A revolving door is the only building entrance that can be always open and always closed.While this apparent contradiction seems more like a Zen riddle than a building component trait, the dual nature of a revolving door makes it attractive to building managers and designers who need to blend access (always open) and security (always closed).Revolving doors perform much like turnstiles at an amusement park, rotating a set number of degrees to permit passage to only one person at a time. As a result, revolving doors are becoming more popular in applications other than just traditional ones, such as big-city retail stores and high-volume office buildings. Thanks to their security benefits, revolving doors are now in demand more often in schools, government buildings, and medium-traffic offices.Among the security and safety features of revolving doors are:Alarm-activated bookfold mechanism. All revolving doors have a bookfold mechanism that allows the door panels to fold outward and provide egress when faced with extraordinary opening force. For added security, inadvertent bookfolding can be avoided with a locking device that is tied into the building’s alarm system. Only after an alarm is activated will the electromagnetic devices used to prevent bookfolding release, allowing door panels to fold outward and permitting mass egress.Activation devices. Electronic devices can be used to control traffic or limit access through a revolving door. Three common designs for interior or exterior activation includeInterior – Push button start. A button switch at accessible height allows a person to activate the revolving door. This can be tied into a security system to provide notification of building access.Interior – Sensor start. An overhead motion detector covering the path of egress can automatically operate the door.Interior or Exterior – Card key start. A mounting bracket for a cardreader can be mounted to the quarter post. An access card carrier would swipe his or her card to activate the door and record entry or exit from the building. Of course, cutting-edge identification devices, such as thumbprint scanners or retina/ iris scanners, can replace the typical cardreader.Pressure sensitive mats. These can be installed to detect non-approved traffic; for example, unauthorized persons entering when an authorized person is exiting. When triggered, the mats will stop the door from rotating and activate voice annunciators from soffit-mounted speakers.Security-grade construction materials. In the move to make buildings bulletproof and blast resistant, revolving doors can be specified with heavy-duty materials to provide additional security. Depending on the level of security desired or required by law, building professionals can request specific types of glass (for example, a glass and polycarbonate mix) and increased thickness of either aluminum or stainless steel.Of course, revolving doors provide a host of other benefits. They help prevent pilferage. They limit the size of packages or carts that can be brought into or out of a building. And they improve HVAC efficiency. These qualities are on top of the aesthetic possibilities and distinctiveness that revolving doors deliver.But when it comes to security considerations, the selection of a revolving door is, quite simply, an open and shut case.Andrew Knarvik is general manager for Lake Bluff, IL-based Crane Revolving Door Co. Inc. (www.cranedoor.com), a division of DORMA Group North America (www.dorma-usa.com), Reamstown, PA.