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6 A problem with sensors

The problem we will look at in this section concerns the analysis of the design of a component used in cars that are fitted with airbags. The airbag has to be inflated rapidly when an electronic circuit in the system decides that a serious collision is taking place. The crucial component in the electronics is the accelerometer, which therefore has to be extremely reliable. Motor manufacturers have turned to a technology called MEMS (micro-electromechanical systems) for these accelerometers, b
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4.5 Model the best solution

In moving from the 'possible solutions' to the 'best solution' box, Figure 12, we have to assume that a certain amount of evaluation has been done in the previous loop. The solution is still on paper, and probably not much more than a sketch, but something is badly wrong if the best solution to come forward has n
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4.4 Evaluate solutions

If the obvious solution has been identified and everyone is in agreement, then a formal evaluation of solutions is unnecessary, and we would move on to modelling the design. However, if there is dissent then some stricter method of elimination is required, and this is usually achieved through a process of rank-ordering. There is little to be lost and potentially much to be gained by returning to the customer at this point for opinion, clarification or guidance.


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4.1 Advancing knowledge

Over the centuries, engineers have faced and solved a huge number of problems of one sort or another. Each time a problem is solved, knowledge is advanced, something usually gets written down, and so today we have a wealth of experience to draw on. Equally, problem-solving techniques have also been developed and evolved through use and refinement, which is rather handy. Not only do we have some idea of existing solutions to similar problems, but we also have an indication of how to go
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Conclusion

This free course provided an introduction to studying Engineering. It took you through a series of exercises designed to develop your approach to study and learning at a distance, and helped to improve your confidence as an independent learner.


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6.3 Tacoma Narrows suspension bridge failure

Such over-design could not be sustained for long and bridge designers gradually pared back their margins of safety. There is elegance and economy in having the lightest structure compatible with function. But history has a habit of repeating itself.

In 1940 a new suspension bridge with a central span of 2800 feet was built over the Tacoma Narrows in the United States. It was soon noticed the bridge deck was prone to oscillate in certain winds. The vertical amplitude of the oscillations
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Myths persist

Many myths still surround the Tay Bridge disaster, the most pervasive being it was brought down by wind action alone. Rothery's report (see Paper 3) should dispel that particular myth, in addition to the numerous examples shown in this unit of the way the structure had deteriorated by the time of the storm in late 1879.

Click 'View document' below to open Paper 3 (35 pages, 39 MB).

5.15 Further investigation is possible

There are still many mysteries that surround the Tay Bridge disaster, largely because so little was recorded at the time of construction. For instance, questions remain about the details of reject rates for the castings, and modifications made to the first designs of the piers and their component parts.

Although enlargement of the BoT set of pictures has helped clarify the various failure modes described by Henry Law and others at the enquiry, it has also revealed yet more mysteries. Wh
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5.13 Conclusion of the BoT enquiry

The BoT enquiry issued two reports at the end of the enquiry, one authored by the chair, Mr Rothery, the other by the two other assessors. The Rothery report is Paper 3, linked below. They agreed about most of the issues in contention, as follows (Paper 3, page 47 of report).

  1. There is no evidence to show that there has been any movement or settlement in the foundations of the pier
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5.12 Pole and Stewart report

Apparently prepared using the same methodology as Law, Pole and Stewart produced a report that calculated the loads at various points in the bridge under live locomotive loads and wind loading at various pressures. Stewart was employed by Bouch to perform the original design calculations for the bridge, while Pole was brought in as an independent expert. He had extensive experience of use of different materials in bridges, and indeed, had written a standard text book for engineers on the subj
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5.10 Bridge stability

Any fracture of the diagonal wind brace tie bars could allow substantial lateral movement at the top of the piers. If these tie bars had already been injured by the previous train to cross the bridge, it would have only taken a little extra effort to complete the process as the mail train arrived over each pier supporting the high girders. Once the wind braces had failed completely, and the struts fractured at their connections each pier would behave as two separate supporting structures.


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5.9 Mechanical tests by David Kirkaldy

In order to determine which of the several parts of the joint were weakest, and gain some idea of the scatter in strength, David Kirkaldy was employed by Henry Law to test various samples he had collected from the bases of the fallen piers. David Kirkaldy had a good reputation for accurate and rigorous mechanical testing of materials using a large tensometer he had designed and built in London (see Input 9, linked below).

Click 'View document' below to open Input 9

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5.8 Design problems

Table 7 summarises the many design problems of the piers uncovered by Mr Law and his team. We have already seen the numerous fractured lugs in the remains of the bridge, shown in Author(s): The Open University

5.6 Casting defects

The first class of defect would have been inferred from examination of fractures in the cast-iron columns, where, for example, the wall thickness would be exposed for measurement. However, some of the casting defects he mentioned in his testimony – and which were to gain some notoriety both in the popular press accounts of the enquiry and in later accounts – are difficult to describe in detail because he did not specify where they were found in the debris, or how exactly they had contribu
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5.1 Overview

The enquiry team set up by the Board of Trade, and sitting in Dundee Court House, held an initial session lasting several days starting on Saturday 3 January 1880. There were three members chaired by Mr Rothery, Commissioner of Wrecks. The others were Colonel Yolland, the Inspector of Railways, and Mr W H Barlow, president of the Institute of Civil Engineers, and a distinguished practising civil engineer.

Henry Rothery was a mathematics graduate but trained as a barrister. He had been a
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4.2 An introduction to the Board of Trade photographs

It is important to bear in mind that these shots show the bridge remains in the state they were in just after the accident, and are almost exactly what the investigators would have seen when they inspected the bridge (see Input 8, linked below).

Click 'View document' below to open Input 8

Disaster!

The train receded into the darkness and the light of the three red tail lamps grew dimmer. Sparks flew from the wheels and merged into a continuous sheet that was dragged to the lee of the bridge parapet. Eyewitnesses would later recall at the inquiry that they saw a bright glow of light from the direction of the train just after it must have passed into the high girders section, and then all went dark.

The train was timed to pass the Dundee signal box at 7.19 pm. When it failed to arri
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3.2 Background

North of Edinburgh, in east Scotland, lie two great estuaries, the Firth of Forth and the Firth of Tay. The Firth of Forth begins at the ancient town of Stirling and runs 50 miles to the east, where it emerges into the North Sea. Edinburgh, the capital city of Scotland, is situated on the southern bank at the mouth of the estuary 30 miles east of Stirling (Author(s): The Open University

3.1 Overview

The Tay Bridge disaster came towards the end of a period of intense development of the railway system in the UK. The bridge had materials that were well known. Cast iron was used for the columns and wrought iron for the trussed girders.

The construction of the bridge was, at the time, the largest single engineering project in Britain, the Tay estuary being about two miles wide near Dundee, and the bridge was the longest in the world.

In the shallower approaches in the estuary, con
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4.2 Articulating your appreciation of complexity

Initially, I would like you to notice whether and how your appreciation of the phrase ‘managing complexity’ has changed since you started the unit. As you work through Section 4 you will encounter a number of ways of thinking about complexity that may be new to you, so it becomes important to record your developing understanding. To help you with this, return to your notes on Author(s): The Open University

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