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# RE: Wind forces

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

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