Basics of Means of Egress Arrangement
Buildings must be designed so that exits are always readily accessible and access to those exits is arranged so that they can be reached at all times. To do this, there are some fundamental design concepts to follow to ensure that the means of egress is arranged for an exit to be reached by occupants in a safe and efficient manner. Means of egress design must consider the distance occupants travel to an exit, how far apart exits are located from one another, and the arrangement of the paths of travel within the means of egress.
Referenced in this blog are design requirements for exits, exit accesses and exit discharge paths. As a reminder, the means of egress is made up of three parts: the exit access, the exit and the exit discharge. Exit access includes all travel within occupied areas of the building leading up to an exit. Exits are those portions of the means of egress that are separated from other building spaces protecting the space from the effects of fire, such as an enclosed exit stair or a door to the outside. Exit discharge is the travel leading from the exit to the public way (designated and approved point of safety.)
What is travel distance?
Travel distance is the maximum permitted distance that occupants are permitted to travel from their location in a building to the nearest exit. Although more than one exit might be required, the travel distance to exits other than the closest exit is not regulated. Excessive travel distances can be hazardous because they increase the time required by occupants to reach the safety of an exit, whether the exit is a door directly to the outside or into an enclosed exit stair from an upper floor of a building. The maximum allowed travel distances are based on factors that include demographics, potential obstructions in the path of travel, number of people in any room or space and the distance to the nearest door opening, the amount and nature of expected combustibles and the speed that fire might spread in that space. Allowable travel distances vary with the type and size of occupancy and the degree of hazard present. For most occupancies, the allowable travel distance can be increased if the building is protected throughout by automatic sprinkler systems.
Travel distance is measured on the floor or other walking surface along the centerline of the natural path of travel, starting from the most remote point subject to occupancy, curving around any corners or obstructions and ends at the center of the doorway or other point at which the exit begins. Where there are stairs included as a component of exit access rather than an exit, the travel over those stairs is included in the travel distance measurement. The natural path of travel is influenced by the contents and occupancy of the building, and a designer should not assume a straight-line measurement for travel distance. Furniture, fixtures, machinery, or storage found in the path of travel can increase the length of travel distance.
What is common path of travel?
It is ideal to always be able to move in different directions from any location, to allow different paths of travel to different exits. However, typical floor layouts and furnishing arrangements often create spaces where travel in a single direction is necessary for a limited distance before it becomes possible to travel in different directions. A common path of travel exists in the initial portion of the exit access where a space is arranged so that occupants within that space can travel in only one direction to reach any of the exits or to reach the point at which they have the choice of two paths of travel to two different remote exits. When an occupant is provided only one direction before reaching a point at which travel in independent direction, all that travel is considered common path. Common path of travel might exist only within rooms and occupied spaces, or it might exist within the combination of room space and corridors, depending on where the point is that two different options to go to two different exits is offered. Travel within rooms or areas with only one door is all considered common.
Like travel distance, maximum permitted common path of travel distances are regulated by the specific occupant chapter. The overall preference in building design is to reduce common path of travel, so the permitted values are not very high. Where occupants are able to travel in only one direction towards an exit, the risk of a fire impacting that egress path and access to exits increases. Common path is permitted only where the risk is reduced by other fire protection features as well as a low risk in the specific scenario.
Common paths of travel and dead-end corridors (explained below) are measured using the same principles used to measure travel distance.
What is a dead end (corridor)?
The terms dead end and common path of travel are commonly used interchangeably and while the concepts of each are similar in practice, they are two different concepts. While a dead end is similar to a common path of travel, a dead end can exist in a path of travel where there is no direct access from an occupied space but can also exist where an occupant enters a corridor thinking there is an exit at the end and, finding none, is forced to retrace their path to reach a choice of exits.
Although relatively short dead-end corridors are permitted for all occupancies, it is a better practice to avoid them as dead-end corridors increase the danger of people becoming trapped during a fire as well as increase the travel time to reaching an exit. Two common types of dead ends in corridors include corridor space beyond an exit, where an occupant moving toward the exit off the corridor mistakenly travels past it into the dead end and also space created by the elevator lobby that does not contain an exit.
It is a principle of life safety in buildings that if multiple exits (as well as exit accesses and exit discharges) are required, they need to be not only separate but also remote from one another and be arranged to minimize the possibility that more than one has the potential to be blocked by any one fire or other emergency condition. Although the objective of this requirement is clear, the term ‘remote’ cannot always be clearly defined.
To be considered remote, the exits, exit accesses and exit discharges in new buildings must be located at a distance from one another not less than one-half (one-third if the building is fully sprinklered) the length of the maximum overall diagonal dimension of the building or area to be served, measured in a straight line between the nearest edge of the exits, exit accesses, or exit discharges. Where exits are located at each end of a long corridor or at each end or side of a building, they qualify as remotely located exits. However, core-type buildings with elevators, service shafts, and stairs in one central or side core introduce some challenging problems with respect to exit remoteness. Other ways of measuring remoteness, utilizing corridors with 1-hour fire separation, also exist. For buildings that are not high-rise, the distance between exit enclosures can be measured along a corridor with a minimum 1-hour separation. In this scenario, although the exit enclosures are physically closer to each other than the dimension measured along the corridor, the exits will perform, under fire conditions, as if they were the corridor length apart.
Proper arrangement of the means of egress ensures that exits are made available to occupants at all times and are located in the building where they can be accessed without traveling too far, for too long, or with the risk of the exits being compromised during an emergency. For more details on the arrangement of the means of egress concepts addressed in this blog as well as additional requirements see NFPA 101®, Life Safety Code®, Sections 7.5 and 7.6.