March 27, 2008

Revolving Doors

Stupid Question ™
July 15, 1999
By John Ruch
© 1999

Q: Why are revolving doors used in skyscrapers, even though they’re tougher to use than regular doors?
—Ralph Portzline

A: Tall buildings are subject to a phenomenon known as the “stack effect.” When their internal temperatures are different from the outside air, the resulting difference in air pressure makes the buildings act like smokestacks of air.

In cool weather, cold air entering the lower floors warms up, becomes less dense and rises rapidly through elevator shafts and stairwells as a strong draft. In warm weather, the buildings’ dense air-conditioned air blows in the opposite direction, pouring out the bottom.

The drafts can be strong enough to suck lobby doors shut or blow them open, depending on the season. The revolving door was invented in 1888, apparently to remedy this problem in the new skyscrapers.

The “wings” of a revolving door are in constant contact with the cylindrical vestibule, creating a perpetual seal even when the door is in use. (Manufacturers like to say a revolving door is “always open, always closed.”)

This air-lock function keeps excess air from leaking in. It greatly reduces the stack effect, avoids the sucking/blowing problems of regular doors and reportedly slashes heating and air-conditioning expenses (even more important in today’s buildings, which are slightly pressurized to keep drafts out).

Revolving doors are required in inflated structures like the Minneapolis Metrodome, where strong winds can squish the pressurized air out regular doors at great force. And revolving doors are found in some airports and hospitals for air-conditioning conservation.

But revolving doors are used more than necessary. The degree of stack effect varies with location, climate and height; revolving doors are probably necessities only in buildings over 50 stories high. (Regular doors at Columbus, Ohio’s Riffe Center were easy to use on a recent hot day.)

The energy-saving value of revolving doors is probably negligible in notoriously inefficient skyscrapers. Building codes don’t require them for skyscrapers; in fact, codes only mention revolving doors as fire hazards and problems for the handicapped. That’s why there are always regular doors next to revolving doors.

And revolving doors are lousy on pedestrians. That’s why Columbus, Ohio’s Port Columbus airport is switching from revolving to wide, sliding doors. Airport spokesperson David Whitaker said the revolving doors confuse travelers and “create bottlenecks at peak times.” The increased convenience will offset the slight reduction in energy efficiency.

Ohio State University architecture professor Kay Bea Jones said the popularity of revolving doors probably has more to do with aesthetics than science. Used in all the coolest skyscrapers, and embodying the trendy notion of erasing architectural boundaries between indoors and outdoors, the revolving door is a prestigious “symbol of modernity.”

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