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Letters to the editor

The Editor welcomes letters at 151 Rosebery Avenue, London EC1R 4QX

fax: (0171) 505 6667 e-mail: nceedit@construct.emap.co.uk

and reserves the right to condense.

Seeking guidance

Alexander Grott (NCE 22 January) can take some comfort from the knowledge that the question of a design and practice guide for ro-ro design is on the agenda of the Maritime Board of ICE for its February meeting as, indeed, it was for the last meeting in November.

Why, then, have we not produced such a guide? Mainly because we want to exhaust all possible avenues to get government to accept some responsibility and assist financially in the production of a British Standard - and it is certainly not through any lack of effort on the part of ICE that we cannot cut through bureaucratic red tape.

The reason for wanting a full blooded British Standard is simple. A DIY store type guide, or even an ICE guide, useful though these documents are, can only be just that - a guide. The only document that really carries any enforceable weight is a British Standard.

What particularly frustrates me is the small amount of money required to produce the BS in relation to the cost of litigation and the fines imposed. One cannot help but feel that resources would be better employed in preventing further accidents rather than dwelling on responsibility for past failures.

In a perverse way, however, the amount of publicity surrounding this affair is probably doing more good in achieving the ultimate aim of producing safe ro-ro systems than any BS or guide. There can surely be no designer of such structures around who is not now fully aware of their complex loading and who will therefore be applying his or her own rigorous analysis.

Look out for the ICE guide!

John Carmichael (F),chairman, ICE Maritime Board.

Parking point

The suggestion to tax private parking spaces in cities strikes me as being quite unethical (NCE 1/8 January and 22 January).

In the 1960s, many of us became involved in the difficult task of designing office buildings in London to accommodate the compulsory off-road parking spaces demanded by the GLC. Most buildings had to have very expensive basements to obtain these spaces.

Now we read that these spaces are to be taxed because the owners complied with those requirements, but the authorities have changed their minds about their priorities. How can that be fair or just?

I suggest that if these spaces are now regarded as undesirable, then compensation should be paid instead of a tax being applied. After all, those cars are parked off the public road, which was the original intention.

GE Bratchell (F), 13 York Close, Kings Langley, Herts WD4 9HX.

Cool concrete

RA Baldwin expresses surprise (NCE 1/8 January) at concrete shell cooling towers in a Second World War movie. It is interesting to note that such structures reached the UK not long after the First World War, as outlined in the Mouchel publication A century of achievement commemorating the consultancy's successes from 1897-1997. 'In 1924 TJ Gueritte (a director of Mouchel from 1907-48) approached the city electrical engineer of Liverpool with proposals for a new type of concrete cooling tower to replace timber towers used at that time.

'Timber towers had a life expectancy of 10-15 years with the ever present risk of fire, and all-steel towers had an even shorter life due to atmospheric corrosion. The Dutch State Lines at Harlem had developed a circular concrete tower narrowing to the top with holes at ground level to allow the passage of air. The walls were built to a hyperbolic curve which also allowed the sloping reinforcing bars to be straight.

'With the knowledge of the success of these towers in Holland, Gueritte persuaded the city engineer to appoint Mouchel to design 12 ferro-concrete cooling towers for the Lister Drive power station, in Liverpool. These towers were 141/2 in thick at the base, reducing progressively to 61/2 in at the top. They were 130ft high and 100ft in diameter at the base. Large as these were at the time, they were small compared to later cooling towers that Mouchel would design which rose to 500ft in height and 400ft diameter at the base.

'Mouchel therefore pioneered the design and introduction of ferro concrete cooling towers into the UK and maintained a commanding hold on this area of construction for the next 40 years both in this country and overseas.

'The first appointment for Lister Drive was quickly followed by appointments for cooling towers at Birmingham, Wolverhampton and Cairo in 1925, and Coventry in 1927.

Alex Morris, Follywood, New Road, Wonersh, Surrey, GU5 0SE.

Salary dissatisfaction

The contretemps about engineers' salary levels (NCE 4 December 1997) betrays the all-too-common failure to appreciate basic statistical principles about collecting reliable survey information of any kind.

Without covering the whole population in question (engineers in this case), the only useful approach is to select a genuinely random sample of suitable size and persist until responses have been obtained from all (or at least nearly all) members of the sample. Even then the survey needs careful detailed design to minimise the chance of biased replies such as reluctance to admit exceptionally low or high salaries.

The sample size needed must be calculated from the level of confidence needed in the results, which leads back to the use to which the information is to be put, ie why is the information being collected in the first place?

Even a large self-selected sample is virtually useless unless it consists of nearly the whole of the relevant population, while a surprisingly small randomly selected one can be sufficient.

The major polling organisations regularly predict election results with impressive accuracy using random samples of 1,000 to 2,000 out of a population of tens of millions.

In my view, ICE would do far better to commission a reputable polling organisation such as NOP to carry out a properly designed sample survey. The full survey need not be annual if too expensive. Annual updates to assess trends using a smaller sample would be an option.

Unless the matter is tackled soundly in future, the exercise would be better discontinued since potentially misleading data is worse than none at all.

None of the above is intended to suggest past surveys have not been run honestly and in good faith, only that the methods used have been misguided.

Stuart Beniston (M), 74 Mapperley Orchard, Arnold, Nottingham, NG5 8AG.

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