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CALCULATING CLEAR OPENING WIDTH

In a previous Tech Tip (March 2002), we discussed the need for AHCs and CDCs to be able to calculate the egress capacity of door openings in a means of egress. In order to correctly work the formula you must determine the nominal clear width dimension of the door openings in question.

The 2003 edition of the Life Safety Code[TM] (NFPA 101) describes the method of measuring the clear opening width of door openings. Chapter 7 establishes the basic requirements for the means of egress in a building. Article 7.2.1.2.1, Egress Capacity Width, describes the method for measuring the clear width of new and existing swinging doors.

Calculating clear width of new swinging doors (7.2.1.2.1(1)) differs from that of existing swinging doors. New swinging doors are opened to 90 degrees, regardless of their full travel (see Figure 1). The measurement is taken between the face of the door (push side of the opening) near the hinge edge and the opposite door stop (or soffit of the frame). However, existing swinging doors (7.2.1.2.1(3)) are opened to their full degree of travel (see Figure 2). The actual measurement is taken in the same manner as new swinging doors. Of course, several components of an opening can affect the clear-width dimension. Door hardware items and door thickness all must be considered when determining clear opening width.

Due to the actual pivot point of a hanging device, hardware items (e.g., full mortise, swing-clear and wide-throw hinges, offset and center-hung pivots, continuous hinges, etc.) used to hang doors greatly affect the position of the doors when opened to 90 degrees. The pivot point of a standard full-mortise hinge is on the centerline between the hinge stile of the door and door rabbet on the frame. However, the pivot point of a swing-clear, full-mortise hinge is positioned approximately 1 5/8"(+/-) off the centerline between the door and frame (away from the centerline of the opening), which is why the door is moved out of the opening when opened to 90 degrees. In this situation, since the door thickness is moved to a position where the face of the door is essentially on the same plane as the frame soffit on the hinge jamb, the net clear opening width dimension of the doorway will be approximately 1 1/4 less than the nominal door width. For example, a 1 3/4"-thick door that is 3 feet wide will have approximately 34 3/4" clear opening width.

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The height of the door stop on the frame can also affect the clear width of an opening. Most hollow metal frames used today are manufactured with a 5/8"-high door stop. Aluminum frames typically have a stop that is 1/2" high. Other hardware items such as weather and sound-control gasketing also affect the clear width of a door opening, especially when installed on top of the frame soffit, which further encroaches on the clear opening width dimension.

Door thickness is another element that can affect the clear opening width of a door in a means of egress. Most doors in a means of egress are 1 3/4" thick, but if a particular opening has a 2 1/4"-thick door (exterior wood door) the stop face of the door opened to 90 degrees is moved 1/2" further into the clear opening of the doorway. Generally speaking, a door that is 1 3/4" thick and 36" wide, installed in a standard hollow metal frame and hung on 4-wide full-mortise hinges, provides approximately 33 1/4" of clear opening width (see Figure 3). This same door hung on 4 1/2" wide full-mortise hinges provides for 33 1/8" of clear opening width. With this in mind, it is safe for you to assume a typical 1 3/4" thick, 3-foot-wide door has a nominal clear with of 33", and thus an egress capacity of approximately 165 occupants in an non-hazardous area.

The following formula can be used to calculate the actual clear-width dimension of a single-door opening:

Single-Door Opening Nominal door opening width         36"

Minus height of door stop on STRIKE JAMB               -5/8"
Minus DOOR THICKNESS                                   -1 3/4"
Minus pivot-point dimension from FACE of DOOR          -5/8"
      (4 1/2"-wide standard full-mortise hinge)
Plus (toward center) or minus (away from center)
     (pivot-point dimension from edge of HINGE STILE)   0"

Remember, when calculating the egress capacity of a door opening the occupant load of the space is predicated on the size and configuration of the room or space. If asked to verify that sufficient egress capacity has been provided for in a design, you need to consider the maximum occupancy load of that space. You may have to take the maximum occupancy load and calculate the total clear opening width dimension required. Generally, you can figure 1" of clear opening width is required per 15 occupants in non-hazardous occupancies. NFPA 101 and each of the model building codes include a table of egress factors that are to be applied to different types of occupancies. Be sure to consult the correct code and use the appropriate egress factor to calculate egress capacity.

RELATED ARTICLE: Using the formula above, answer the following questions:

1. What is the clear opening width dimension of a new single 1 3/4" X 4'0" X 7"2" HMD installed in a standard hollow metal frame with 5/8" high door stops and hung on 1 1/2" offset pivots?

2. What is the clear opening width dimension of a new single 2 1/4" X 3'6" X 7'0" STC-rated SCWD installed in a Cased Open HMF frame with 1 1/4" high sound gasketing and hung on 4 1/2" wide, standard full mortise hinges?

3. What is the clear opening width dimension of a new single 1 3/4" X 3'0" X 8'0" SCWD installed in an WDF with 3/4-high stops and hung on 6 wide throw hinges?

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