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RE: Factor of Safety against Uplift

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> -----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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