Thank you for your thoughts and the information links. It sounds like your
Aussie codes have it a little more together than Codes here in the States,
but that isn't too hard. We are still working out the bugs from the
processes of merging 3 model building codes and a huge host of specialty
codes and integrating with all the material standards. Good progress is
being made, but we still have a long way to go.
There is much less consistency in the science of our load and material
strength design information. I don't know the particular statistical
measure, but I think the material standards are trending toward some
consistency, although there are still differences in how "strength" is
measured, depending on if the material is brittle or ductile, and how
sensitive it is to installation conditions, etc. Most of our loads are
based on different return periods. There is some logic to this, because
different loads have different distributions and pose different risks to the
building occupants. Snow loads occur regularly and may often be near design
levels. When the design load is exceeded, it may only be by a small
percentage. Seismic loads may only occur 1 or 2 times in the life of the
structure, and when the design load is exceeded, it could be by a very large
percentage. There is also an element of predictability and control for some
load types and not others. If floor live load is exceeded, it may be easy
to observe the situation (sagging beams) and control it (remove some of
filing cabinets from that room). Seismic loads cannot be predicted or
controlled, so the structure simply has to withstand the load. Thus there
is some logic in designing for a longer return period for seismic loads than
other load types to achieve a consistent reliability. This is all part of
the science of load estimation, and is reflected (partially) in the load
factors and combinations.
I wonder if your preference for burying the factors of safety in the design
equations to avoid a lawsuit is typical of structural engineers. This seems
to be one possible explanation for the way the equations are currently
configured. This is a very unfortunate situation that we have to
overcomplicate an already complicated issue in order to reduce public
scrutiny.
Dmitri Wright, PE
Cascade Engineering, Inc.
245 SE 4th Ave, Suite A
Hillsboro, OR 97123-4033
dmitri@cascade-structural.com
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