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

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Conrad
Brevity is the soul of wit.

Conrad Harrison wrote:
Gil,

I am not aware of any mandatory serviceability criteria in the steel
structures codes. Though they do suggest a need to recalculate all section
properties for deflection checks. And getting nonlinear analysis to converge
can get a structure to be a lot stiffer than would otherwise need be. The
main serviceability criteria I have come across are those in the crane code,
and the industrial stairways and platforms code, and industrial racking
code: but these aren't buildings and AS1170 is primarily for buildings and
its serviceability criteria are informative suggestions. Also aware of
criteria for communications antenna and electricity power poles, these based
on acceptable loss of service: but once again not buildings.

Mandatory is also a matter of perspective. It is relative to the needs of a
particular design. If check of deflections at the ultimate strength loads
doesn't increase member size, then no need to carry out analysis for smaller
wind loads. For small structures this may be the case because cannot find a
smaller section to use to meet strength requirements alone. Also changing
return period, changes Vz, which in turn changes qz (no longer in code), so
can ratio serviceability and ultimate strength loads based on ratio of
square of the velocities. Only complication is serviceability load factors
and combinations. For simple structures not a major problem, and more
complex structures tend to use analysis software which permits easy
combination of load cases: and people have different ways in which they use
such load case combination features of software: some inefficient. Also
depends on how pedantic the building officials are, and if everything has to
be written out to the letter of the code: like qz not in code therefore
cannot use.

Also I don't believe, alternative return periods has anything to do with
accuracy. It is more to do with serviceability being a subjective judgment
and dependent on the needs of the particular situation. Thus the mandatory
20 year mean return period for serviceability in the 1989 code, changed to
recommendation in the 2002 code.

A steel fabricators building with the main doors open in the rain and wind,
needs to be serviceable under such conditions, a 20 year mean return period
may not be adequate. But then the now 500 year mean return period for
ultimate strength design may achieve adequate level of serviceability at the
desired operational wind load.

The limit states: stability, strength and serviceability are really broad
classes of states . There is a whole spectrum of limit states for a building
or other structure, and each has differing performance criteria. It is the
designers responsibility to determine what limit states and performance
criteria need to be assessed. The loading code and BCA simply mandate the
performance criteria for ultimate strength.

Codes increase in complexity, so that application of more complex theories
can be granted approval for compliance. But the individual users can
collapse that complexity down to suit the needs of their particular
activity. Competent building officials, can see the relationship between the
simplifications and the complexity of the code. Day to day design needs to
be practical, but do need some control for those more complex proposals so
that not declared noncompliant. In that respect I think there should be
multiple tiers to the codes.

As for statistics. We have to deal with variance, uncertainty and risk. We
have simply progressed : lies, damned lies and statistics. So now using
statistics instead of lying about provision of safety. Safety never present
only risk.

Risk based design, fits better with your earlier view about over regulation
to protect people tripping over own two feet. If choose to walk, then have
the risk of tripping: if don't accept the risk, then don't walk.
Or the case of two youths fighting in stairwell. They fall down the steps
and one becomes a paraplegic. The designer was held responsible because the
handrail was 50mm lower than the standards specify. Such ignores the
statistical variation in heights of people and their arm length. The code
specified heights can be a hazard to life for many people: if they slip the
handrail could prove to be either too low or too high for the individual
concerned to regain their balance: vertical infill rails may provide
something to grab. Specifying a specific height or height range implies the
provision of safety to the lawyers and public. Further more the height of
the handrail, may be uncomfortable for the user and be the direct cause of a
fall.

Quality robust design, aims to find a design solution which can accommodate
variance in the operating environment of the end-product, as well as
variance in the production process, and variance in the design process. It
requires a adding greater level of focus on the qualitative aspects of
design, not just aesthetics, but the functional requirements, and not just
function as regards strength and stiffness, but the point for making the
thing in the first place. What is the purpose of a hand rail and guard rail,
can they be integrated into the one object, or do they need to be kept
separate? What is a house, does it simply provide shelter, and shelter from
what?

Routine design is simply to go with the codes, and follow tradition.
Progress comes from questioning the status quo, and changing perspective:
design a car or mode of transport, design a house or a shelter. Regression
also comes from same approach.

Variance all around. Some think changes to wind loading code stupid, others
don't. The writers of a quality robust code would have taken that variance
into consideration when writing the code, to reduce opposition to it. I
guess they didn't. Therefore code not quality robust: complexity opposed by
many, and therefore likelihood of unwarranted errors in design creeping in.
Using the code becomes risky.

On the other hand the mandated requirements are also less certain, because
code accommodates a greater variety of circumstances, for which designers,
suppliers and end-users are all made more accountable. Therefore variance
from compliance with the code more difficult to detect: providing designers
with a lot more scope for innovation. Far better than mandated prescription,
yet the designers have opportunity to create and develop more prescriptive
solutions for efficiency in production.

If codes are highly prescriptive and lock a great deal in, then no need to
keep on producing calculations, only need to produce once, since variance
from the input parameters and the results are not permitted. In which case
remove time consuming prescription on calculations, and simply specify
prescription on what can construct. No, no, too much regulation, want to
innovate!



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



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