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RE: Wind forces[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Wind forces
- From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
- Date: Fri, 19 Dec 2008 15:54:47 +0000
Major evolution of the codes has been because of real problems in the past. More current evolutions have happened as we try to look at uniform risk and performance design. |
I recall with longing, nostalgic affection at the 60 pages that comprised the ANSI A58.1-1972 (which was the revision of the A58.1-1955), or even further back to the Code of Hamurabi. Note that about 40 years ago we were on a 17 year update cycle. For those young snappers of whipper, the ANSI A58.1 evolved into the ASCE 7 in 1988.
Regards, Harold Sprague
> From: sch.tectonic(--nospam--at)bigpond.com
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Wind forces
> Date: Fri, 19 Dec 2008 11:47:32 +1030
> The codes are complicated so that those who want to push the limits can do
> so and have a chance of getting approval.
> But individual designers and approving authorities are quite capable of
> creating their own simplifications, without the need to create another
> simplified code.
> As I said here most read the wind code once, and have never read it since.
> They adopt net pressure coefficient 1.2, and typically calculate qz less
> than 1kPa. Which since 2002, our code hasn't had qz, but everyone calculates
> anyway rather than repeat the _expression_ everywhere as the code now does.
> Thus get resultant pressure of 1.2kPa, which surprisingly approximates the
> 25 psf that Ralph mentioned.
> Both designers and the certifying authority are quite capable of making a
> qualitative assessment that the loads adopted are within the requirements of
> the code: the values may not exactly equal, but are conservative. The code
> can be applied and lower values obtained, but few want to expend the time.
> They only expend the time, when advised the net pressure coefficient adopted
> is too low. In which case the effort is generally expended demonstrating
> otherwise, and keeping to a net of 1.2. Adopting higher for buildings tends
> to get a response of too expensive.
> So if 15 psf, or 25 psf is the answer, then you use ASCE7-05 to show that is
> so, then forget about the code, until circumstances of a project demand
> otherwise. (Everyone should be able to see it without constantly working
> through it. Write it up once.)
> If engineers produce the designs, and engineers certify/approve the designs,
> and all agree the code is too cumbersome for daily use on common structures,
> then where is the problem? The designer should be capable of presenting an
> argument the approving authority finds acceptable. Easier if certifying
> engineers are also design engineers, and they want their life to be easier
> when designing.
> Because as many have said there is not a history of wind destroying
> buildings all over the place in non-cyclonic regions. The major damage to
> buildings during a storm comes when trees are uprooted, fall on power lines
> or crash through house roofs, little actual direct wind damage to houses.
> The direct damage which does occur is to cladding and ornamental decoration
> of the building. If people want the ornamental decoration then they have to
> accept the risk of damage.
> Designers and regulators have to be realistic in their application of the
> codes. And to start with the codes probably only cover at most 80% of
> requirements. So design purely to the code is defective. Choosing to use the
> code in the first place is a judgement on the part of the designer: the
> designer has to be capable of deciding the code is not adequate and
> demanding assessment against appropriate performance criteria to suit their
> application. Equally well determine the code is too demanding.
> It is the job of the designer to achieve a design-solution which complies
> with appropriate performance criteria, to produce adequate
> evidence-of-suitability and justify their decisions. The design-solution
> needs to be found compliant with the code: that is performance equal to or
> in excess of. Some list members seem to think have to achieve exactly equal
> to or won't get approval: which is highly impractical.
> My point is should be capable of making a quick review of ASCE7-05 and
> pulling out the values required to get simple expressions and loads with
> magnitudes similar to those you are familiar with from earlier simpler
> codes. (eg. most variables have a default value of 1, most of the time the
> effort generates a reduction, occasionally an increase results.)
> Of course may not want to put the effort into reviewing a more complex code
> when previously had simpler codes. Now that's a different story.
> Conrad Harrison
> B.Tech (mfg & mech), MIIE, gradTIEAust
> South Australia
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