Slip and Trip Type Falls

LEVEL WALKING

After years of practice, most people learn to walk while giving the walking task very little conscious thought. The difficulties of normal walking on level slip resistant surfaces are largely overcome in early childhood by learning, practice, and experience, coupled with physical growth and development.

Although walking may at first be characterized as merely putting one foot in front of the other, human walking is an extremely complex locomotor activity. The human body has a relatively high center of gravity combined with a relatively small base of support, producing a rather unstable object in terms of fall prevention. To remain upright, one must maintain the center of gravity over and within one's base of support. If a slip or other misplacement of a foot moves the center of gravity outside the base of support, a fall will occur.

During the walking process, as the center of gravity moves forward, weight and balance are continually being transferred from one foot to the other. As each foot contacts the walking surface, and at every other phase of the walking cycle, coordination of muscle and bone, weight and balance, are continuously and precisely adjusted and timed to prevent the loss of stability. At the same time, the user of various walking surfaces must contend with a variety of surface characteristics such as surface friction, slope, texture, and subtle protrusions. Physical characteristics of walking surfaces may assist people in maintaining stability by offering design features which are compatible with human needs, or they may seriously hamper stability and create conditions which are clearly or deceptively dangerous.

Analysis of how people walk has shown that when walking, the forward leg slows at the termination of its swing and then appears to bound vigorously onto the walking surface, instantly beginning to support body weight. As the forward foot is initially placed on the walking surface, it contacts the surface so that only the rear edge of the heel strikes first. At the same time the trailing foot remains in contact with the walkway, continuing to bear part of the load until the forward heel rocks forward, gradually accepting the full load as the entire foot contacts the walking surface.

During the walking process, the upper body's center of gravity moves forward at a fairly constant rate, its support being continuously shifted from leg to leg. More than this, the upper body actually "commits" itself in a forward direction and then relies on the successful movement and proper placement of the forward leg in order to maintain (regain) support and prevent a fall. Walking can be thought of as the process of repeatedly losing and gaining body support by one leg and then the other, with one leg taking over support of the upper body as the other leg begins to lose support.

Balance of the upper body during walking activity is also a dynamic process. Not only is balance of the whole body being shifted from leg to leg while walking, such balance must be continuously adjusted on the stationary leg during the time of opposite leg swing.

SLIP TYPE FALL AND THE IMPORTANCE OF SURFACE FRICTION

A major characteristic of a walking/working surface that impacts on user performance and safety is the coefficient of friction (COF) of the surface material. Coefficient of friction is used as a measure of slipperiness. Coefficient of friction is the ratio of the force required to move one surface over another, overcoming friction, divided by the vertical or normal force pressing the surfaces together. Thus:

COEFFICIENT OF FRICTION =

HORIZONTAL FORCE WHEN SLIPPAGE BEGINS / VERTICAL FORCE

The horizontal component of the force exerted by the forward leg on a level walking surface during the normal walking process reaches a maximum in the forward direction shortly after the heel makes contact with the walkway. This horizontal component is the force that must be counteracted by the friction characteristics of the walking surface in order to avoid slipping. A "slip" occurs when shoe sole to surface friction is not great enough to overcome horizontal component forces.

The generally accepted COF standard today relative to slipperiness is that a static coefficient of friction of 0.50 or above is relatively safe on a dry surface. A COF reading below 0.50 indicates an unsafe surface in terms of friction. While only select coefficient of friction measuring devices are reliable when used on wet surfaces, measurement is typically unnecessary regarding common (Class 2) surface materials (such as vinyl and ceramic tile, marble, smooth troweled concrete, and other such common or similar flooring materials), which are well recognized as being moderately slip resistant when dry, but inherently slippery when wet.

The coefficient of friction of a walkway/working surface is a function of many factors including the surface material used, its finish, its characteristics of wear, the presence of foreign material, and how it is maintained.

Some of these factors may change over time. A change in the coefficient of friction may occur as a result of surface damage, due to cleaning and maintenance techniques or a lack of cleaning and maintenance, or from exposure to environmental conditions. Wear, or the introduction of foreign material on the surface, will also change its coefficient of friction. In many cases, a buildup of sand, fine dirt, water, and general "street grime" can act as a lubricant on some surfaces to make them extremely dangerous.

WALKING/WORKING SURFACE MATERIALS

Primary considerations in walking/working surface materials with respect to fall protection are slip resistance, resistance to the harboring of contamination that would affect slip resistance, and the relative ease of cleaning when contaminated to restore slip resistant characteristics.

The slip resistance of a walking or working surface depends largely on its surface characteristics (how rough or smooth the surface material is).   On wet surfaces, the slipperiness (slip index) of a walking surface is a function of the ability of surface projections (call asperities) to penetrate through such water (or other contaminants) to grip a persons shoe to prevent heel-slid.  The sharpness of the asperities is the primary factor in slip resistance.  

Surface materials can be divided into three classes: (Class 1) "slippery," such as smooth-surfaced steel plate and other similar smooth metal surfaces, freshly painted metal or concrete, polished concrete, polished marble, or polished ceramic tile, (Class 2) "moderately slippery," such as ordinary vinyl tile, marble, relatively smooth ceramic tile, smooth troweled concrete, polished or smooth painted wood, and other such common and relatively smooth flooring materials, and (Class 3) "non-slip," such as rough troweled cement, textured rubber, sharp-edged diamond plate steel, sharp edged self-cleaning metal grating, and surfaces where high abrasive (paint or grit) treatments have been applied.

The class of slippery surface materials (Class 1 surface materials) must be considered slippery and hazardous when they are wet or dry. The best remedy for such slippery surfaces is to replace them with safe ones (Class 2 or 3). However, in some cases it is enough to treat them with abrasive applications such as abrasive paint, a special abrasive coating, or adhesive abrasive strips. When these methods of control are used, appropriate inspections of the surfaces should be made on a timely basis to repair or replace abrasive surface treatments as needed.

The class of moderately slippery surface materials (Class 2 surface materials) is considered acceptable in terms of slip resistance when clean and dry, but must be considered slippery and hazardous when wet. It is not uncommon for the coefficient of friction of Class 2 walking surfaces to "drop" by a factor of .08 to .12 when wet. It is therefore extremely important to provide Class 3 flooring surfaces where moisture may occasionally be present, and to keep Class 2 surfaces free of water, grease, oil, food, sawdust, soap, and other foreign materials.

Spills on Class 2 surfaces must be cleaned up promptly and use of the surface restricted until dry. Replacement of Class 2 surfaces with a safer (Class 3) material or treatment to make it highly abrasive is recommended when spills are not likely to be detected immediately, and are not followed by prompt clean-up or barricading of the spill area.

That is, if it is reasonably anticipated that a Class 2 flooring will not remain dry under all reasonably foreseeable conditions of use, a Class 3 floor material is warranted (or the proper use of matting must be utilized).

The class of non-slip surface materials (Class 3 surface materials) affords relatively good footing even under adverse conditions and may be considered acceptable in terms of slip resistance even when wet. Nevertheless, every effort should be made to keep such surfaces free of foreign materials such as oil, grease, mud or algae, which can make even the best of slip resistant materials slippery. Taking into account that the coefficient of friction of a dry surface will be significantly reduced when the surface is wet, a dry coefficient of friction well above 0.50 (such as 0.65 to 1.00, depending on circumstance) is required for this surface class.

 Special note:  A "concrete" walking surface can fall into any one of the three classes of flooring depending on the finish given to the surface.  "Racked" or "coarse broom" finishes of relatively dry concrete mixture can produce high slip resistant Class 3 walking surfaces.  A "rough" (or minimally) troweled finish of relatively dry concrete pourings can produce a minimum but acceptable Class 3 surface where only water is anticipated to be present.  Typical finishing (troweling) of relatively wet mixture of concrete will commonly produce a Class 2 walking surface.  This type of surface can be acceptable if located indoors and away from potential surface moisture.  Finally, relatively wet mixture of concrete with an excessively smooth troweled finish, can produce a Class 1 walking surface, which can be inherently slippery for some shoe sole materials and "treacherous" in terms of slipperiness in the presence of the slightest moisture.

 Special note:  "Diamond plate" is not magically slip resistant because the word "diamond" is used in its name or because the visual pattern of the surface resembles a diamond shape.  Rather, true diamond plate is slip resistant because it has "sharp" edges" that "cut" into the shoe sole material to hold a person's foot in place.  If the mineral we call "diamond" is compared to "diamond plate," it is vital to keep in mind that there are both "cut" diamonds and "uncut" diamonds, as well as slippery diamond plate and slip-resistant diamond plate.

 Depending on its degree of sharpness, diamond plate can actually fall into any one of the three walking surface classes:  Class 1, such as diamond plate which has raised "diamond shaped" sections that, rather than being sharp-edged (due to design, wear, or the misuse of paint) are smooth and rounded or have their sharp features recessed or positioned where they have no effect on slip-resistance (or worse, provide an inherently slippery surface); Class 2, such as "ordinary" diamond plate that may be moderately sharp (which also means moderately slippery); and Class 3, which requires highly raised, accessible, and very sharp edges to be incorporated into the diamond shaped surface components.  It must be noted, however, that the most slip resistant diamond plate is less slip resistant than most sharp edged metal grating.

WALK-OFF MATS

Fall prevention mats are designed to provide a slip resistant walking surface in areas where moisture or other contaminants are likely to be present and result in a slippery floor.  Such moisture or contamination may come from nearby equipment or wet processes, or be transferred from other locations by pedestrian traffic.

Probably the most critical area for initial slip and fall prevention in buildings is the first 15 to 20 feet inside the entrance.  Walk-off mats placed in these entrance areas can be a very effective fall prevention measure.  Walk-off mats are designed to remove debris or moisture while providing a slip resistant walking surface.  Moisture absorbent matting should be considered for use at all building entrances and near all wet processes and equipment.

In wet weather, the necessity of using walk-off mats at building entrances to remove pedestrian traffic debris and moisture is self-evident.  In dry weather, the use of entrance matting can also have a positive affect on the prevention of falls.  In wet or dry weather, entrance mats will allow persons time to initially scan the eye-level interior environment in order to perceive the nature of the interior surroundings while walking on a relatively safe surface material.  Further, entrance mats can remove and collect "tracked in" debris that will soil interior flooring.  The removal of debris before it can soil interior floors will reduce the need for periodic cleaning.  Liquid cleaning materials, along with other factors, can increase the risk of persons experiencing injury producing slip and fall type mishaps.

Debris removal mats (typically placed outside building entranceways or in intermediate foyers or vestibules) are made of coarse material to clean dirt and other materials from pedestrian shoes as they enter buildings.  This type mat should be at least 4 feet long so the bottom of a person's shoes will make contact with the mat at least once.  Longer debris mats can only provide added benefit from this minimum

Moisture removal mats (typically placed immediately inside building entranceways) are made of an absorbent material to dry moisture from the bottom surface of people's shoes before it can be tracked to floor surfaces that become hazardous when wet.  A moisture removal mat should be long enough so the bottom of a person's shoes will make contact with the mat at least twice (four or five normal steps or strides).  Typically, the moisture removal mat would need to be at least 12 feet long.  Longer moisture absorbent mats of 15 to 20 feet in length can only provide added benefit from this minimum

In addition to building entrances, moisture removal mats can be an effective safety device in the interior of buildings where floor moisture or other slippery substances may potentially be encountered.  Such interior locations might include food display or service areas, drink dispensing areas, plant and flower display and service areas, areas near drinking fountains or ice machines, areas adjacent to refrigeration units, and areas where spills from relatively slippery sales or production products can be reasonably anticipated.

High moisture content produce in grocery stores, automotive and other products that contain various forms of oil, and various granular or spherically shaped powder products (to name but a few) are particularly hazardous.  Mats to protect against falls caused from the presence of such products must be large enough to contain the products when they spill and prevent them from being tracked to unprotected floor areas.

When debris or moisture removal mats become saturated, they should be removed and replaced immediately.  A saturated debris mat no longer removes debris, and a saturated moisture mat (with an excess of moisture and other contaminants) can be as hazardous as no mat at all.

THE "TRIP" TYPE FALLS

On any walking surface, a trip type fall occurs when an object, unevenness, or other characteristic of the walking surface prevents or even momentarily delays the trailing leg from swinging forward to achieve a timely and accurate positioning of the foot on the surface ahead for proper body support and balance at the critical moment of contact. Since the process of walking requires split second timing in the transfer of support and balance from leg to leg, the slightest disruption of this process, or the rhythm of this process, can produce imbalance of the upper body and result in a fall produced as the upper body continues to move forward, beyond a base of support, according to the law of inertia.

The trip and fall type accident has been well recognized in the safety literature and through common experience. Not only have large objects been recognized as tripping hazards but small objects have been recognized as particularly dangerous due to their relative "hidden" nature. Traditionally noted small tripping hazards include loose floor boards, protruding wall molding, improperly sloped rug moldings, non-flat or turned-up floor matting, electric extension cords, raised floor sidewalk sections, bent floor plates, and uneven floor cracks or repairs.

Tripping hazards are particularly difficult to detect when potential victims are engaged in any task or activity that involves their attention away from the specific location of such hazards.

INSPECTION AND MAINTENANCE

Because of the frequency and potential severity of falls, walking surfaces must be inspected on a systematic and timely basis. How quickly a defect or unsafe condition can develop will dictate the frequency of inspection. Discovered defects must be repaired or barricaded immediately.