Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Wind forces

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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.
>
>
>
>
> Regards
> Conrad Harrison
> B.Tech (mfg & mech), MIIE, gradTIEAust
> mailto:sch.tectonic(--nospam--at)bigpond.com
> Adelaide
> South Australia
>
>
>
>
> ******* ****** ******* ******** ******* ******* ******* ***
> * Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> *
> * This email was sent to you via Structural Engineers
> * Association of Southern California (SEAOSC) server. To
> * subscribe (no fee) or UnSubscribe, please go to:
> *
> * http://www.seaint.org/sealist1.asp
> *
> * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> * send to the list is public domain and may be re-posted
> * without your permission. Make sure you visit our web
> * site at: http://www.seaint.org
> ******* ****** ****** ****** ******* ****** ****** ********



It’s the same Hotmail®. If by “same” you mean up to 70% faster. Get your account now.