Gaining Traction: Standards for Footwear

Updated standards currently in development will address issues related to protective footwear and walkway friction testing.
Jack Maxwell

For most people planning to buy a new pair of casual shoes, the most important factor is usually aesthetics — how they look. Functional considerations may come into play for activity-specific footwear like running shoes or winter boots, but product performance in areas like impact protection and slip resistance is generally low on the list of purchasing criteria.

These attributes are, however, of great interest to those individuals who earn their living in challenging work environments like factories and construction sites. Their shoes must protect against falling objects and encounters with heavy machinery, as well as deliver reliable footing in conditions that are less than ideal.

The members of ASTM International’s committee on pedestrian/walkway safety and footwear (F13) spend a lot of time thinking about these attributes, particularly as they relate to protective work shoes and boots. They also study the flooring products upon which people walk. The goal? Maximize traction, minimize slip-and-falls.

Safety Footwear

Sneakers, pumps, wingtips, sandals, ballet flats, loafers, clogs — the list goes on. There are many types of shoes out there. However, within this vast universe, there is a category that demands extra attention: safety footwear. It is here that the dedicated people who volunteer their time on the footwear subcommittee (F13.30) have traditionally focused their attention.

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The subcommittee was formed in the wake of the dissolution of the American National Standards Institute’s safety footwear technical committee. “The members involved in that committee, who created the safety footwear portion of ANSI Z41, moved to ASTM to continue their work, and published their first standards in 2005,” says Matt Piotrowski, a senior manager for product engineering at VFC. He co-chairs F13.30. “The subcommittee includes volunteers from safety footwear manufacturers and component suppliers, test laboratories, insurance agencies, academia, and end users.”

Since the publication of those first standards 18 years ago, the subcommittee has been busy, and currently oversees six separate standards.

The standard test methods for foot protection (F2412) and the specification for performance requirements for protective (safety) toe cap footwear (F2413) are the most widely referenced of these standards. Along with the specification for performance requirements for soft toe protective footwear (non-safety/non-protective toe) (F2892), they are in the process of being updated. Bill Ells, the incoming chair of ASTM’s Board of Directors, explains goals for the revisions. “We’re looking to add better clarity of sizing of test specimens, making certain that both men’s and women’s sizes are properly represented in testing, and that the end product has improved labeling. We’re also re-examining the test method as it relates to the static dissipative  function.” Ells is also a member-at-large of F13’s executive committee and vice president of sales at Vibram S.p.A.

The footwear subcommittee is also addressing the issue of metatarsal protection, in partnership with the University of Waterloo’s Biomechanics of Human Mobility Laboratory. “The university is in the second phase of a multiyear project that is utilizing cadaveric testing and computer modeling to better understand metatarsal protection,” says Ells. “The goal is to create an improved test method and performance requirement, built upon scientific biomechanical data.”

The final results of this research might not be available in time to be incorporated into the pending update of F2412, potentially requiring a midterm revision of the standard to reflect new information.

One of the six standards F13.30 is responsible for is the standard test method for determining the longitudinal load required to detach a high heel from the rest of the shoe (F2232). Lori Hyllengren, laboratory manager at Red Wing Shoes and footwear subcommittee co-chair, cites this as an example of their expanding purview.

“The committee has been predominantly focused on creating and maintaining standards for safety footwear, but we have recently changed our scope to include development of standards for all footwear,” she says. “For example, we are in the very early stages of developing a stand-alone water-resistance test method that could apply to general footwear in addition to safety footwear.

Additionally, members of the committee are involved in other global footwear committees and bring the latest developments from around the world to be evaluated and/or implemented in our
future work.”

Friction Measurement

The shoes we wear and the flooring surfaces we walk upon come together with every step we take. For a typical gait cycle, the sole of the shoe will contact the ground for between 0.75 and 1.5 seconds.

That’s little more than the blink of an eye, and the vast majority of the time these steps are uneventful. But when the friction between sole and surface is insufficient, falls can occur, and sometimes people get hurt. That’s why the work of the traction subcommittee (F13.10) is so important.

Numerous standards go into the design and manufacture of the soles many take for granted.

“This group is basically concerned with friction measurement techniques, devices, and methods,” says John Leffler, vice chair of F13 and chair of F13.10. “There’s also the subcommittee on walkway surfaces (F13.50) — those are the materials you test, while the traction subcommittee covers the devices you test with.”

The current focus of F13.10 is a suite of connected standards based around an in-revision F2508, currently titled validation of walkway tribometers using research-based reference materials. But what, many may ask, is a tribometer? defines it as: “A generic name for a device which is used to simulate friction and wear at the interface between surfaces in a relative motion under controlled conditions.”

The impetus for the thorough revision of F2508 is the challenge of correlating walkway friction testing carried out with a tribometer to the frictional requirements of a human being in motion. “Friction test devices don’t slip and fall,” Leffler notes. “We only really find the information from them to be useful in the context of how they can tell whether a surface is slippery for humans.”

Both around the world and in the U.S., different standards organizations have grappled with the relationship between friction testing and human friction requirements. But, as Leffler points out, there’s really no way to do a direct formula that links the friction measurable by a machine (like a tribometer) to the friction required by a human.

“When people take steps, they apply a certain force profile,” Leffler explains. “There are vertical forces and there are horizontal shearing forces that are resisted by the friction of the surface. If you have a little friction, you’ll slide. If you have a lot of friction, you won’t. A machine isn’t in a position to measure the same thing that humans require from the surface.”

A New Set of Reference Tiles

To be meaningful, walkway tribometer test results must be validated against the performance of humans walking over floor surfaces and contaminants that vary in slip potential. This is what the initial 2011 iteration of F2508 attempted to do.

The first version of the standard was based on the results of a University of Southern California (USC) research study designed to identify a set of reference surfaces, also known as RSs, that humans could rank and differentiate according to slipperiness (or lack thereof). Eighty young adults were split into four groups, and 20 people each walked across one of four RS types multiple times.

The RSs were set into one area of a gait analysis “walkway” in the USC lab. They wore motion-capture reflectors and fall-protection harnesses, and were warned that falling was a possibility. At some point during the multiple passes each human made along the walkway and across the RS, unbeknownst to the test subject, a coating of water was applied to the RS surface. The numbers of heel slips, toe slips, and “no slips” were then compiled to rank the slipperiness of the tiles.

Four RSs remained after many were eliminated for being too slippery, too grippy, or too similar. Leffler picks up the story. “The researchers found that most everybody slipped on one, 15 people slipped on another, seven people slipped on another, and nobody slipped on the fourth one. The F13 members did a tribometer study, where they operated a dozen different models to see if they would rank and differentiate the tiles in the same order the humans had. A few were able to, but the majority of them were not.”

This study served as the foundation of the original version of F2508. In order to determine if a particular tribometer was able to match the study results, the user would operate it on copies of the same four RSs (available from ASTM). If the results lined up, the tribometer achieved validation under the standard.

Though it was hailed as a technological advancement at the time, there were issues with the original USC research. Among them: inconsistencies in the actual RSs and variability in the walking speed of the subjects.

“The problem with many manufactured flooring materials is that their market has never demanded they be made with the consistency that a tribometer has the ability to measure,” Leffler says. “There’s never been a need for manufacturers to really buckle down and make products that are homogenous and consistent.”

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Coming up with a more consistent set of reference tiles was one of the primary goals of a similar USC study completed in 2021. Another addressed the walking speed issue. This latter objective was met by working with test subjects to maintain a pace of approximately 1.4 meters/second.

An Improved Standard

As of last November, Leffler’s hope was that balloting on the new version of F2508 would begin early in 2023, with final approval sometime later in the year. What might the revised standard look like?

One change is based on the results of the most recent USC study. Again, many candidate reference surfaces were evaluated in early pilot testing, and four were chosen for the study’s human testing. But the new set of reference surfaces that will be available from ASTM for tribometer validation contains three tiles. “The researchers found that humans weren’t really able to differentiate between two of the surfaces, and the tribometers weren’t either. So it was decided to eliminate one of them,” Leffler explains.

The revised standard will also incorporate adjustments in the statistical approach used to determine if a tribometer ranks and differentiates the reference surfaces correctly. Another important modification: increased emphasis on having the end user of a tribometer validate its performance.

“In the first generation of F2508, the supplier, distributor, or manufacturer completed this important task,” says Leffler. “But further research showed that the individual differences between how people operate tribometers and how they interpret the test methods can make a real difference in the measurements, and that can go to how well structured the testing method is, how clear it is to understand, and how easy it is to achieve repeatability and reproducibility.” (Reproducibility is a statistical analysis that allows you to determine how measurements with your device should be expected to compare with somebody else’s measurements using a different unit of the same device when testing on the same surface.)

It’s important to note that although the new version of the standard will encourage end users to validate their particular tribometers, it will still leave open the option of having suppliers do it. “However, by necessity, the statistical burden is higher, because we have to be looking at reproducibility versus the repeatability of when the user does it,” says Leffler.

Friction Ratings

The proposed changes to F2508, as useful as they will be, nonetheless represent only part of the story. “The larger question is about the traction subcommittee’s plan for its connected suite of standards,” Leffler says. “The need in the industry, and around the world, is for there to be a technically sound foundation for friction ratings of different flooring materials that allow end users to know if the flooring is going to be suitable for their application. There’s nothing like that right now.”

One reason for the absence of this type of rating system is the reluctance of private entities like manufacturers or distributors to attach their name to a friction threshold, because they could be found liable if someone slips on their product. Leffler believes these ratings must be developed through an organization like ASTM. “That way there will be the opportunity, through the consensus process, for people from across the spectrum of interest to provide information that should lead to a defensible standard.”

In Leffler’s view, the standard must establish a minimum level of friction (a “threshold”) for safe usage of a flooring surface, and it has to be tribometer-specific. “One thing the USC research and other studies have shown is that different tribometers get different measurements on the same surface,” he says. “That’s not unexpected, because different tribometers operate in different ways.”

“So there will be a tribometer-specific threshold standard in place,” says Leffler. “It will be something that provides the framework for a user to determine what the threshold will be for any particular tribometer that meets the statistical demands of F2508. It will also be important for any standards to accommodate both U.S. tribometers, which tend to be proprietary designs, and tribometers used around the world, such as the popular British Pendulum, which are not proprietary designs.”

“This is something that we have to take into account, because there’s no point in specifying a particular friction threshold or a particular friction level for a flooring material, and then saying: ‘OK, you’re good forever.’ It doesn’t happen that way,” says Leffler. “The ultimate goal is to have something that is usable for all common types of flooring material,” he says. “So that a design professional or homeowner can go into a store and see that this gets an A for friction, that gets a B for friction, the other gets a C for friction. So if C has average friction, maybe I don’t want to use this in my bathroom or kitchen. This one gets a B for friction, so it has higher friction but is not so coarse as to be uncleanable. This third one gets an A, it’s got tons of friction, but maybe it’s harder to clean.”

Leffler and his colleagues in F13 recognize that the suite of standards they’re putting together will never really be able to encapsulate all of the variability that occurs with human frictional demands, footwear frictional characteristics, and tribometer operation.

In the words of Bill Ells, “The pairing of shoe sole designs, the materials those soles are made of, the surface — whether it be commercial or residential flooring or exterior walkways — and the composition of those materials make for endless combinations. The goal for all involved is to work toward finding combinations of materials, test methods, and performance requirements that bring a level of confidence to the consumer. It’s the very process that brings together ASTM members representing so many stakeholders that makes ASTM standards as compelling as they are.” ■

Jack Maxwell is a freelance writer based in Westmont, NJ.

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