Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

4.5 Afterword

Given this conclusion, the Legal Aid Board – which funds many cases brought by injured claimants – decided the case did not stand a good chance of success if it did proceed to trial. The victim of the accident did not therefore receive compensation.

The moral of this forensic story is that initial perceptions of failure may not always survive the scrutiny of probing analysis. The investigator should always examine the circumstances surrounding a product failure. The fracture was just one piece of evidence contributing to a reasonable interpretation based on all the evidence. Ladder accidents are common, and a site visit is often essential to gather more evidence of the incident. In this case, the contact evidence on the wall was important for the construction of a working hypotheses to explain the accident. In addition, practical reconstruction of the accident using the failed ladder demonstrated stick-slip motion above the critical angle of repose, and helped explain the contact evidence. However, the answer to the problem lay not in the tips but the feet of the ladder, because it is the feet that play the most important part in the stability of ladders.

4.5.1 Ladder accidents

After car accidents, accidents involving falls are one of the most common causes of death and serious injury in the UK, according to the Department of Trade and Industry. The DTI monitors accident statistics for one common cause of falls, those from ladders, and the figures show 42 per cent of accidents involved the victim falling from the ladder. The ladder broke or collapsed in 16 per cent of cases, and the ladder fell while the victim was on it in 22 per cent of cases. The victim tripped over the ladder, caught fingers on the ladder and so on, in the remaining 20 per cent of cases. Wooden and metal ladders were about equally represented.

What can be done to lower the accident rate? Informing users of the potential dangers of ladders and the best way of using them. Indeed, new ladders have one of the most elaborate warning notices posted on any product (Figure 61).

Figure 61: Warning notice on stile of failure ladder advising user on precautions

Owing to the instability of lean-to ladders, they should be stabilised wherever possible, by tying the feet or tips to a solid support, or by having a colleague standing on the feet to prevent slippage. Ladders should never be used when the floor is wet owing to the lowering of the coefficient of friction, and the floor should also be level so that both feet are in contact.

Ladders must be inspected at regular intervals, and the critical feet and tips renewed when worn or damaged, a service many ladder manufacturers actually provide for free.

But can ladder design be improved so as to lower the chances of slippage accidents of the kind considered here?

4.5.2 Design of ladders

One of the problems of the accident ladder (Figure 54), is that it possessed fixed attitude feet, so that any deviation from a repose angle of 75° lowered the contact area with the ground. As the feet are the key to the stability of ladders, one solution is to provide feet that rotate as the angle of repose is changed (Figure 62). Then the full area of the feet is always in complete contact with the ground, so providing an extra margin of safety.

Figure 62: An alternative design: a ladder fitted with feet that stay in contact with the ground at various angles of repose

The design of the plastic tips and feet can also be improved for ladders of the specific design considered here. Although the tip broke during the accident rather than causing it, the design possesses several stress concentrators: one by design (sharp corners), the other from manufacture (internal void). Both could easily be eliminated, the first by simply smoothing the corners of the metal tools, the other by more careful control of the moulding process. Both procedures would increase the strength of the product substantially.

While the investigation did not support the claim against the manufacturer of the ladder, it did expose areas where this specific design could be improved, and also pointed to other designs of extension ladder that gave greater protection to the user. This is usually a common feature of many forensic investigations, simply because they probe not just the specific circumstances of particular accidents, but because they compare the design of products that either have failed, or appear to have failed at first glance, and suggest ways product design can be improved for future users. Product designers can therefore use the reports of such investigations directly to improve product performance. It can also aid bodies such as the Consumers Association in evaluating the effects of product design on accident statistics, so that the public can be made aware of the dangers and limitations of existing designs.

SAQ 16

List the way the ladder investigation progressed, stating the various questions that the investigation threw up, and which the investigator posed as a natural outcome of his analysis of the failure. What were the major lessons concerning the investigative procedure? What role did the analysis of the statics of a ladder leaning against a wall have on the investigation? Indicate the importance of external literature on the interpretation of the experiments.


The investigation developed as follows.

  1. User injured when ladder slips.

  2. Broken tip found at accident site, so user infers that ladder accident may have been caused by product fracture.

  3. Solicitor consulted by injured user of ladder.

  4. Solicitor commissions investigation and report.

  5. Investigator examines broken tip. (Questions thrown up: How did fracture occur? Where was origin? What features on the tip initiated the fracture?)

  6. Fracture analysis shows defects in moulded tip, but corroboration of accident conditions needed. (Questions thrown up: What corroborative evidence is needed? What was angle of repose of ladder? How high was sill from ground? Was the ladder extended or not? Any other trace or contact evidence from accident site?)

  7. Site visit to measure various distances and heights, inspect damage to wall, inspect for traces of contact of feet with ground, and so on.

  8. Contact evidence seems to indicate ladder tip may have broken when it hit a lower sill, so further research needed.

  9. Reconstruction of accident organised. (Questions thrown up: What was the coefficient of friction of the plastic feet? What was the critical angle of repose of the ladder? What was the weight of the user and on what rung was he standing when the accident occurred? How does the coefficient of friction of polymers change with loading conditions?)

  10. Experiments show ladder slipped at low angle of repose. Stick-slip motion demonstrated, so explaining marks on brick wall at user's home. (Questions thrown up: How did the abrasion damage to tips of ladders occur? How does this evidence fit with existing theories of the accident?)

  11. Re-examination of tips reveals both tips show abrasion damage, confirming tip broke after contact with wall rather than initiating accident.

  12. Results indicate user did not use ladder under recommended conditions, so Legal Aid Board refuse to support further action.

  13. Experiments also indicate improved designs for ladder feet, so that contact with ground constant at different repose angles.

In hindsight, the investigation showed the importance of corroboration. It was tempting to jump to the conclusion that because the tip fractured from the inherent defect of the void, the tip therefore caused the accident. The investigation showed the interpretation of one piece of evidence – the broken tip – is simply not enough to establish a case: all the evidence must be examined to construct a sequence of events. In this case, the evidence of abrasion to both the intact and broken tips could not support the initial hypothesis.

The examination of the stability of ladders was important in showing there was generally a critical angle of repose, above which the ladder was apparently stable, below which a ladder could slip uncontrollably. At or near the critical repose angle, instability could induce a stick-slip downward motion of the ladder. Experimental reconstruction of the accident showed exactly what angle of repose had been used, and how unstable the ladder was at this repose angle. Static analysis also showed the importance of the feet in providing a high coefficient of friction, and hence maintaining stability.

External literature showed that the coefficient of friction of polymers is not constant, as conventional mechanics suggests, but varies substantially and significantly with applied load. When a ladder has an adult standing on a rung, the coefficient drops, and the critical repose angle rises, making the ladder more unstable and hence increasing the risk of slippage.


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