note decides roof equipment

What the note says is short. A machine on a track or a davit on the roof must be able to place a platform within reach of every panel, and the building must supply the guides, anchors and structure that hold the platform against the face. The federal rule for powered platforms on buildings spells out the pieces, and it defines the tie in guides as the portion of the building that keeps a platform in continuous engagement with the face of the building while it travels.

That makes the guides part of the facade package rather than part of the roof package. A track set into a mullion, a button fixed to a panel frame, a recess left in a spandrel: each is drawn by the facade team, tested with the full size facade mockup, and expected to survive being touched by a platform now and then.

The sequence matters here. The facade team fixes the guide locations first, because they repeat on every floor and every panel type, and then the roof team looks for a machine whose reach and weight fit the pattern the facade has already committed to. When that order is reversed the guides land where the machine wants them rather than where the panels can carry them, and the panels get redesigned late.

The note leaves one thing open, and it is the thing that matters. It does not say where the building maintenance unit parks or how far it reaches. The drawing is silent on both, and the roof answers them.

Where does the maintenance unit live on a slender tower?

On the roof, usually on a track or a turntable near the edge, close to the mechanical plant and on some towers close to the damper. On a slender tower that is the most crowded square footage in the building, and the machine has to be parked, serviced and reached around everything else.

A roof on a slender tower is small because the floor plate is small. The machine needs a parking position, a track or turntable that lets it swing over the edge, and clear space for a crew to hook up. The mechanical equipment and the structure of the crown want the same ground, so the facade access consultant, the mechanical engineer and the architect trade square feet until each can live with what is left.

Wind enters the trade too. A machine reaching over the edge of a tall roof is a lever with a platform on the end of it, and the structural engineer wants to know what it adds at the top, which is one more reason the crown gets the same attention as the shaft, a habit I described when writing about why a finished tower is still measured for wind.

A tower built as slender as 111 West 57th Street gives the roof very little area to give away. I think about the access question there as a question of area before I think about it as a question of machinery.

Can a machine reach every panel of a setback tower?

No. A facade that steps in and out on the way down puts some panels away from the line a platform hangs along, and those panels are reached from a second machine, from a rope or from inside.

The geometry makes the problem. A platform hangs from the roof on wire rope, so a facade that steps in or out puts panels away from the line the rope falls along. Reaching them takes a second machine on a lower roof, extra anchor points on the face, or a person on a rope.

Window cleaners spread across the gridded facade of a tall building.

The accepted compromise is that on a facade with enough movement some panels are reached by rope access technicians and by nothing else. I have accepted that on drawings, because the alternative is to flatten the facade until a machine reaches all of it.

A tower flattened for a machine is a tower the architect did not design.

A facade consultant can fairly tell me I am too relaxed about this. Rope access is slower, depends on the weather and leaves more of the inspection to the eye of the person on the rope, and a rule that every panel be reachable by machine would remove those variables at a stroke. There is something in that, and on a simple facade I would draw it that way.

On a facade with setbacks I draw it the other way, because the extra guides and second machines that full coverage demands take their own space on a roof that has none to spare. Both readings are defensible, and the right one depends on which constraint binds first on the site.

The inspection is why the access exists

The access exists because someone has to look at the facade up close. New York requires buildings above a certain height to have their exterior walls inspected on a recurring cycle. Only a licensed engineer or a registered architect may perform that critical examination.

An inspector who must see a panel closely needs a way to reach it every cycle for the life of the building. That turns a convenience for window cleaners into the practical means of meeting a legal requirement, and it is why the note in the corner of the sheet carries more weight than it appears to.

The professional to retain is a facade access consultant, early, with a structural engineer reading the roof loads and a licensed engineer or registered architect reading the city rule for the specific building. I am describing how the pieces connect and not the code for any site.

The note in the corner of the sheet is the cheapest line on the drawing and the last one a tower can afford to get wrong.

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

Photos by Roman and Emanuele Bono on Unsplash

Sources

1910.66 Powered platforms for building maintenance
Facade and Local Law Buildings

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