Expert Witness Blog

In Analysis and Testing In Accident Reconstruction, accident reconstruction expert witnesses at Technology Associates explain the nature of engineering analysis:

Thus, in the absence of reliable injury-data based on relevant accident records, the subject danger can be evaluated only by analysis and not by testing. Similar reasoning can be applied to many different cases, in spite of differences in detail.

Engineering analysis is not always as easy as in the above case. As an extreme example, consider the structural design of a skyscraper, which requires involved and sophisticated calculations (whether done by computer or otherwise). Here again testing is impractical, and prior experience is of little value unless gained from similar structures, which have been in use over an extended period of time. Thus, again the role of analysis is predominant.

The danger of falling arises when the unsuspecting climber shifts his center of gravity, causing the ladder’s elevated rear leg, to impact the ground. This is likely to occur when the user lifts one foot while stepping from one level to the next or shifts his weight while working. When this happens, large and rapid forces and the user’s overcompensating reflexes can cause him to lose his balance and fall….

Based on our research, Type II racking can easily lead to the…three-legged condition, even when a stepladder meets the present ANSI (Type-I) racking standard. Based on dynamic testing, a vertical rear leg lift of as little as 1 inch is sufficient to cause a ladder user to fall upon unanticipated crossover. This scenario is consistent with many accident investigations and offers a likely explanation for stepladder fall accidents when there is no obvious cause. We have found that the minimum leg lift-off required to cause a ladder user to fall upon crossover is less than 1 inch and also depends greatly on the agility of the ladder climber.

Seat belt and airbag expert witnesses at Technology Associates describe “whiplash”:

Testing has shown that the maximum loading to a rear-ended car was amplified about two and a half times when it reached the heads of the occupants. The testing also revealed that this occurred about a fourth of a second after impact.

The momentum and loading to cars which are involved in a rear-end impact (of low enough impact velocity so that there is no permanent deformation of the bumpers) can be fairly accurately modeled as a mass-spring system. This enables determination of the loading effects on the cars and heads of the occupants, by input of known quantities (masses of the cars, bumper stiffness, relative velocity between the cars at time of impact).

In Analysis and Testing In Accident Reconstruction, accident reconstruction expert witnesses at Technology Associates explain the nature of engineering analysis:

Straightforward as the above reasoning is (see 7/16/09 blog entry), it nevertheless constitutes a valid (though simple) example of engineering analysis. Now let us consider what it would take to demonstrate the defect of the steps by testing rather than by analysis. To do this, there must first be devised a suitable test procedure, and this can be arrived at only by further analysis-which is another word for organized and systematic thinking with relevant technical considerations taken into account. From this analysis, there emerged the following requirements:

1. The tests must be done with different subjects, who must not know they are being tested and must not observe each other performing the descent of the stairs-else their performance will be affected, and so will not represent “normal” descent of the stairs by an unwarned person.

Ladders and scaffolds expert witnesses at Technology Associates write on stepladder instability:
Three-leg contact can develop under a number of situations such as set-up on an uneven surface or when climbing, sliding, pivoting or “walking” a flexible ladder along the ground as the user’s work progresses. Dr. John Morse has cited a subtle type of unperceived three-leg stepladder contact named “Type-II racking”, which occurs during climbing as follows.

After the climber has one foot on the floor with his other foot on the first step, and one or both hands on the front rails at chest height, he pulls himself upward with one arm and attempts to keep his body straight. This imposes a torque about a vertical axis to the ladder. This torque, combined with the climber’s pulling force (which is necessary to raise his weight to the next step), tends to unload the rear legs. With these legs offloaded, and while this torque is still applied, the ladder twists or “racks” in the direction of the applied torque. When the climber’s foot then leaves the floor and reaches the first step, weight is shifted back onto the rear legs of the racked (slightly twisted) ladder. When this occurs, only one of the ladder’s rear legs can contact the ground. If this goes undetected, the climber has unknowingly created a three-legged ladder and the potential for instability, should center of gravity diagonal-crossover occur later after subsequent climbing or use.

Seat belt and airbag expert witnesses at Technology Associates describe “whiplash” injuries:

Unfortunately, the effects of whiplash are often downplayed, and its sufferer thought to be malingering, on the grounds that injury isn’t visible. In addition, experiments have shown that the forces to the neck during whiplash are not much greater then those occurring during normal activities (e.g. “plopping down into a seat”, “hopping onto a step”, and even “sneezing”). However, unlike whiplash, normal events do not take a person by surprise, so one can instinctively brace the neck muscles in anticipation, and control the force transmitted to the cervical soft tissues. With whiplash, the force to the neck is violent and sudden, and is not filtered through the neck musculature. Hence, those with thinner or weakened necks (i.e. women and those who have had prior neck injury) are more prone to the effects of whiplash, which can occur from an impact to the car as low as 3G’s.

A problem facing investigators of a whiplash case is that the impact velocity of the striking (rear) car is typically not known with certainty, and this value is needed for determining resulting forces. A conservative estimate of the speed can be surmised by using the damage threshold of the cars’ bumpers (because whiplash injury is caused by low speed impacts involving no (or minimal) damage to the bumpers; hence most of the shock is transmitted to the passengers’ necks). Testing has shown the damage threshold of bumpers of many cars to be about 5 mph; thus lash forces to the neck based on a maximum 5 mph impact velocity to the struck car. However, most crash testing involves the car impacting a rigid barrier, which does not yield in any way, rather than a relatively flexible bumper of another car. Hence, the crash testing can be more severe than an actual impact with another car, and can, in fact, be equivalent to the car’s being struck with another car at up to twice the velocity used for the barrier test.