It's How You Ask
Runnel ledge vs. runnel trench: Why clarity and precision are key when seeking NFPA technical support
BY RON COTÉ
An NFPA member recently sent us a technical question related to calculating the capacity of an egress component in accordance with NFPA 101®, Life Safety Code®. As with other questions submitted via the technical questions tab on the document information page, I was left to answer the inquiry without the benefit of clarification that often occurs with questions received live by telephone. I prepared and sent an answer to the written question I believed the member had asked. In hindsight, I am having second thoughts.
The capacity of an egress component, like the portion of an exit access corridor leading to an exit stair enclosure, is the number of persons that can be accommodated by the component over the entire emergency evacuation or relocation process. The capacity of a stair serving multiple stories of a building is the maximum number of persons, from any one floor, that can be credited with using the stair for evacuation or relocation under fire or similar emergency conditions.
Capacity is calculated by dividing the clear width—the usable width for occupants to pass through—of the egress component by a code-specified egress capacity factor, typically 0.2 inches (5 millimeters) per person for level components and ramps and 0.3 inches (7.6 millimeters) per person for stairways. Clear width is generally measured at the narrowest point of the egress component under consideration. However, for other than door openings, clear width is permitted to have projections of not more than 4.5 inches (114 millimeters) on each side at a height of 38 inches (965 millimeters) and below. On a stair, the permitted projections usually take the form of handrails, wainscoting, and pan tread stair stringers.
The member’s technical question asked whether runnels can be considered as projections eligible for the 4.5-inch exemption to the clear width measurement. I was unfamiliar with the term “runnel,” and found photos and descriptions online. I learned that the traditional runnel is a narrow channel used to convey liquid. I recalled seeing such runnels at the sides of an outdoor stair. The stair treads were designed to shed water to the sides where the runnel ran at an elevation lower than the nosing run of the stair, to divert water to a safe accumulation area. The runnels, therefore, were not projections—like the stringers at the sides of the stair—but rather trenches into which a foot could slip. I answered that the runnel width needed to be subtracted from the nominal stair width so as to arrive at the clear width to be used in an egress capacity calculation.
Shortly after answering the question, I read a Web article on the popularity of bicycle stair runnels. The runnel, in this case, takes the form of an extra-wide stringer. The runnel creates a narrow ledge at an elevation higher than the nosing run of the stair. Bicycle users are able to ascend or descend the stair on foot while pushing the bicycle along the runnel. Had I known the difference between an elevated runnel ledge and a depressed runnel trench when I received the technical question, I would have asked the submitter for clarification before answering.
I expect some of you will visit various Web knowledge bases and search for “pan tread stair stringers” or “bicycle stair runnels” for images and clarification of these and other terms used in this column. Precision and clarity are important when asking a question and expecting a relevant and correct answer.
RON COTÉ, P.E., is lead engineer, life safety, at NFPA.