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Why do some doors open inward and some open outward? | August 30, 2010

 
Interviewee

Welcome to BrainStuff from HowStuffWorks.com, where Smart happens.

Marshall

Hi, I’m Marshall Brain with today’s question. Why do the entry doors to most homes open inward while in most public buildings the entry doors open outward? The basic doorway in your house has a very simple structure. There’s a large board attached to a frame using a few pin hinges. This design has a number of advantages. It’s easy to build, easy to install, easy to repair. Of course, it’s also very easy to disassemble. Removing the hinge pins completely detaches the door from the frame. While this might help you out as a homeowner – for example, you can remove the door in order to squeeze in an oversized couch – it’s not something you want to make assessable to intruders. For this reason, the hinge mechanism needs to be positioned inside the house. With a standard hinge design, this means that the door is going to open inward; that’s the natural way for it to open in this configuration.

Public buildings have the same kind of security concerns, but they also have to consider other safety factors that you really don’t have to think about with a house. Unlike a private home, a public building is likely to have large numbers of people in it. In the case of a fire or other emergency, these people need to be evacuated as quickly as possible. When a mob of people rushes an exit, it’s very hard for somebody to open the door inward. Everyone pushes up against the door, and there’s no room for it to open. Therefore an effective emergency door has to open outward, moving with the force of the mob. This is also why a lot of emergency exits are built with wide panic bars instead of ordinary door knobs. The basic idea is to build the exit so even the most out of control mob will be able to escape.

To maintain perimeter security, public exits are typically build with concealed or protected hinges, which are much harder to detach than a simple pin hinge. These doors are more expensive to install and repair, making them impractical for anybody’s normal house. And as long as you don’t have an unruly mob living with you, these outward opening doors don’t offer any real advantage in your home.

There’s another reason for outside doors as well. Any public building that contains a lot of people is normally required to pump in extra outside air to keep oxygen levels high and carbon dioxide levels low. This air is just brought in from the outside through the heating and air conditioning system. This usually over-pressurizes large buildings, so if you had an inward opening door, that over-pressurization would tend to push the door shut and make it really hard to open. With an outward opening door, it’s much easier to get the door open against that over-pressurization. The air pressure works with you to open the door.

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