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RE: ASCE 7 Wind Loads on Bare Steel Framing - Opinions Wanted

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Bill,

>From the commentary to ASCE7-05, apparently AS1170.2 is one of the sources
of coefficients for canopies: from what I can tell those coefficients have
been in AS1170.2 since the 1970's: so some history of being reasonable.
AS1170.2 also includes drag coefficients for airflow over corrugated and
ribbed cladding for walls and roofs. I cannot seem to find equivalent in
ASCE7-05.

As for open frames AS1170.2 contains drag force coefficients for various
structual sections: I-beams, channels, etc. The code also permits shielding,
but when the wind is at 45 degrees to the orthogonal axes of the structure
little shielding is available for simple rectangular frames (eg. carport, or
building frames during construction).

I cannot find similar coefficients in ASCE7-05, it seems to be focused on
the section of the chimney, or tower. It seems to me such approach may under
estimate the forces on the individual framing elements.

Drag forces on individual elements or whole frames can be high. I've seen a
photo of  6 gable frames (in the UK), they were left over the weekend:
Monday morning they were all folded to the ground about the I-beam weak
axis. (The beams looked fairly large about 610mm deep, possibly bigger.)

If I could find the picture again it would be good to show the builders who
keep telling me: for 30 years they've never had a problem so why should they
prop and brace the frames during construction.

As for which code to use. I prefer at least two approaches where possible.
Older codes hopefully provide calibration for newer codes. If the older code
produces more conservative results go with the older code: at least the
older code is tried and tested in the field. Often reduction in newer codes
are a consequence of missing out important constraints and conditions, which
are later reinstated. If newer code more conservative go with that.

Otherwise it's a risk management issue, if there is no mandated legal
requirement to use one code or the other. If older code easier to use, and
faster, and more expertise available to check its application, then the
older code may be the better choice. Possibly incorporate additional design
factor of ignorance to allow for what don't know about the new code.

It also depends on the critical factor. In Australia wind loading in
AS1170.2 is ultimate strength. Separate codes for cranes, antennas, power
poles, and industrial racking define serviceability requirements. The wind
loads in these codes seem low, but when the deflection limits imposed are
taken into account the ultimate strength of the structure can end up much
greater, than if we started the design with ultimate strength load: but not
always the case. Since 2002, AS1170.2 has permitted the calculation of basic
wind speed for any return period or annual probablity of exceedance. I
expect the more specialised codes to remove their low design wind speeds,
which tend to be seen as non-compliance with AS1170.2 and replace with
probability requirements. Thus achieving inceased consistency.

The way I see it, in similar manner to AS1170.2,  ASCE7-05 is becoming a
single point reference for all loading requirements: the expected reference
to be used by all structural engineers. The market for engineering services
is variable, an engineer who can only use simplifications in IBC, would take
time to get upto speed when building market drops and industrial market
expands. If only use the one code than already upto speed. Also the market
place for engineering services is becoming more international, and ASCE7-05
is more aligned with the ISO guidelines: thus AS1170.2 and ASCE7-05 are
similar but not equal. Thus industrial/petrochemical structures designed to
ASCE7-05 in the USA and imported into Australia likely to be found compliant
with AS1170.2, those designed to UBC/IBC: more likely to be rejected or
involve extensive recalculation before accepted.

So whilst have the experience available for the older code and experience of
acceptable design-solutions, run with the new code (ASCE7-05), check and
calibrate design-solutions against the old code (ASCE Petrochemical
facilities), and where necessary set company policy and design procedure:
assuming resources permit. Thus eventually move over to ASCE7-05 and be
better placed internationally.

Just a few thoughts for consideration.



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




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