Tribometers and Coefficient of Friction
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People slip, trip, and fall on walkways, sidewalks, paths, parking lots, steps, and stairways for many reasons. Among the more common reasons are clumsiness, a slippery surface, not paying attention, and defects in the surface or steps where the person fell. Many people walk in places where there are defects, but not all slip, trip, and fall injuries occur because of negligence.
This is not a criticism of the concept of coefficient of friction measurements or a comment on their value when properly used. This discussion purely applies to the widespread misuse of coefficient of friction measurements in evaluating slip and fall incidents. This is not applicable to properly made slip resistance measurements used, for example, by flooring manufacturers.
Coefficient of friction is the ratio of the force of friction between two bodies and the force pressing them together. Static friction is the friction force when an object is not moving. Static friction is the force that prevents a block from sliding down an incline. Kinetic friction (Sometimes called dynamic friction) is the force when two objects are moving relative to each other. Generally, the kinetic coefficient of friction is less than the static coefficient of friction.
Coefficient of friction and slip resistance are often used interchangeable. Although they are related concepts, they are not identical. Slip resistance is what one is looking at in slip and fall incidents.
A slippery surface can cause a fall. Years ago, a block of a known weight was dragged across the surface to measure the static slip resistance. However, slip, trip, and fall incidents occur when people are walking, so the dynamic slip resistance appears to be more appropriate.
A tribometer is a device for measuring tribological quantities. Tribology is the science of the interaction between surfaces in motion relative to each other. Tribology includes the application of the principles of friction, lubrication, fluids and fluid flow, materials properties. In slip and fall incidents, human dynamics and human reaction to a perceived loss of friction also play a roll. Tribology includes mechanical engineering, materials science and engineering, fluid flow, physics, and chemistry. When analyzing slip and fall incidents using tribology, human factors must also be included.
Various methods of measuring dynamic slip resistance. have been devised and some have been accept by Courts. In many cases, these measurements are made by self-proclaimed safety experts with no Engineering background. The device used to make these measurements is often referred to as a tribometer (sometimes called a slipometer), although there are other trade names by which a tribometer is called. In some cases, Courts and Insurance companies have accepted these measurements blindly, and used them to decide negligence or pay a settlement.
Tribometers do not slip and fall. People slip and fall. It is important to remember this when looking at tribometer slip resistance measurements. There are many problems with blindly taken dynamic slip resistance measurements:
- There is no objective standard for measuring slip resistance on existing steps, paths, walkways, floors, driveways, etc. Standards exist for the measuring slip resistance of manufactured flooring, and these standards have been misapplied to existing construction.
- Many people do not know exactly where they fell, or where their foot was located at the moment before they fell. The measured slip resistance can be different at different points on a surface.
- The measuring devices often do not accurately reproduce the conditions when the person fell. The operator of the measurement device often cannot accurately reproduce the motion of the person's foot before the slip, trip, and fall incident.
- The slip resistance measurement can be influenced by the person taking the measurement. All surfaces have some deterioration. Based on irregularities in the surface, one can set up the tribometer to show a higher or lower slip resistance measurement. In a Court trial, there may be two experts who testify to significantly different slip resistance measurements on the same surface.
- The actual slip resistance depends on the person's footwear. Often, the measurement is taken with a generic material.
- Because of irregularities in the surface, the size of the contact area is critical. Someone's shoe may be contacting different sections of a surface with different slip resistance measurements.
- Many people slip on wet surfaces, so it has become a common practice to pour water on a surface and measure the coefficient of friction. The quantity of water and impurities in the water can influence slip resistance. Spilling a cup of water on a surface does not accurately reproduce the conditions that existed when a person fell. All that is often determined is that a wet surface is more slippery than a dry surface.
- The tribometer does not accurately measure the slip resistance on a wet surface even if the exact liquid and floor conditions can be reproduced. This is because the dynamics of the tribometer on a liquid surface are considerably different than the dynamics of a person's footwear.
- Slip resistance measurements do not take into account hydroplaning-type phenomenon. (Hydroplaning and aquaplaning are generally reserved for the behavior of rolling objects like tires). Hydroplaning occurs when a film of water builds up between a tire and the road, causing the tire to loose contact with the road. A similar phenomenon occurs when there is a layer of water between shoes and a floor.
- Slip resistance measurement ignores the fact that when a human being walks, it is a feedback process. The human gait is a closed-loop feedback system, where the brain makes adjustments based on expected conditions and conditions found. Measuring the dynamic slip resistance ignores the feedback part of a human's walk.
In some cases, measurement of slip resistance (often by non-Engineer self-proclaimed safety experts) has replaced an evaluation of the defects and cause of the slip, trip, and fall accident by a Professional Engineer. Just measuring the slip resistance, without determining why the person slipped and fell, is bad engineering.
The Professional Engineers of Heimer Engineering do not use questionable slip resistance measurements in evaluating the cause of a slip, trip, and fall injury. Rather, actual defects in the design or maintenance are reported. In many cases, these defects are code violations that provide objective evidence as to the cause of the slip, trip, and fall injury.
Building Codes and
It is important to accurately determine which building codes (and other codes, regulations, and standards) are applicable to a specific incident. When a building is constructed, it is constructed under the codes, regulations, and standards are in force in the municipality at the time of construction.
Heimer Engineering has
an Extensive Library of
Historical Building Codes
Codes, regulations, and standards change over time, and modifications are made to existing buildings and sites. It is important to examine the history of the building to determine what codes, standards, and regulations are applicable. The Engineer's report should be based on applicable codes, regulations, and standards.
The term "grandfathered" is often applied to existing construction. Saying a building is "grandfathered" is often inaccurate. The term "grandfathered" only applies to original and unaltered construction. "Grandfathering" requires that no changes were made without required permits and approvals. Once a change is made to a building, newer building codes, standards, and municipal regulations become applicable. For example, making a change to the interior of a building might require that exterior steps be brought up to modern codes.
Heimer Engineering has an extensive library of searchable historical building codes, dating back 90 years for The State of New York and over 140 years for The City of New York. We also have electronic access to hundreds of current state, town, and village codes.
Just because a building
complied with the applicable
codes at the time
constructed does not mean that
hazardous defects are permitted.
Even if the building or site is unaltered, it may not have complied with the codes, standards, and regulations in effect at the time the building was constructed. Many older codes, standards, and regulations are not as clearly written as their more modern counterparts.
The Engineer needs to consider both the wording and the correct application of the code, standard, or regulation. In addition, the injury may have been caused by faulty maintenance, which is covered under modern codes.
Just because a building complied with the applicable codes at the time it was constructed does not mean hazardous defects are permitted. Failing to maintain a building or site in a safe condition is never "grandfathered".
Proximate cause is often defined as the immediate or closest cause. When examining a site or building where a personal injury accident occurred, it is important to determine the proximate cause of the accident.
We have seen many reports where defects were cited, but are irrelevant to the accident that caused the injury. For example, if a client fell at the top of a set of steps, defects near the bottom of the steps are irrelevant.
Unfortunately, there are many experts who look to find any defect and never determine if it is the proximate cause of the accident. Heimer Engineering only includes relevant facts in the reports. Engineering reports that include irrelevant code violations may look good, but these reports can be defeated in summary judgment because the cited defects are irrelevant to the incident.
De Minimis Defects
Who decides if a defect is de minimis? Is it decided by the Attorneys, the law, or the Engineer? Engineers need to take into account whether a defect is de minimis in determining the proximate cause of an accident and subsequent injury. It a Professional Engineer determines that a defect is the proximate cause of an injury, then it is axiomatic that the defect is not de minimis.
Foreseeable Incidents and Reasonable Care
The cause of injury is often the defendant's failure to use reasonable care in the maintenance or repair of a building. Engineers need to look at the site conditions, and determine if the owners created a hazard by failing to use reasonable care. It is also important to determine if it is foreseeable that the condition could cause an injury.
"Old School" Engineers
After World War II, Engineering moved from war-related work to developing products for consumers and of national interest (such as the space program). There were few universally accepted standards other than building codes and mechanical standards (for example, screw sizes or wire sizes). Many Engineers who graduated before the mid-1970s were used to creating their own standards, and possibly changing these standards from project to project. These "Old School" Engineers worked according to rules and standards created on an as-needed basis.
Although organizations such as the American National Standards Institute (ANSI) began developing standards after World War I, many of these standards did not aquire the force of law until the 1960s when were incorporated into Building Codes. In addition, organizations such as the Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHTSA) set and enforced standards. At the same time, organizations such as ANSI and the Consumer Product Safety Commission (CPSC) set standards that were universally followed.
Some of the "Old School" Engineers retired from their full-time employment and went on to be consulting Engineers. The problem is that they continued to analyze the way they worked in the past. These Engineers approached accident investigations the same way they had worked their whole life. The result was reports that reflected the Engineer's standards or the Engineer's interpretation of Building Codes without regard to the realities of how the Department of Buildings interprets codes or accepted practices.
An example of the difference between old self-created standards universally accepted standards can be seen in the change in automobile safety in the last 50 years. In the late 1950s, automobile manufacturers began to talk about how safe their cars were. The standards for safety were set by automobile manufacturers. General Motors stated their cars had a "safety X-frame" in advertising. People bought these "safe" cars.
The Insurance Institute for Highway Safety released a video in 2009 that showed a frontal offset collision between a 1959 Chevrolet Impala and a 2009 Chevrolet Malibu. Analysis after the collision showed that the driver of the 1959 Impala would have died instantly, and the driver of the 2009 Chevrolet Malibu might have sustained a knee injury.
This video illustrates the difference between self-defined standards and universally developed and accepted standards. By the self-defined standards of 1959, the Impala was "safe". By today's universally accepted standards, the 1959 Impala offers little safety.
When choosing a Professional Engineer, be careful of Engineers who define their own standards. While they may sound good, they will hold up no better under cross-examination based on today's standards than the 1959 Chevrolet Impala held up against the 2009 Chevrolet Malibu.
Engineer Experts or Hired Guns?
Some expert witnesses are "hired guns". These experts provide a report in favor of their client regardless of the facts. Heimer Engineering's Engineers never exaggerate or embellish their findings. If there is no defect, that is what we report. There are Attorneys who will never use Heimer Engineering's services because we do not embellish the reports in favor of their clients.
Heimer Engineering never
exaggerates or embellishes
Some experts do whatever the Attorney asks them to do, and never bother to evaluate the situation. Heimer Engineering evaluates the information provided and advises, based on Engineering experience, as to what defects exist and the cause of the accident. As an Attorney, you will be provided our professional opinion so that you know how to proceed with the litigation.
Embellishing a report or relying on a code reference that is not applicable sometimes works if the insurance company settles based on an exaggerated report. In other cases, the expert's statements in the inaccurate embellished report cannot stand up to summary judgment or under cross-examination in a Court of Law. With Heimer Engineering, you can be sure you are receiving a professional opinion based on the facts as provided to the Engineer. Heimer Engineering will be able to defend the Engineer's report under cross examination.
Heimer Engineering never
codes that are not applicable
Accident or Incident
Accident has no clear meaning. An accident could be an injury caused by a property owner's negligence. An accident could be a mistake by the injured party. An accident could be caused by a contractor's negligence. There are many potential caused of an accident.
Heimer Engineering's Professional Engineers evaluate incidents. Someone is injured, and that is the incident. It is only after the incident is evaluated that negligence can be established.
When an incident is evaluated, there is no pre-conceived notion of who is negligent. The incident is evaluated from an unbiased point of view, including the application of Engineering principles. Only after all the facts related to the incident are determined can negligence and blame be determined.
Expert Witness Court Testimony
Clearly written reports help make it clear how defects at a site contributed to an accident and injury. Clearly written accurate reports can help you settle a case and avoid the time and expense of a trial.
If the case has to be litigated in a court of law, Heimer Engineering's Professional Engineers serve as expert witnesses, and have qualified as expert witnesses in United States Federal Courts and at all levels of State of New York Courts.
For questions regarding Professional Engineer expert witness services, call 631.820.4500.
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Heimer Engineering℠ performs home inspections, building inspections, condominium inspections, and co-op inspections in the State of New York. We do not perform inspections or recommend Inspectors or Engineers outside of the State of New York. We provide Licensed Professional Engineer consultation services including hurricane and storm damage and damage from adjoining construction. Expert witness services are provided regarding playground injuries, parking lot, walkway, and stairway slip, trip, and fall.
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