In a fire or other emergency situation, a large number of people may need to exit a room or area quickly. A large group of people funneled into a hallway may count on one door to allow them an immediate exit from the building. Obviously a doorknob and deadbolt is not the best hardware to handle this...
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When the door needs to be locked again, the dogging key or for cylinder dogging, the mortise cylinder key is used to release the bar, allowing the latch(s) to re-engage the strike plate. In this position it is necessary to push the bar from the inside or turn the outside trim or key to gain entry.
Fire-rated exit hardware offers the same features as other exit devices in appearance, finish and operational advantages (except dogging). However, fire-rated hardware is constructed to remain secure for at least three hours under test conditions.
Fire-rated devices can't be locked down at any time because positive latching of the door is required at all times. The material of the latch and mechanism must be able to hold up under fire conditions and remain latched. A fire rated exit device must meet the standards of Underwriters Laboratory to receive the UL listing and must be clearly labeled as such.
The final determination to require a fire rated device can be determined by your local Authority Having Jurisdiction (AHJ), usually a local fire marshal or building inspector.
Exit devices fall into three basic categories: Rim Exit Devices, Vertical Rod Exit Devices and Mortise Exit Devices. Almost every manufacturer's exit device is offered in a standard and fire rated version.
RIM EXIT DEVICES
Most single doors use a rim device. This single point mechanism uses a spring-loaded latch (many offer a Pullman style bolt) to secure the door. This may or may not incorporate a deadlatch feature.
The use of outside trim varies from none at all to a simple pull plate to open the door when the bar is dogged down, a pull plate with a keyed rim cylinder to open the door from the outside, or a completely keyed lever or knob with lock cylinder mounted into a trim plate that can be similar in appearance to a conventional lever or knob cylindrical or mortise lock.
VERTICAL ROD EXIT DEVICES
There are two styles of vertical rod exit devices. The standard vertical rod exit device is surface mounted. There is also a concealed version, where the vertical rods and the latch mechanism(s) are installed within the body of the door.
In addition to the standard and the concealed versions, there are also Top Rod Only models of vertical rod exit devices. Top Rod only exit devices do not have a bottom rod mechanism. Depending upon the manufacturer and the application, when a Top Rod Only vertical rod exit device is installed, a pop-out latch may be required to be installed into the lower portion of the door which will extend and secure the lower portion of the door should there be a fire.
Probably the most common application for vertical rod exit devices is double-door applications where there is no center mullion. The standard top and bottom vertical rod exit device provides latching action at the top and bottom edges of the door. Depending upon the door's configurations, in a standard double door application, each of the doors can operate independently and open or close with no affect on the other door.
When a fixed center mullion is in place, a pair of double doors can be treated as two single doors, using rim devices on each door that latch to a strike plate mounted onto the sides of the mullion. Another alternative is a removable center mullion, used in schools and auditoriums where large equipment must be moved in and out. By removing the mullion the double-wide opening can be utilized. When the mullion is put back in position, all hardware acts in the normal fashion.
When using a vertical rod device, it may be surface-mounted or concealed. In a new door situation, the door can be factory prepped to accept the concealed vertical rods inside the door cavity with only the bar itself visible on the surface of the door. While this makes a clean appearance, existing door installations usually end up with surface mounted rods.
In special situations, a Less Bottom Rod (LBR) installation may be called for. Because only the top rod is securing the door, approval from the AHJ is required.
Dogging is again allowed in a non fire-rated application. By depressing the bar, the rods are drawn down, retracting both top and bottom latch bolts. Operation is identical to a dogged down rim device until the bar is released to the active position.