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RE: Paying for clarification

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Dennis and others,

For an example of simplified wind loading take a look at:

AS4055: Wind loads for housing
AS1684.1 : Residential timber-framed construction : Design Criteria
AS1684.2 : Residential timber-framed construction : Non-cyclonic areas
AS1684.3 : Residential timber-framed construction : cyclonic areas

These are derived from and dependent on: 

AS1170.2 : Structural Design Actions Part 2: Wind Actions
AS1720 : Timber Structures Code

AS1170.2 is similar to ASCE7-05, in the office I can trace it back to 1973,
which seems to be the first metric edition, prior to that seems to have been
called CA34. I assume ASCE7 has similar vintage and it didn't just spring
out off nowhere in 2005. Personally I'd expect licensed SE's to use ASCE7
and have significant experience using it, and be capable of simplifying for

I don't have the wood framed construction manual (WFCM), but I have been
assuming it is similar to AS1684.1 and presents a prescriptive application
of ASCE7, identifying the pressure coefficients to use for rafters, studs
etc. If it has pressures like IBC:2003 (ps30), then it is too limited in
application. Though IR 16-7 seems to be heading in right direction.

Also given that the WFCM guides refer to V and exposure B, as does IBC:2003,
does suggest an industry wide lack of understanding of ASCE7 and the
calculation of qz and the relative constancy of pressure coefficients for a
given structural application. Whilst qz is a trivial calculation to suit the
site/building combination.

Using AS1684.2 a timber estimator, builder or architect can look up
pressures to apply to projected areas or directly look up forces for
tie-down and bracing. Once they have the applied force they can also look up
resistances and select appropriate connection and bracing details.
(Comparing against IBC:2003, I'd say connections given in IBC are not
suitable for V=100mph)

AS1684 = Simplified: no engineer required. 

Though the tables in AS168.2 only go up to wind class N4. However wind class
N3 to N6 have same reference design speed (Vzu, qzu) as cyclonic classes C1
to C4. So for wind classes N5 to N6 could use the cyclonic prescriptions.

Being simplified AS4055 also tends to be conservative, and often builders
get engineers to check to AS1170.2 to reduce wind load and thus tie-down and
bracing requirements. Builders however, take member sizes from the span
tables, because, it would take far longer than 30 minutes to engineer all
the timber in a typical house. (AS1684 is only valid for 2 storeys)

And as an engineering technologist, with formal education in industrial,
manufacturing, and mechanical engineering, and having to learn structural
codes on the job: I most certainly expect a registered structural engineer
(NPER) to know more about AS1170.2 than I do. All my calc's are checked and
certified by engineers: though development act currently calls up
NPER(Civil): but structural relatively recent addition to NPER.

Whilst ASCE7-05 is not my code, I don't really see any difficulty learning
the wind loading requirements. The only difficulty I am having is
understanding the potential misapplication of empirical research and the
inappropriate prescriptive measures in the use of C&C pressures.

For if the way everyone wants to apply ASCE7-05 C&C pressures is correct,
then for years wind loads have been too low, and the MWFRS pressures are a
waste of time. Why calculate a gust factor G which comes out less than 0.85,
if all components (rafters, wall studs, trusses) have to be designed loaded
along entire span with C&C pressures?

The MWFRS Cp=+0.8 (Fig6-6) for windward wall does not occur over all the
surface of the wall at the same time, it is a peak value. Also say Cp=-0.4
on windward edge during transverse loading does not occur over entire
surface of a single roof plane. Further more these extremes do not occur at
the same time on all the surfaces being considered. The pressures based on
therefore should not be unconservative for design of a truss, or rafter or
wall stud.

There is a significant difference between applying external Cpe(W,L) for
roof planes of Cp(-0.4, -0.6), then deciding to be conservative and adopting
Cp(-0.6,-0.6) and then going ultra-conservative and replacing with magnified
C&C pressures Cp(-1.2,-1.2), assuming magnification factor from MWFRS to C&C
is 2. Which is effectively the consequence of applying C&C pressures to
design of a truss. Obviously the truss manufacturers will oppose: when
Cp(-0.4,-0.6) has otherwise been adequate for years: and implication is did
survive cyclone Larry if internal pressure set to Cpi=+0.7 (windows broken),
and topographical features considered: buildings not so designed were
amongst the wreckage.

However, difficult to have sensible discussion about wind loading
requirements when far too many are declaring ASCE7-05 far too complex.
Rather than referring to texts on the subject, and suggesting conservative
and practical simplifications which comply with ASCE7.

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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