For big civils projects the specified "design life" can be 100+ years, and for the Humber Bridge and Channel Tunnel it was 120 years. This means they should be fit for purpose for a minimum of 120 years, before some major refurbishment or rebuilding is required. They should hopefully last longer than that if a maintenance regime is followed.
I dont know whether the designers of Victorian bridges had an intended lifespan for their constructions?
The Iron Bridge at Ironbridge is still standing after 227 years, but now with limited traffic allowed on it.
However things don't always pan out as intended. The Forth Road Bridge built in the sixties had a design life of 120 years but the cables are currently snapping due to much increased traffic density; and according to this article it may need to be decomissioned in 2014, having lasted only 50 years.
www.thecourier.co.uk/output/2006/11/18/newsstory89...p
news.scotsman.com/scotland.cfm?id=481652005
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major engineering structures are built e.g Suspension bridges, Channel Tunnel etc - what's sort of lifespan are they expected to have
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as rich 9-3 says, the design life can vary. in the days before computer assisted design became the norm, the calculations used to be done manually (assisted by the slide rule when that was in fashion). engineers used to build in safety/error margins in their calculations depending on the normal standards of the day. nowadays, with supposed advanced in computing power and mathmatical modelling etc., the calculations are supposed to be more accurate and safety/error margins have been reduced (not least to make the product as economic as possible).
in some cases, the assumptions made or the behaviour of concrete/steel or the environment have not been what was expected. examples are corrosion in uk's gas-cooled nuclear power station reactors, and concrete-steel "cancer" corrosion in many sixties/seventies projects.
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Many of the Victorian structures were over-engineered by the standards of today, when as Dalglish says safety margins can be predicted to a very accurate degree.
As they say, "Any fool can build a bridge that will stand up, but it takes an engineer to build a bridge that will only just stand up".
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Or wobble.
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I'm sure I read somewhere that the Victorian terrace houses ( I live in one BTW ) were only intended to last 30 -40 years - they're certainly more robust than the modern ones.
Anyone know the expected lifespan of the Channel Tunnel ?
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100 year according to some well buried Hansard report on the thing.
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>>As they say, "Any fool can build a bridge that will stand up, but it takes an engineer to build a bridge that will only just stand up".
I know which I would prefer.
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rustbucket (the original)
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Wasn't it Colin Chapman's philosophy regarding GP cars that they should only be built well enough to just finish a race.
Otherwise they were over-engineered, ie too heavy and thus too slow. Think quite a few race drivers belatedly found out that was not a good idea.
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I wasna fu but just had plenty.
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Chapman apparantly once berated a mechanic for fitting washers to the bolts during an engine assembly. He told the fellow that they were adding unnescessary weight to the car.
As someone who throws away the valve caps on cycle innertubes to save weight, I can sympathise. Though it makes naff all difference.
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Bridges like the one in Ironbridge are still standing not because they were built heavily, but because they were built to be predominantly in compression when loaded. In general, fatigue cracks don't grow in compression, and concerns about lifetime are limited by corrosion, which for some materials means a very long life can be ensured.
Despite the pointed use of the description "supposed" when referring to advances in design and technology, what is perhaps being forgotten is that there were many catastrophic failures of structures like bridges and boilers which until the discipline of fracture mechanics was developed, forced the use of large margins of safety - and even these were sometimes unsuccessful.
If there are modern weaknesses in design, I think one of them is that engineers aren't exposed to sufficient materials teaching, and in many cases are barely taught about fracture mechanics. As an example, it came as news to me during a design review this week, that when cleaned with methanol, Titanium parts become weakened via the mechanism of stresscorrosion cracking - luckily, for the parts I am responsible for, I had specified a cleaning technique using propanone rather than methanol, but it was a matter of luck rather than skill on my part.
The other, perhaps obvious, weakness that springs to mind is how much engineering is being done by slavishly following standards and software user manuals. The wobbly bridge complied with the applicable standards, and was properly designed and analysed.
Engineering design software is now almost too easy to use, and I frequently see the results of analysis which are either incorrect in approach, unfit for purpose, or incorrectly interpreted because the analysis software is seen as an add on to a CAD package, and designers are trying to carry out analysis which is beyond their skills.
One other issue that may not be obvious to the layman is that once you have a large or complex structure, you also will have cracks and defects in that structure - even from brand new. The management of these cracks, and the decisions as to whether to monitor, repair, or replace is an expensive and sometimes difficult ongoing task for those responsible for engineered structures like aircraft, trains, ships, oil rigs, nuclear power plants, bridges, etc, etc. The tools for the management of these cracks simply did not exist until comparatively recently - consider, for example the Alexander Keiland disaster in the 70's.
One of the odd things that couldn't have been understood by previous generations of engineers is that due to the statistical distribuion of flaws and defects in even new materials, there is a reduction in the allowable fatigue stress for thick materials, so, as you beef up a part, you rapidly move into a regime of diminishing returns.
Number_Cruncher
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Would it be fair to say that the two Menai crossings in Wales are over-engineered by their original builders ? I know that there have been changes in the road decks but the basic structures carry weights that were inconceivable to the original designers, Stevenson and Telford. Imagine it the rail crossing carries trains that must be substantially heavier than the originals and then you have the weight of the road deck and all the traffic it carries (yes I know they binned the original tubes after the fire in the 70s).
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.. the pointed use of the description "supposed" when referring to advances in design and technology ..
number cruncher -
just to clarify. i used the term advisedly. i have been party to negotiations for multi-million pound damage claims arising from such failures in design/manufature/construction/commisioning/operation. howeever, due to client confidentiality as well as legal agreements, i am unable to even hint which "big construction" projects were involved.
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Well, the supposed advances are *absolutely* real - there's no comparison between the analysis and design techniques available today and those available in yesteryear.
However, there's lots more to engineering than just computer power, modelling and analysis. It's all virtually worthless without some skilled interpretation, and that involves common sense and experience. So, while the analysis techniques today are very accurate and sophisticated, this in itself doesn't guarantee a good design or implementation.
Despite my respect for mathematical analysis, I fight this nonsensical excessive blind dependence upon computer based analysis on an almost daily basis - I'm being pressured (by whom, I cannot say!) to produce analysis results for a system I'm working on which won't really add any value, or reduce any risk for the finished item* - this almost needless analysis work will have to be done at the cost of doing some useful , but less glamorous work which will really reduce the risk of failure. Because aerospace qualification is largely a standards driven, tick-box discipline, I am becoming unpopular in the project for not conforming, and concentrating my efforts where they can make some real improvement to the design.
Number_Cruncher
* some "fag-packet" calculations have already demonstrated that the design is OK, and has plenty of design margin available.
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"one pence coin"
ONE PENCE is an impossibility, two or more are not!
Roger. (Costa del Sol, España)
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The Tamar Bridge linking the Town of Saltash to the city of Plymouth along the A38 that was built in 1961 is actually inverior in capcity to a 400 year old stone built single lane bridge on the A390 some 20 miles away, also linking Devon and Cornwall. This 400 year old bridge is actually capable of carrying the biggest commercial vehicles on our roads today with NO strenthening at all, whereas the Tamar Bridge IIRC needed extra bracing to cope with the heavier trucks that have evolved since the 60s.
The design and costrution of the Tamar uses components in both compression and tension (the latter being mainly the steel cables etc IIAR). The 400 year old bridge I refer to is of a design based soley on compression.
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To build on Dalglish's point, and also Hugo's, we are fortunate that 17th, 18th and 19th century bridge builders did not have CAM, if they had they would have built bridges capable of taking a couple of haycarts pulled by shire horses, as it is they massively over engineered the bridges to the extent that many can take 500 horse power 38 tonne articulated trucks.
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CAD to be correct Chedder. CAM is Computer Aided Manufacturing, a term mainly given to CNC machine tools and the like. CAD is Computer Aided Design that has weened draughtpeople off drawing boards to workstations that can not only instantly carry out spacial calcs to help the engineer decide what will fit in where but can also carry out instant stress calcs. It's this technology that Number Cruncher (quite understandably in my opinion) is so scathing about.
As NC says, they are good tools provided that our reliance on them is not total. For example when you learned to use a calculator at school did your teacher suggest that you should always see if the answer is sensible? We don't just take the answer at face value as being right. It has to look realistic.
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CAD to be correct Chedder. CAM is Computer Aided Manufacturing, a term mainly given to CNC machine tools and the like.
Yes thanks, I meant CAD, I am too used to typing either.
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>>If there are modern weaknesses in design, I think one of them is that engineers aren't exposed to sufficient materials teaching, and in many cases are barely taught about fracture mechanics.
Good Lord. If they aren't taught about microscopic phenomena, what hope for them when they try to apply theories macroscopically. Did somebody say 'Comet'?
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>>Did somebody say 'Comet'?
To give a more recent example, I was thinking about the inappropriate material choice for the support bearings of the motorway bridge over the Manchester ship canal. A grade of stainless steel was used which ordinarily is OK, but which suffers stress corrosion cracking when exposed to a chloride environment (did anybody think that we might be spreading a little salt in winter?). During an inspection one bearing was found with a crack almost all the way through - hence the long running roadworks, while all the bearings were sorted out.
Number_Cruncher
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