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Re: Snow Load + Seismic

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Paul,

You misunderstood me, I believe.  In both cases, I was trying to say that
if the design snow load was greater than 30 psf, then one must use 100% of
the design snow load, UNLESS the building official permits a reduction and
the building official can permit a maximum reduction of 75% (in
otherwords, the smallest amount of the design snow load that can be used
for either calculation the seismic masss or for the snow gravity load in
the load combination is 25% of the design snow load, but ONLY after the
code official approves it).  And in both cases, if the design snow load is
less then 30 psf, then it need not be included.

The other "side" thing that I pointed it is the why it is likely that the
building official would use the same reduction in both situations (if the
design snow load was greater than 30 psf), the code does not specifically
say that he/she must.  In otherwords, the building code official could
require a 40 psf (to use Gerard's case) for use in hte load combination,
but only require 10 psf to used in the determining the seismic mass (i.e.
allow the full 75% reduction).  In otherwords, while the wording is almost
identical, there is NOTHING in the code that directly links the two
situations, other than common sense.

Regards,

Scott
Adrian, MI


On Sun, 23 Jul 2006, Paul Ransom wrote:

> > From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
>
> > The point is that to me section 1630.1.1 means that by the 1997 UBC code,
> > you must use 100% of the design snow load as part of the seismic dead load
> > if that seismic design load is greater than 30 pdf UNLESS the builing code
> > offical gives you the approval to use something less that 100% of that
> > load, but even the building official cannot let you use less than 25%.
> > That deals with the seismic dead load that is used to determine the
> > seismic loading (i.e. V).
> >
> > Then if you choose to use a load combination from section 1612.3.2 (i.e.
> > an alternative ASD load combination) that has snow load in combination
> > with seismic (i.e. equation 12-16), then the exception allows you to use
> > zero (0) psf for the S part of the combination if the design snow load is
> > 30 psf or less.  If the design snow load is greater than 30 psf, then you
> > must use 100% of the snow design load for the S part of the load
> > combination, UNLESS the building official give you the approval to use
> > something less than 100%, but again the builing official cannot let you
> > less that 25% of the snow design load.
>
> What is the rationale for including a seismic mass if it doesn't also
> exist similarly as a gravity load (e.g. applying different ratios of
> snow load effect in the same load combination)?
>
> Is it reasonable to assume the maximum snow gravity effect (1 in 50 yr),
> as either gravity mass or horizontal inertial mass, to occur at the same
> time as the maximum seismic effect (1 in 500 yr)? Doesn't this blow the
> statistical basis into microscopically small probability ranges?
>
> I always figured that
> a) the snow mass reduction in E was a statistical twiddle for seismic.
> Also, snow does not necessarily act as a rigid body during a dynamic
> event, and,
> b) applying the full snow gravity load in combinations with E was,
> generally, a conservative simplification and I could avoid calculating
> different S for use in different combinations.
>
> Therefore:
> f(r*S) <= E <= f(1.0*S) when S => 30 psf
> or
> E = f(0*S) when S < 30 psf
>
> Now I interpret your remarks to be a reverse of this:
> 0 <= r2*(S < 30 psf), E => f(r*S)
> or
> 1.0*(S => 30 psf), E => f(r*S)
> r is the reduced factor for the snow mass applied to E
> r2 is the reduced factor for snow gravity load effects applied in
> combination with E.
> (dependent on reductions permitted in each load effect)
>
> Canadian code was conservative if applied prima facie but latest edition
> decided to explicitly use a consistent mass effect for both E and S in
> the same load combination
> e.g. Load effects = E(0.25*S) & 0.25*S.
>
> --
> R. Paul Ransom, P. Eng.
> Civil/Structural/Project/International
> Burlington, Ontario, Canada
> <mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>
>
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