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Re: Factor of Safety against Uplift

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Peter,

	Thank you for intervening.  I think your explanation provides a little
more precise explanation than the speculative description I provided for
Syad.

				Regards,

				H. Daryl Richardson

Peter James wrote:
> 
> > -----Original Message-----
> > From: Sherman, William
> >
> > John MacClean wrote:
> >
> > > I'm glad that Daryl mentioned limit states design for overturning which
> > IMHO
> > > is far superior to the allowable stress method at least for overturning
> > > (otherwise it's mostly a pain).
> >
> > Why do you consider limit states design superior to allowable
> > stress design
> > for overturning? Personally, I have never heard of using limit states load
> > factors in stability calculations (overturning, uplift, sliding, etc). In
> > Chapter 18 of IBC 2000, it states to use allowable stress design load
> > combinations with formulas in that Chapter and in Section 1610.2 it
> > recommends a factor of safety of 1.5 against overturning,
> > sliding, and water
> > uplift of retaining walls. Presumably this safety factor is for service
> > loads as it would be excessive to apply it to factored loads.
> > (This section
> > should be clarified that it applies to service loads, and it also
> > should not
> > apply to seismic design conditions.) IMHO limit states should only be
> > applied to "strength" design (material stresses) and not to "stability"
> > design.
> >
> 
> I think there is some confusion relating to the terminology and design
> methods used in different countries. John, Daryl and I are all in Canada,
> where the design philosophy and all load factors are defined by the National
> Building Code, versions of which are adopted by each province.
> 
> The NBC defines "Working Stress Design", and "Limit States Design". Both
> design methods cover strength issues and stability issues. However, most
> material codes are based on limit states design only - working stress codes
> have been phased out.
> 
> The steel code provides an explicit summary:
> 
> "Limit states define the various types of collapse and unserviceability that
> are to be avoided; those concerning safety are called the ultimate limit
> states (strength, overturning, sliding and fatigue) and those concerning
> serviceability are called the serviceability limit states (deflections,
> vibration, and permanent deformation)."
> 
> There is more background to limit states design in Canadian Building Digest
> 221, at:
> 
> http://www.nrc.ca/irc/cbd/cbd221e.html
> 
> I think that another purpose of bringing the strength analysis and the
> stability analysis together is to provide more emphasis on stability - as
> structures have become lighter, and as lateral loads have increased,
> stability has become a more significant design issue that it once was.
> 
> In comparing the various code requirements it was interesting to note that
> the NBC specifically requires that dead load which is counteracting _any_
> live load (not just wind and earthquake) be factored by 0.85. IBC 2000 only
> seems to require a dead load reduction when counteracting wind and
> earthquake loads - is that correct?
> 
> Peter James
> 
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