Dear Paola, (01)
MW: Well I am an engineer (Chemical rather than Civil as it happens,
but this is of little relevance here. (02)
> Hi Ed
> thanks for the extensive illustrations
>
> My point is simply that we cannot assume that a sound model which is
> 'stable' according to conventional calculus, is stable, and likely to
> remain stable indefinitelyin real conditions
>
> The x factor I was suggesting means simply: assume the bridge can
> become unstable
> in certain conditions in the future - which is something I think
> should be written on either side of the bridge.
>
>
> > But you cannot in any way capture in a model what you don't know
> > that you don't know
>
> A model should allow for uncertainty, the product of interaction and
> change, I am not sure all models of buildings do
>
> >
> > >> But the surprise failures are those that involve a
> factor that was not
> > >> considered at all, and not commonly considered in the trade.
> > >> How do you build> an "X factor" defense for that?
>
> That you are trying to identify the x factor, and that is
> near impossible
> But you can easily assume that something can go wrong and
> prepare for that (03)
MW: It is perhaps worth reflecting on the approach that engineers take
to safety. (04)
MW: It is assumed that some disaster will happen, and then on the one
hand look for the possible causes of such a disaster, and then seek
to eliminate the causes, and on the other hand to mitigate the
consequences. (05)
MW: To take an example from my own field: If you have a tank that holds
petrol, it is very bad if it overflows and petrol vapour reaches a source
of ignition (think of Buncefield). So tanks are provided with guages
and alarms (and possibly even trips) to warn or prevent overflow.
However, it is also assumed that it will non-the less happen, so tanks
are provided with bunds, so that if the tank fails, the contents are
contained adjacent to the tank, rather than spreading through the
surrounding environment. (06)
MW: Actually, the question engineers are starting to worry about is
how many simultaneous things going wrong should be taken into account?
Most disasters I have seen (that are not primarily down to human
negligence) have happend as a result of multiple factors coming into
play, when each on its own was designed for. (07)
Regards (08)
Matthew West
BSc, PhD, DMS, CEng, CSci, MIChemE, MCMI, MBCS, CITP
Reference Data Architecture and Standards Manager
Shell International Petroleum Company Limited
Registered in England and Wales
Registered number: 621148
Registered office: Shell Centre, London SE1 7NA, United Kingdom (09)
Tel: +44 20 7934 4490 Mobile: +44 7796 336538
Email: matthew.west@xxxxxxxxx
<http://www.shell.com/>
<http://www.matthew-west.org.uk/> (010)
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