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RE: Wind load and pile depths for wooden fences (San Jose)

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Dmitri,

My understanding is that here in Australia we discarded factor of safety and
safety margins, and moved over to limit state design, with a soft conversion
of our codes, but ultimately approaching a more probabilistic based design.
The view is that a factor of safety is misleading, it declares safety, if
you said it was safe, the end-user may not be happy when it fails. If you
say it has a risk of failure, the end-user can decide whether to accept the
risk or not. Though many of articles written, make reference to medical
profession and their presentation that the procedure has a 10% probability
of success.

All strengths or resistances have to be 5 percentile characteristic
strengths. Whilst loads are typically for a 95 percentile loading, or 5%
probability of being exceeded.

So when our wind loading code AS1170.2 went to limit state in 1989, the mean
return period for ultimate strength design was based on 1000 year mean
return period, derived from 5% risk of exceedance for 50 year life
expectancy. The commentary provided some probabilistic models for adjusting
the return period, and I made use of a fair bit when assessing existing. In
2002, the mean return period was reduced to 500 years for normal buildings,
and probabilistic models were provided for all regions, and added to the
main body of the code.

Tell someone that there is approximately a 10% risk of the wind speed being
exceeded for a building life expectancy of 50 years. They think the risk is
too high, and the life expectancy too low, and they stop complaining that
the design wind speed is too high and never occurs around here.

Thus far the main criticism is that the imposed live loadings have not been
adjusted to 95 percentile loads, and have a typical load factor of 1.5.
However if a load is derived probabilistically then it has a load factor of
1.

Since there is variation in strength of materials and dimension of
materials, the capacity reduction factor (phi) should account for variations
in the section properties (Z, A), whilst material properties are the 5th
percentile properties.

As for load combinations the 0.9G should account for the 5th percentile low
estimate of the gravity loads, whilst 1.2G should account for the 95th
percentile high estimate of the gravity loads.

Few Australian codes have mandatory serviceability loads, or conditions,
just a blanket requirement that a structure has to be serviceable. The
designers and end users determine the serviceability criteria. There are
recommendations in the codes, but not mandatory.

With reference to ultimate strength, expect materials to have yielded and
deformations to be permanent. Thus unless have a post disaster facility, do
not expect the typical building to remain serviceable after experiencing the
ultimate strength load condition. Which is where the US codes appear
different since many have said that wind loading is considered a service
level loading.

Here I don't believe it is: after an hurricane do not expect typical
building to be functional or fit for habitation, and expect significant
repair, possibly demolition and rebuild. Also don't take shelter in a
typical building during the design level event: whilst the building is not
permitted to collapse at the design load, the design load may be exceeded,
and the movement and noise of the structure is likely to be unnerving.

Since we can determine design wind speeds for different mean return periods,
and also different risks and building life expectancies. It is feasible to
define different performance criteria, for different states of the building.
Thus could have annual maintenance schedule, 5 yearly schedule and 10 yearly
schedule. Or building can be open, say warehouse open and operating under
one set of conditions, but closed when impossible to continue work outside.

Make the assessment as complex as you want. But effort to reduce load, and
to reduce installed resistance, also potentially reduces the robustness of
the structure. Because the real loading the structure is likely to
experience is unknown, and so is the actual resistance of the structure.
Though robustness is more a qualitative aspect of the design, rather than
quantitative. For example may want to carry out quality functional
deployment (QFD), or failure modes effects criticality analysis (FMECA), and
attempt to design the structure such that when it fails it poses the minimum
risk to life.

I don't agree with putting the factor of safety in the expressions, I don't
want to see a factor of safety any where. The structures are not safe, and
they certainly are not twice as strong as they need to be. If you have told
the owner it is safe, then expect to get sued when such claim turns out
false.

Though I believe more individual factors should be introduced. Here the
authors of the timber structures code seem, to have confused the capacity
reduction factor (phi), and used it to maintain some concept of an
importance factor, when importance factors otherwise discarded. But as I say
our conversion to limit state is a soft conversion of our permissible stress
codes. So we are still in process of slowly converting.

If you really like probability check out:

Joint Committee on Structural Safety:

http://www.jcss.ethz.ch/

Probabilistic Model Code

http://www.jcss.ethz.ch/publications/publications_pmc.html



Regards
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
Adelaide
South Australia

 



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