New Zealand Engineering 1998 May

How Lang Should Your Lum Reek?

"Lang may your lum reek" (long may your chimney smoke) is believed by non-Scots to be the Scottish equivalent of "best wishes for your future happiness" or "may you have a thousand sons" or whatever corresponds to their own ethnicity. Personally, I suspect that the phrase was invented by the comedian Sir Harry Lauder, along with such other catchphrases as "mony a mickle maks a muckle" and "it's a fine braw moonlicht nicht the nicht". One theory is that he was trying to see how far over the top he could go before English audiences realised that he was having them on. He never succeeded; people will believe anything about other people's cultures. Be that as it may, and setting aside the fascinating question of whether some contemporary New Zealand personalities are doing much the same thing but without the intentional humour, "lang may your lum reek" touches a matter of vital interest to most engineers. How long should a chimney last, or a motor car, a sewer system, an electricity supply cable, or any other engineering project?

It is certainly a topical subject for me.

As an employee of the Building Industry Authority, I have been crossing pens about it with consulting engineer Esli J Forrest in the journals of the Structural Engineers Association and the Heavy Engineering Research Association. Essentially, his question is "Why does the Building Act prevent legal claims more than 10 years after a building is constructed when it requires minimum durability periods of 15 years and more?"

As a lawyer, I look forward to some durability cases under the Consumer Guarantees Act. That Act says in effect that the suppliers of certain goods and services must guarantee that they come up to certain standards. One guarantee is to the effect that consumer goods will be as durable as a reasonable consumer would regard as reasonable in the circumstances. Obviously, that leaves room for argument, but so far the arguments seem to have been concluded more-or-less happily because as far as I know they have not reached the courts.

As a Wellingtonian I have been watching the Mercury Energy debacle with sympathetic fascination. One of the interesting features has been the frequent assertion that four cables failed "well within their reasonably expected lifetimes" (a phrase used by economic consultant Dr E A Hudson in the 20 March 1998 issue of The National Business Review). What basis was there for that expectation? Of course, we do not yet know whether there was in fact a durability problem with the cables (and perhaps we never will) but nevertheless the episode is a dramatic illustration of what can happen when an engineering project unexpectedly reaches the end of its useful life. Another, tragic, example is the comparatively recent collapse of a farm bridge resulting in death.

Obviously, durability involves different considerations in different branches of engineering, but I can comment in respect of buildings only.

It is unusual to hear of a building that has simply died of old age while still in use. Perhaps that is because of our generally high quality of building, possibly too high in today's economic climate. Almost all of the new building products and processes introduced over the last 30 years or so have performed well up till now, but that does not necessarily mean that they will continue to do so for as long as we expect.

Specific legal requirements for the durability of buildings and parts of buildings were introduced under the Building Act, which came into full force in 1993. However, the relevant New Zealand Standards have always contained implicit durability requirements with such things as minimum concrete cover to reinforcing steel, treatment of timber against insect and fungal attack, self-cleansing gradients for drains, and so on.

Those implicit requirements presumably corresponded to the general public's expectations of how frequently various building elements will need to be replaced, in other words of their useful lives. Corrugated steel roofing, for example, might be generally recognised as needing to be repainted after a certain number of years and to be replaced after another number of years even when regularly repainted. The general public expectation could well depend on how obvious it will be that a particular building element needs replacing, how easy it is to replace, and how dangerous the building will become if the element is not replaced at the end of its useful life. Foundations, for example, or concealed plumbing, could well be expected to last the life of the building.

For most buildings, that life is specified under the Building Act as being "indefinite". That seems realistic, we would be in a bad way if all New Zealand buildings older than, say, 50 or even 100 years needed replacement simply because of their age. On the other hand, is it credible to expect many New Zealand buildings to survive for the 500 and more years commonly experienced in Europe?

Scientists are devoting increased efforts to improving their forecasts of the durability of both established and new building materials used in a range of circumstances. On the one hand, increased understanding has led to some doubts about whether certain concrete structures are as durable as we thought. On the other hand, increased understanding has led to the use of untreated timber in certain circumstances where we previously thought treatment was needed.

For well-established building materials and methods of construction the expected durabilities will be based on experience and in most cases are likely to correspond reasonably closely to the durabilities which will actually be achieved.Those expectations are reflected in the durability periods required under the Building Act. Nevertheless, I have heard it said that durability is very difficult to assess at the time of construction so that it would have been better not to include durability as a performance requirement because of uncertainty and liability issues. However, I have not heard any suggestion to remove the implicit durability requirements from New Zealand Standards, and those requirements need to be legally enforceable. So what else might we be doing? Perhaps there should be more recognition of the fact that some buildings do not need to be designed for an indefinite life. The Building Act provides for "temporary" buildings (where "temporary" means a specified period not exceeding 50 years) but those provisions do not appear to be used as widely as perhaps they might. Anticipated obsolescence could be a significant factor in the design of buildings as it is in the design of cars and household appliances.

Perhaps there should be more attention to the provision of maintenance manuals. Again, cars and household appliances are better served than buildings. The Building Act's durability requirements assume "normal maintenance". More specific maintenance provisions are specified in compliance schedules under the Act, but those mainly apply to automatic systems such as lifts and sprinklers where the need has always been recognised.

Some manufacturers produce excellent instructions for the routine maintenance of their products. But how many designers could identify all of the routine maintenance procedures they implicitly rely on to ensure that a particular building will achieve its required durability?

Most of the information is readily available, and professional building managers are well aware of it. Many owners are not. Perhaps designers should include a complete maintenance manual as part of the plans and specifications for new buildings.

Then perhaps a designer could say something like: "Your lum will reek indefinitely, provided you use only coal, coke, and clean kindling (no salt-impregnated driftwood), call in the chimney sweep every other year, and replace the flue liner after every 10,000 hours of fire burning time." Mind you, clear air legislation will probably forbid reeking lums before the designer can be proved wrong.