The Effect of Open Area on the “Self-Cleaning” Characteristic of Vehicular Walking Surfaces 872286

A review of past and current codes, standards and specifications reveals that while “self-cleaning” is a desirable characteristic of vehicle access and walking surfaces, its importance and definition needs further study. A method is proposed to aid in determining if a step or walking surface can be said to be “self-cleaning”. This method attempts to relate the percentage of open area to whether foreign substances can readily build up on the walking surface and degrade friction between the walking surface and the sole of a shoe.

FEDERAL HIGHWAY ADMINISTRATION regulations require that “steps and deck plates shall be of a slip resistant design which minimizes the accumulation of foreign material. Whenever practicable self-cleaning material should be used” (49 CFR subsection 399.207(b)(3)). It is not clear how the term “self-cleaning” modifies the requirement that steps and deck plates shall be of a design that minimizes the accumulation of foreign materials. Does self-cleaning suggest an absolute standard of total self-cleaning? While the standards for minimizing accumulation of foreign material and “self-c1eaning” are not set out, intuitively these appear to be desirable characteristics of vehicle walking surfaces in as much as the accumulation of foreign materials such as snow or mud may cause slips and falls from vehicles. A review of past and current codes, standards and specifications reveal that the tenn self-cleaning is not defined adequately.

Many standards, codes and regulations use the term self-cleaning in reference to stairways, steps, walkways and general walking surfaces. Self-cleaning is often cited as an example of ways to make a surface safe for pedestrian and industrial foot traffic or to improve existing walking surfaces. Below are some examples of “self-cleaning” requirements and recommendations.

…Solutions for slippery metal steps include: improved housekeeping, the use of self-cleaning materials, and if possible, appropriate footwear with slip resistant properties… (1)*

…Tread surfaces of all walkways and platforms shall have high slip resistance as well as self-cleaning properties … (2)

…Step design should minimize the accumulation of debris and aid in the cleaning of mud and debris from the shoe sole… (3)

…The steps will be constructed of, or covered with, a self-cleaning safety material (ie., expanded or pierced metal, grating, etc.)… (4)

…In very few cases can shoe bottoms be designed to provide adequate slip resistance to foreign materials. Instead, walkway surfaces sbould be self-cleaning as they are used by pedestrians… (5)

The regulations that mention self-cleaning do not specify whether the self-cleaning property applies to the ability of a surface to automatically rid itself of foreign material or the action of the walking surface scraping foreign material off of the shoe bottom. In some cases, the inference is that the walking surface should do both! For the purposes of this paper, it was assumed that the “self-cleaning” property desired is that of the step being able to clean itself, ie. not allow the buildup of foreign material to the point of degrading or eliminating sole-to-surface friction.

For this paper, self-cleaning was defined to be “the ability of a surface to purge itself of foreign material under the worst expected design conditions”.

Design conditions and walking surface applications and needs vary widely, and at times may indicate that a non-self-cleaning surface is more desirable. To narrow the scope of this paper, a severe outdoor open exposure situation that emphasizes the need for a clean walking surface was chosen for examination. This application might be, for example, an unprotected step exposed to severe weather.