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RE: Photovoltaic Panels & Manufactured Metal Plated Roof Truss - Addendum

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Also here I believe there is still an on going debate that the output from
the truss manufacturer's software is inadequate. I'm not sure about other
states but in South Australia, we require an independent technical check
before approval can be granted. Certifying engineers cannot sensibly do that
based on the information output from the software; further more there are
some doubts that the software can actually design the joints properly.
{There is a tendency for building officials to reject manufacturers
catalogues produced and certified by the same engineer interstate.}

After all the old truss design manual provided templates of each nail plate,
so that the number of teeth in each side of the joint can be determined and
find an appropriate orientation for the nail plate. There have been
incidences of engineers specifying custom trusses which are too shallow and
practical to fabricate, and relying on the truss manufacturers to design by
software. Or if can fabricate there is not enough timber on at least one of
the members to accommodate adequate number of nails. So whilst the software
may be able to pick a nail plate which as potential, there is no one sitting
down and doing the detailing to verify it.

We have had some improvements in terms of the software output, and
requirements on the part of the manufacturer for someone to take
responsibility for the relevance to the individual project. But still no
proper workshop detailing and design to check for irregularities. For most
residential trusses detailing would be a waste, they are all much the same.
But I believe the irregular or otherwise less commonly used trusses should
be looked at more closely.

It sounds like in the US there is detailing of the trusses, beyond software
output. Could someone confirm?

More trouble than timber is proprietary steel framing. The framing
manufacturers are not interested in retrofitted items, they cannot do the
checks using the information they have. They may be willing to provide a
framing manual but getting section properties of the cold-formed sections is
more difficult.

One benefit we do have here is our simplified wind classification system for
products. So a truss and house framing would typically be designed for say
wind class N2, for which the design wind speed is 40 m/s, a typical
assessment to AS1170.2 would produce a design wind speed of around 37 m/s
and therefore some reserve available. Wind class N1 having design speed of
34 m/s, and N3 speed of 50m/s. (or N1=W28, N2=W33, N3=W41 using old
permissible stress wind speed maps.)

{NB: we use design wind speeds: basic regional of 45m/s is reduced or
magnified to match conditions at the site.}

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

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