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RE: STRESS AND FORCE

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Steve,

Why design to the code? The codes are not written for the purpose of design,
they are written for assessing the achievement of an acceptable level of
performance. It is not necessary to do calculations verbatim to the code,
only to remain within the guidelines of the code.

As for forces or stress, the choice depends on what you wish to do. It is
easier to compare moment capacities across sections and materials. Since
higher strength material doesn't need the same second moment of area, to
achieve the same resistance. So it becomes easy to compare high material
strength c-section against lower material strength hot-rolled section, or
even timber or aluminium. Also once have member capacity curves it is easy
to over lay load capacity curve for an application and determine the
limitations for a range of sections. Once got such system-curves printed,
don't really need the spreadsheets, can get the answer before windows has
booted up.

Most of the calculations for building structures are a waste, an unwarranted
delay. Point-value calculations for small variations in dimension from one
project to the next is not really helpful, when all really needed to do was
find the limiting span for the application, and then pay attention to when
the structural model for the application is invalidated by the current
building/structure: especially if simple framing.

And if work for manufacturers then really need to know what structural
sections are most economical to order in bulk and suitable for the majority
of projects which come their way. Also if a roll-former then what structural
section can you design with a reasonable market (range of application) which
say can get into the residential area and be used alongside say timber.

Historically there weren't stresses, just physical testing and breaking
loads or limits on deflections.

The codes are there to assess performance and fitness-for-function, I don't
believe there is any mandate that detailed calculations to the code have to
be completed for each and every like project. That most civil/structural
engineers do so, probably explains the lack of productivity in building and
construction. The engineers keep re-inventing the wheel, thus contributing
little of added value to the industry.

Consider this the WFCM guide for 130 mph, implies it is only of use for
regions with a 130 mph basic wind speed. Which is not entirely true, the
structure has a given resistance, and that resistance is suitable for a
multitude of wind conditions which generate the same reference wind pressure
qz, or if in Australia reference wind speed Vzu. Different terrain, and
topography can develop similar design wind speeds at different sites. By
applying the 'k' factors to the basic wind speed a reference design wind
speed can be calculated for a site and/or a pre-engineered building. A
suitable manufactured building can thus be easily selected for a site, based
on its design wind speed or wind class. Of course there are other issues to
check such as seismic and snow loading, but once again some simple
assessment against a reference load could be worked out.

Also I hazard a guess that some 80% of structural components can be assessed
once by each industry or enterprise in each code cycle, whilst the 20% which
requires individual assessment represents those novel and innovative
situations which give rise to the codes becoming more complex.

And as others have implied do the structural calculations really benefit
anyone. The builder or fabricator has some left over steel from previous
project, over sized for current project, but it is suitable. And does
varying nail spacing between 4" and 6" centres for plywood bracing really
save the builder or the building owner anything in the over all scheme of
things? Why not simply provide nailing to match the capacity of the panel?
If doesn't save time, or reduce costs, why waste time on calculations for
reduced resistance, when can provide a higher resistance/performance
building with little effort?

By the way, in the USA, do you have plywood and hardboard panels sold as
bracing, pre-printed with fastener spacing, and with known resistance?

More attention to the reason for the calculations and the value of such
calculations would benefit the building industry. I believe there is a great
deal of wasted effort in design offices.


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




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