At Apple, I ran engineering on the largest sliding glass doors ever put in a retail store. Fifty feet tall, twenty feet wide, 27,000 pounds each, and there were two of them.
Everybody assumes the hard part was that nobody had built one before. Here is the part that actually mattered: figuring out that it was not a door.
That sounds like wordplay. It is not. It is the single decision that made the project possible, and it happened before anybody was hired.
Take the problem in front of you and write it down without using the noun your industry uses. For the door, it came out like this. A 27,000 pound mass has to accelerate from rest, travel 20 feet, and come to a complete stop at a repeatable position, thousands of times, in public, with people standing on both sides of it. Then do it again, because there were two of them.
If 27,000 pounds does not land, a 40 foot city bus weighs about 27,700. That is the object, and it is made of glass.
Nothing in that sentence says door, storefront, glazing, or hardware. It is a motion control problem wearing a door costume.
Once you see that, the hiring question answers itself. You do not go find the best storefront company. Storefront companies are extraordinary at storefronts, and a storefront is a thing that holds still. You go find the people who move heavy things and stop them safely for a living.
Here is the physics behind why that matters so much.
Holding up 27,000 pounds of glass is the easy half. Structural engineers do that every day. The hard half is that a moving mass carries energy, and that energy scales with the square of its speed. Double the speed and you have four times as much to get rid of. All of that energy has to go somewhere when the door stops, and there are only two places it can go. Into a braking system you designed on purpose, or into the glass.
Glass does not forgive. It is strong in compression and it fails without warning when you load it at a point. Every hard stop sends a shock through the panel and into the fittings that hold it. Do that ten thousand times and you are not designing a door, you are running a fatigue test on the most expensive part of the building.
So the real design problem is deceleration. How fast can you slow this thing without spiking the load on the connections, and without jerking it at the end of travel in a way anyone standing there can feel. That is exactly, precisely, the problem a theme park ride engineer solves every day of their career. A ride is a heavy mass that must accelerate, travel, and stop, smoothly, in public, with humans in the loop, forever. They have spent decades on acceleration curves and controlled stops. Kinetic and motion control specialists live in the same territory from the controls side.
None of those people are in the storefront phone book. That is the whole point.
Now the part I get asked about less. What changed between the first set and the last one.
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The first set moved 20 feet in about three minutes. The last one did it in 45 seconds. Same mass, same distance, four times the speed.
I want to be honest about why the first one was slow. It was slow because we did not know where the energy was going. When you cannot predict the failure, you buy margin with time. Creep it along, keep the energy low, accept that the door feels like it is thinking about it. That is not timidity. That is the correct engineering answer when your knowledge is thin.
By the last set we had run the thing enough to know what actually loaded up and what did not. We knew which parts of the travel were forgiving and which ones were not. And the moment you can predict the failure, you can stop paying for it with speed. The margin moved out of the schedule and into the design.
That is the general rule, and it is worth more than the door story. Early conservatism is not a permanent tax. It is a loan you take out against your own ignorance, and you pay it back with data.
So here is the test I use now, on buildings and on deals. When I hit something I have never seen, I write the problem down without the industry noun. Then I read it to somebody in my own field. If they nod and reach for a familiar product, I am fine. If they go quiet, or start describing a bigger version of the thing I already have, I am in someone else’s discipline and I need to go find them.
The expensive mistake is not hiring the wrong specialist. It is hiring the right specialist for the wrong problem, and then spending a year getting an excellent answer to a question you were never asking.
—Jon