Roll-over shape is introduced as a significant characteristic of prosthetic feet. The roll-over shapes of the Flexwalk, Quantum, SACH, and SAFE prosthetic feet were determined using three methods; two involving quasi-static loading and one dynamic loading. The results show that foot roll-over shape properties obtained by quasi-static and by dynamic methods are similar.Relationships between foot roll-over shape and the alignment of trans-tibial prostheses are introduced that suggest ways to align trans-tibial prostheses without walking trials and iterations. The relationships may explain what prosthetists attempt to accomplish when they dynamically align a trans-tibial limb. They also explain why prosthetic feet with different mechanical properties usually necessitate different alignments, and may explain why a number of gait studies of trans-tibial amputees do not show major gait differences when walking is executed on various kinds of prosthetic feet.
When a user falls from a step ladder, the accident can be accompanied by damage to the ladder. A common area of damage is in the vicinity of the connections between the front side rails and the lowermost step. When determining the cause of a fall, it is important to understand how this damage occurs and whether it may be causal to the accident or a result of the accident. Commonly, engineers investigating such accidents have relied on three methods of structural analysis: classic analytical methods (so-called “hand calculations”), computational methods (finite element analysis) and laboratory testing. These three methods each have strengths and weaknesses that affect how the results should be used and interpreted by the investigating engineer. Factors such as the assumptions and simplifications used as input to an analysis, the type and amount of results available as output and cost are examined. These issues are discussed in the frame work of a case study wherein all three methods are applied to the analysis of a step ladder damaged in the field. The results show that, while step ladders may, at first, appear to be relatively simple structures they are, in fact, quite complex. As a consequence, it becomes very important to understand the analysis technique being used and its inherent limitations. Without consideration of these factors, the investigating engineer can be drawn to an incorrect understanding of the damage and its cause. This, in turn, may lead to an erroneous determination of the cause of the accident.
It is estimated that over 100,000 people each year are injured as a result of falls from ladders. Stepladders are one of the many different types of portable ladders, and are used frequently both on the job and around the home. Many times a fall from a stepladder is accompanied by a damaged ladder. In particular, one or both front side rails are often bent inward below the lowest step, and the lowest step buckled upward. This paper investigates the root cause of these types of structural failures through multiple analytical methods, including calculations, strain gage analysis, computer modeling, specialized strength and stability testing, and analysis of the user dynamics on the ladder. Results include data collected from various ladder makes (wood, aluminum, or fiberglass), sizes, and duty ratings (200 pounds, 225 pounds, 250 pounds, and 300 pounds). Additionally, the results were compared to the requirements of the existing ladder safety standards. The results demonstrate that the above described damage patterns do not occur when the ladder is upright on all four legs under normal use circumstances. Instead, the analyses conducted herein show that the damage results from the dynamic forces of the user impacting the ladder after the ladder has already substantially tipped over.
For extension ladders and other non-self supporting ladders, setting the angle of inclination of the ladder is a critical step in the set up process. The angle of the ladder influences: 1) the potential for a ladder to slide out at the base, 2) the overall strength of the system, and 3) the biomechanics of the climber/user on the ladder. A field study was conducted where over 120 real world ladder installations were documented, including angle of inclination, site conditions, and use. The average angle of inclination in the field study was 67.7 ± 4.9 degrees. Research was further conducted to assess the effectiveness of existing ladder labels for setting the ladder at the instructed 75.5 degree angle of inclination. 45 participants set up extension ladders following the "anthropometric" label (appendices of current ANSI A14 ladder safety standards), and/or the backward "L" label (historic ANSI A14 label). The average set up angle for the "anthropometric" label was 74.1 ± 3.2 degrees. This is consistent with calculations of the expected angle of inclination using anthropometric data. The calculated angles ranged from 74.9 to 75.2 degrees. The average angle of inclination for the backward "L" label was 72.7 ± 2.7 degrees. These results indicate that the current "anthropometric" label resulted in a set up angle consistent with anthropometric calculations and significantly higher than documented in real world ladder use.
Ladder accidents involving stepladders usually reveal damage to the spreader bars. This paper addresses the sufficiency of the present stepladder safety standards, design and testing requirements related to spreader bars. Spreader bars are the hinge members affixed to the sides of the stepladder that facilitate folding. Post-accident observation of buckled spreader bars or detachment from the side rails is frequently suggested as the cause of a user’s fall and injury. In addition to complete detachment at an end of one or both spreaders, several different bending configurations to varying degrees have been observed during accident investigations. These include bars bent into an “S” shape, bars bowed out/in, and bars with compound bending. In order to study these various post accident spreader conditions, stepladders of different size, weight ratings (i.e. types III (200 lb.), II (225 lb.), I (250 lb.), etc.) and material (wood, aluminum and fiberglass) were instrumented with strain gages in relevant locations to monitor stresses during normal use and misuse, as well as during various load tests and during live user falls from ladders. This extensive measurement experience of multiple loading configurations empirically demonstrated that spreader bar forces were minimal both in normal use, and even some circumstances of misuse. The resulting stress does not result in disconnection or deformation. Conversely, the loading of the stepladder structure that occurs in a tip over accident was observed to be more than sufficient to cause the frequently-identified post accident spreader bar damage patterns. On ladders that meet the applicable safety standards, all post accident spreader bar damage was found to be the result of the accident and not the cause.
Accident reconstruction involving consumer products and industrial equipment often requires biomechanical and/or human factors analyses to help determine the root cause of an accident scenario. A systematic method has been established which incorporates numerous components of the sciences of biomechanics and human factors and uses the scientific method as the framework for evaluating competing theories. Using this method, available data are gathered pertaining to the accident or incident and organized in a modified Haddon matrix, with categories for Man [person(s) involved in the accident], Product/Machine, and Environment. Information about the person(s) is separated further into injury and human factors components. The injuries are viewed as physical evidence, where each injury occurred as a result of being exposed to a specific combination of energy, force, motion/deflection, acceleration, etc. The injuries are evaluated with known injury research and categorized with a specific type, location, mechanism, and injury threshold. This injury evidence is then reconciled with the other physical evidence developed from the accident environment and product/machine categories. Human factors evaluations of body size, posture, capabilities, sensory perception, reaction time, and movements create similar information that is also reconciled with the rest of the evidence from an accidental circumstance. At the core of this method is developing scientific data or information that can be used to support or refute accident reconstruction conclusions. An accurate and complete accident reconstruction using the available data must be consistent with the laws of physics, and the physics of interaction between the man, product/machine, and environment.
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