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RE: Wind load and pile depths for wooden fences (San Jose)

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Conrad,

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(--nospam--at)cascade-structural.com



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