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The Loretto Chapel staircase: A lesson in physics, not miracles

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By Tim Carter
Saturday, January 16, 2010

DEAR TIM: I recently was vacationing in Santa Fe, N.M., and saw the Loretto Chapel staircase. It's a beautiful wooden circular staircase thought to be built between 1877 and 1881 by an unknown carpenter. There is no center support, and nothing seems to be holding up this staircase but the will of God, or so suggest the owners of this building. Is the miraculous staircase just that, or is there a scientific explanation for why it's not a pile of splinters on the chapel floor? -- Michelle B., Minneapolis

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DEAR MICHELLE: I've also seen the Loretto Chapel spiral staircase in person. It's a magnificent work of art that humbles me as a master carpenter. To create a staircase like this using modern tools would be a feat. It's mind-boggling to think about constructing such a marvel with crude hand tools, no electricity and minimal resources.

But that doesn't make it impossible or less complex than many other building and engineering masterpieces that are clearly visible in old structures around the world. The private owners of the Loretto Chapel state on their Web site: "The stairway confounds architects, engineers and master craftsmen. It makes two complete 360-degree turns, stands 20 [feet] tall and has no center support. It rests solely on its base and against the choir loft. The risers of the 33 steps are all of the same height. Made of an apparently extinct wood species, it was constructed with only square wooden pegs without glue or nails."

My guess is the owners of the chapel have not talked to many architects, engineers or master craftsmen. It's obvious to me that there is nothing miraculous holding this staircase in position. It's subject to the same laws of physics as any staircase in your home.

This will all make sense if you allow me to explain the basic structural components in a house. The talented carpenter who built the Loretto Chapel staircase just happened to know what I and a few others know about beams.

If you live in a house with a wood floor system, the floor you walk on supports you. It doesn't collapse under your weight, the weight of the furniture and any guests you may have during a large party. The floor system is made up of joists and wood that covers the joists. The floor joists under your feet are beams not unlike the steel I-beams in a skyscraper.

In a typical floor system, these joists run parallel with one another on 16-inch centers. The joists are covered with plywood or other wood sheeting. They rest on a foundation wall or, in some cases, another beam that rests on a foundation wall. The weight of the entire system and anything on it is transferred to the foundation by the joists.

Imagine if you have a narrow floor, meaning just two joists, and you cover these with plywood. This would be very similar to a narrow footbridge. You could walk across it, and the platform or bridge would not collapse. Now, let's drop one end of this narrow platform to the ground, leaving one end up on the foundation wall. Now you have a ramp.

The normal staircase in your home is just like this, but instead of a ramp, the builder made flat steps or treads so you could climb it without slipping or sliding. A simple staircase has two beams, called stringers, and the treads of the staircase rest on these beams or are connected to them. The stair treads are no different than the wood that covers a flat floor. When the carpenter set your steps in place, the weight of the staircase was transferred to where the two stringers touch the floor.

The only difference with the staircase at the Loretto Chapel is these beams or stringers have been twisted into a helix. If you took the staircase apart and just allowed the inner and outer stringers to stand there by themselves, they would do so like the flagpole just outdoors on the plaza, even though each stringer is made up of several pieces of wood glued and pegged together. It's that simple. It's not a miracle at all.

Tim Carter is a columnist for Tribune Media Services. He can be contacted via his Web site at http://www.askthebuilder.com/printer_Submit_Question.shtml.


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