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@bigpond.com
> Adelaide
> South Australia
>
>
>
>
>
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