At Safety Direct America, we are experts in slip resistance (or coefficient of friction) testing. There is little information available in regards to what COF is required for different angles of pedestrian ramps.This blog entry discusses some published information on the subject of how much coefficient of friction is needed on various types of ramps and why.
The minimum safe coefficient of friction for a ramp, inclined surface, or slope depends on the angle from the horizontal as well as another factor if the pedestrian is controlling a separate load such as a wheelchair, shopping cart, etc: the ratio of the total weight of the load to the weight of the pedestrian.
For simplicity, common floor friction safety standards for ramps are “one size fits all” — they’re based on the assumption that all ramps require the same increased coefficient relative to a level floor, regardless of the angle of the ramp. Thus a three-degree ramp would require the same increase in friction coefficient as a seven-degree ramp, e.g. an increase from 0.60 to 0.80 for static friction or, for dynamic friction, from 0.42 to 0.45. This can place a restriction on small-angle ramps that’s more stringent than necessary.
Standards may also be based on the assumption that the pedestrian is not controlling a separate weight, such as a shopping cart, wheelchair, wheeled suitcase, or gurney. This is of course often a poor assumption. When a separate weight is involved a higher minimum coefficient of friction is required, whether the floor is wet or dry.
The way coefficient of friction is defined (friction force divided by applied force, or weight) means that the more weight that there is on the pedestrian’s shoes, the higher the friction force is. This works fine when a pedestrian is carrying something — for instance, two heavy shopping bags. The pedestrian gets added friction from the added weight on the shoes, which helps maintain safety despite the added weight that’s being controlled.
However, when added weight is not on the pedestrian’s shoes, but instead on a separate or wheeled load, the shoes don’t get the benefit of added traction. This means that the floor’s minimum coefficient of friction (COF) for safety needs to be higher.
How much higher? Well, an analysis published by Sotter, Stone and McCarthy (ISOES Sotter et al) in Proceedings of the International Occupational Ergonomics and Safety Conference indicates that the increase in the minimum COF is related to the ratio of the weight of the load to the weight of the pedestrian controlling the load. For a 110-pound nurse controlling a 220-pound load of patient + wheelchair, that ratio is 220/110 = 2.0. The paper’s Figure 2 (see below) shows that for a the maximum ADA ramp angle of 7.12 degrees and a weight ratio of 2.0, the safe coefficient of friction is 0.98, which is 0.38 higher than the assumed level-floor minimum safe COF of 0.60. In Fig. 2 the assumption is that the force exerted by the pedestrian is parallel to the ramp — probably a reasonable assumption for a wheelchair or shopping cart.
Should the force (push or pull) exerted by the pedestrian be upward at a 45-degree angle — perhaps true in the case of a manual pallet truck.. The required minimum COF is lower, other things being equal. This is because pulling the load upward exerts a downward force on the pedestrian’s shoes, increasing the friction.
Using a minimum COF that’s based on reasonable physics makes it possible to provide a safe environment for pedestrians while potentially making things easier on the designer, architect and property owner. The choice of flooring materials is much greater when an unreasonably high COF is not required. However, if loads are being controlled, such as in airports, hospitals and supermarkets, a significant increase in required coefficient of friction for the flooring might be needed.
For more information on the amount of coefficient of friction needed for various angled ramps, and for floor slip resistance testing on your ramp (either at your business or in our lab), contact Safety Direct America at 1(800)988-6721.